Table of contents

We employed edge-magnetoplasmon (EMP) resonance technique to study magneto-transport properties of 2DES where edge states play an important role. We measured EMP frequencies and linewidths of 2DES on liquid helium surface under various lateral con nement potentials. The experimental results show that as the lateral con nement potential is reduced measured EMP linewidth takes minimum at a certain strength of con nement potential and broad signal was obtained on further potential reduction. This broadening behavior is absolutely unexpected from the exsisting theories of conventional EMP. We consider that an oscillation mode transition is responsible for the linewidth broadening. The linewidth behavior in the strong con nement region is reasonably explained by conventional EMP, while the broad signals in the weak con nement region is not. We show our experimental data and discuss the origin of the line broadening.

By using luminescence spectroscopy, we have established the influence of low-temperature uniaxial deformation on the configuration of a self-trapped exciton (STE) in alkali halide crystals (AHC) at the instant of radiative relaxation. In face-centered crystals, there occurs the redistribution of the luminescence intensity from the asymmetric configuration of STE to the symmetric one (III → II → I- types), whereas in volume-centered crystals, on the contrary, the evidence counts in favour of the asymmetric configuration of STE (I→ II–types). The external deformation in the <100> direction leads to an effective sliding of anions in the <110> direction that coincides with the direction of the STE squeeze which classically makes for the creation, of the symmetric STE configuration in the <110> direction, acting perpendicularly to the STE' length results in their stretching, which in turn brings into the creation of the asymmetric STE configuration with a higher degree of the polarization.

We have investigated the effect of asymmetry in tunnelling resistance of individual normal metal-insulator-superconductor (NIS) tunnel junctions that constitute a SINIS pair, both experimentally and theoretically. Ours results clearly demonstrate that any finite asymmetry in the tunnelling resistance gives rise to an excess current, as compared to its symmetric counterpart, both below and around the gap edge. The signature of this excess current is visible almost up to the critical temperature. We find that this apparent broadening of the density of states is purely electrical in origin. Our calculations also show that any finite resistance that is in series with the tunnelling resistance, such as the resistance of normal metal island or the line resistance in case of two probe measurements, leads to a suppression of the conductance maxima at the gap edge. This is a manifestation of the finite voltage drop across the series resistor. Our experimental results validate our theoretical prediction.

Transport measurements of as-grown and annealed (Ga,Mn)As samples were conducted under pulsed high magnetic field up to 40T. Assuming that the magnetization follows modified Brillouin function, we obtained the hole densities of the (Ga,Mn)As samples at various temperatures, which generally increase with temperature and become two times larger after annealing process. The hole density determined by electrical transport measurement at low magnetic field up to 1.5T is found to be underestimated.

Magnetoresistance components ρ_xx and ρ_xy were measured in two p-Si/SiGe/Si quantum well samples with an anisotropic g-factor in a tilted magnetic field of up to 18T as a function of temperature (20 mK-2 K) and tilt angle. We analyzed dependences of the conductivity, the activation energy ΔE, and the filling factor ν on the tilt angle ⊝. In the sample with density p = 2 × 10¹¹ cm⁻² in the vicinity of ν = 2 ΔE(⊝) undergoes a minima at ⊝ ≍ 60^o, while ν(⊝) shows a sharp jump. These facts allowed us to conclude that at ⊝ ≍ 60^o and ν ≍ 2 a crossing of the Landau levels 0↑ and 1↓ occurs. It leads to the first order ferromagnetic-paramagnetic (F-P) phase transition. A coexistence of two phases at the transition point also supports the idea. However, in another sample, with p = 7.2 × 10¹⁰ cm⁻², no transition was observed. For both samples we have obtained the dependences of the effective g-factor on the tilt angle, which led us to the conclusion that the F-P transition in the p-Si/SiGe/Si structure in a tilted magnetic field is a result of a violation of the g∗-factor axial symmetry due to disorder.

Superfluid state of a magnetoexciton gas in bilayers is studied with reference to graphene-dielectric-graphene structures subjected by a perpendicular to graphene layers magnetic field B. We find that in difference with quantum Hall bilayers with the total filling factor v_T = 1, an imbalance of filling factors of graphene layers is required. An imbalance can be created by an electrostatic field E applied perpendicular to graphene layers. We determine the range of B and E where magnetoexciton superfluidity can be realized. The dependence of critical temperature and critical current on magnetic field is computed. It is found that the maximum critical temperature is reached at B ≍ 0.5ø₀ød², where ø₀ is the magnetic flux quantum, and d is the interlayer distance. It is shown that the interaction of electrons with impurities reduces the critical temperature. The critical concentration of impurities is determined. Stationary waves in a superfluid magnetoexciton gas are considered. The waves are induced by counter-propagating electrical currents that flow in a bilayer with a point obstacle. It is found that the stationary wave pattern is modified qualitatively under variation of B.

We investigate tunneling through a resonant level formed in a carbon nanotube quantum dot contacted by resistive metal wires. These contacts create a dissipative environment for the electrons tunneling across the nanotube, thus suppressing the tunneling rate. We study the shape of the resonant peak in the nanotube conductance, with the expectation that the peak width and height, both dependent on the tunneling rate, will be suppressed. Instead, we find that the behavior crucially depends on the ratio of the tunneling rates from the resonant level to the two contacts. We discuss the implication of our findings for a boundary quantum phase transition in this system.

We carried out magnetotransport experiments in the v = 1 bilayer quantum Hall state (BQHS) using a GaAs/AlAs double-quantum-well structure with tunneling energy as small as 1 K. We focus on measurements of not only activation energies but also onset temperatures of the BQHS for a wide range of the total density and the layer density imbalance. We have found that the dependency of onset temperature on the total density is different from that of the activation energy. We discuss physical interpretations of the onset temperature with relationship to finite-temperature phase transitions in the v = 1 BQHS.

A two-dimensional electron gas in a tilted magnetic field with Rashba spin-orbit interaction (RSOI) was studied. The RSOI is accredited to the asymmetry of the heterostructure where the two-dimensional electron gas is found. The effects of the disorder-attributed Landau level broadening and the RSOI on the spin splitting were identified by simulating the density of states which was assumed to take a Gaussian shape. Increased Landau level broadening obscures the spin splitting and increases the overlap between spin states resulting to stout Gaussian peaks. On the other hand, stronger RSOI amplifies the splitting and lessens the overlap between spin states of the Landau levels. The splitting, however, results to stouter peaks. The similarity in the RSOI and Landau level broadening effects can be explained by recognizing that the asymmetry of the heterostructure is in itself a form of structural disorder.

We have investigated how the Andreev-reflection hole current at ballistic point contacts responds to a large bias voltage. Its strong suppression could be explained by the drag excerted by the non-equilibrium phonon wind generated by high-energy electrons flowing through the contact. The hole - phonon interaction leads to scattering lengths of the low-energetic holes down to 100 nm, thereby destroying the coherent retracing of the electron path by the Andreev-reflected holes.

We have investigated break junctions of normal non-magnetic metals as well as ferromagnets at low temperatures. The point contacts with radii 0.15—15 nm showed zero-bias anomalies which can be attributed to Kondo scattering at a single Kondo impurity at the contact or to the switching of a single conducting channel. The Kondo temperatures derived from the width of the anomalies varied between 10 and 1000 K. These results agree well with literature data on atomic-size contacts of the ferromagnets as well as with spear-anvil type contacts on a wide variety of metals.

We study single-electron transport through a double quantum dot (DQD) monitored by a capacitively coupled quantum point-contact (QPC) electrometer. We derive the full counting statistics for the coupled DQD - QPC system and obtain the joint probability distribution of the charges transferred through the DQD and the QPC consistent with the fluctuation theorem for four terminal system. For the two-terminal DQD system, the FT is not necessarily satisfied. It is due to the back action caused by the shot noise of the QPC, and the FT for the DQD is modified with an "effective temperature".

We report the experimental results and theoretical understanding of the Quantum Point Contact Transistor - a fully ballistic one-dimensional (1D) Field-Effect Transistor (FET). Experimentally obtained voltage gain greater than 1 in our Quantum-Point-Contact transistors at 4.2 K can be explained with the help of an analytical modeling based on the Landauer-Büttiker approach in mesosopic physics: the lowest 1D subband and the band gap play the key role in increasing its transconductance, especially by reducing its output conductance, and thus achieving a voltage gain higher than 1. This work provides a general basis for devising future ballistic FETs and the quantum limits found in this work may be used to estimate normalized transconductance and channel resistance in future two-dimensional (2D) ballistic FETs.

For the model of a linearly driven quantum anharmonic oscillator, the role of damping is investigated. We compare the position of the stable points in phase space obtained from a classical analysis to the result of a quantum mechanical analysis. The solution of the full master equation shows that the stable points behave qualitatively similar to the classical solution but with small modifications. Both the quantum effects and additional effects of temperature can be described by renormalizing the damping.

We study a many-body ground state of graphene in perpendicular magnetic fields. Chiral symmetry in graphene enables us to determine the many-body ground state, which turns out to be a doubly degenerate chiral condensate for the half-filled (undoped) case. In the ground state a prominent charge accumulation emerges along zigzag edges. We also show that gapless excitations are absent despite the presence of the robust edge modes, which is consistent with the Chern number C = 0.

Current rectification in (Ga,Mn)As tri-layer magnetic tunnel junctions (MTJs) is found to be controllable through the alignment of magnetizations, which can be changed with small current injections. The tunneling magnetoresistance (TMR) at 4.2K is 120% in amplitude, showing three step structure, which corresponds to the alignment of magnetizations. With a minor field loop, the alignment of magnetization can be anti-parallel for the top and the bottom layers and then current injections with alternative direction can reverse the direction of the magnetization in the middle layer. The threshold current is as low as 2 × 10⁴A/cm². We have found the junctions have small rectification effect up to 8GHz, which is strongly dependent on the alignment of the magnetization. Hence the direction of the rectification as well as the amplitude can be switched by the bi-directional current injections. The rectification can be explained within the Julliere model with enegy dependence of the density of states. To check this we performed tunneling measurements and obtained positive results.

We study the effect of disorder in the normal and superconducting phases of the bilayer graphene. We choose the Bernal stacking configuration to characterize the graphene bilayer, and introduce the kernel polynomial method to provide numerical results for the evolution of optical conductivity with the increase of disorder. The Drude weight is found to decrease very rapidly with the enhancement of disorder and drop to zero at a critical strength W_c, suggesting that Anderson metal-insulator transition can take place. In the superconducting phase, the distribution of the inhomogeneous superconducting gap is obtained by solving self-consistently the Bogoliubov-de Gennes equations under the mean-field description, and the significant suppression of the Drude weight is obtained in the weak disorder region.

Superconducting Nb films with honeycomb array of holes are studied using transport measurements. The oscillating magneto-resistance curves are observed up to large flux density. Two types of resistance minima with different field intervals are observed, indicating the reconfiguration of the overall flux lattice from honeycomb to triangular arrangement. Moreover, hysteretic effects are found in a very large field span from H = 2H₁ to H = 8.5H₁. It is revealed that the hysteresis is related to the presence of interstitial vortices.

We report the tunneling spectroscopy investigation in an atomic scale gap filled with liquid ⁴He using mechanically controllable break junction (MCBJ) technique. In order to assure the filling of liquid ⁴He into the gap, we construct a cryostat with an inner chamber for the tunneling spectroscopy inside the vacuum jacket of the liquid ⁴He bath. MCBJ apparatus is installed in the inner chamber with a flexible bellows. After filling inner chamber with liquid ⁴He below 4.2 K, Au electrical electrodes were stretched by the mechanical force generated by a piezo device. We observed the increase of the tunnel conductance through liquid ⁴He compared to that in the vacuum environment.

Magnetic properties of edge states in the chiral p-wave superconductor Sr₂RuO₄ are investigated based on a multi-band model restricting to the two bands derived from the 4d-t_2g d_yz/d_zx-orbitals. While generally low-energy gapless Andreev bound states appear at the edges which carry spontaneous charge current, they are not topologically protected. Including spin-orbit coupling of the two orbitals we find additionally edge spin currents which are present in the normal as well as the superconducting phase. They contribute to the anomalous and the spin Hall effect and generate in the superconducting phase a finite spin polarization at the surface. Repulsive onsite interaction strengthens the spin polarization through the coupling to a nesting driven incipient magnetic phase of the system. We speculate that the magnetizations of the spin polarization and the chiral edge currents could partially compensate each other, in view of recent experimental results.

Low-energy electronic states in a topological insulator nanowire is studied theoretically. We highlight the role of spin Berry phase which distinguishes transport characteristics of such a nanowire from those of a single-wall carbon nanotube.

We present an anisotropy of the hysteretic transport due to the dynamic nuclear polarization (DNP) around the spin transition point at Landau level filling factor ν = 2/3 in tilted magnetic field. When the direction of the in-plane component of the magnetic field B_|| is normal to the probe current J, a strong hysteresis related with DNP occurs. On the other hand, as B_|| is parallel to I, the hysteresis almost disappears. To clear the origin of the anisotropy of DNP, we study the nuclear spin-lattice relaxation and nuclear spin polarization rates at the transition point and find out that they are anisotropic for a relative angle between B_|| and I.

We propose a renormalization scheme of the Kubo formula for the electrical conductivity with multiple backscatterings contributing to the electron-hole irreducible vertex derived from the asymptotic limit to high spatial dimensions. We use this vertex to represent the two-particle Green function via a symmetrized Bethe-Salpeter equation in momentum space. We further utilize the dominance of a pole in the irreducible vertex to an approximate diagonalization of the Bethe-Salpeter equation and a non-perturbative representation of the electron-hole correlation function. The latter function is then used to derive a compact representation for the electrical conductivity at zero temperature without the necessity to evaluate separately the Drude term and vertex corrections. The electrical conductivity calculated in this way remains nonnegative also in the strongly disordered regime where the localization effects become significant and the negative vertex corrections in the standard Kubo formula overweight the Drude term.

We report on the observation of a modulation on the circular photogalvanic effect (CPGE) imposed by an extra optical radiation in a GaAs-based two dimensional electron gas system. The wavelength of the radiation for exciting the CPGE is 1064 nm and the wavelength of the modulation is 532 nm. The experiment is carried out from 77 K up to room temperature. The 1064 nm induced CPGE modulated by the 532 nm radiation increases as the increasing temperature. We also vary the power of the modulation beam to investigate the intensity dependence of the modulation effect. The modulation exhibits a linear dependence at low intensity. As the intensity increasing, we observe a saturation at certain level of the intensity and a suppression of the modulation when the intensity is further increased. The investigation of photoconductivity reveals that the change of the photoexcited charge carrier density has little contribution to the radiation modulation effect. Therefore, the microscopic mechanism of the radiation modulation effect can be attributed to the modulation of spin-orbit interaction in the structure.

We study a double quantum dot system coherently coupled to an electromagnetic resonator. By suitably biasing the system, a population inversion can be created between the dot levels. The resulting lasing state exists within a narrow resonance window, where the transport current correlates with the lasing state. It allows probing the lasing state via a current measurement. Moreover, the resulting narrow current peak opens perspective for applications of the setup for high resolution measurements.

We discuss our recent theoretical proposal to detect interactions in the electron transport through a nano-scale system by measuring high-order factorial cumulants of the full counting statistics. Our proposal is based on theoretical studies which have demonstrated that the zeros of the generating function for the full counting statistics are always real and negative for non-interacting electrons in a two-terminal scattering setup. As we have shown, this implies that the factorial cumulants do not oscillate as functions of any system parameter. Interactions, however, can cause the zeros to move away from the negative real axis into the complex plane. This transition is clearly visible in the factorial cumulants which start oscillating. We illustrate our findings with a model of transport through a two-level Coulomb blockade quantum dot, which we analyze both for finite times and in the long-time limit, and we discuss possible experimental implementations to test our predictions.

We use a superconducting quantum interference device (SQUID) to detect the phase of an InAs quantum dot Josephson junction (QDJJ) in the Kondo regime. The QDJJ phase is derived from the measurement of superconducting interference. The π-junction behavior is observed for a QD with an odd electron number and the phase transition from π to 0 is observed by changing the parity of the electron number. In the Kondo regimes, we find that the QDJJ becomes a 0-junction even though the electron number is odd.

We have discussed anomalus excitations in the electron state of the semiconductor dot from viewpoint of field-theoretical formula. It is suggested strongly that exotic excitations with fractional charges might exist in the semiconductor-dot with the domain wall shell.

An intriguing anomalous dielectric behavior is observed in nanoparticle (NP) La₂O₃: SiO₂ nano-glass composite system synthesized via sol-gel route at different calcination temperatures. Temperature dependent dielectric properties exhibit a notable dielectric broadening, indicating of diffuse phase transition with high ∊', quite different from and much higher than pure bulk La₂O₃ and SiO₂. We postulate such dielectric effect in the context of the oxygen vacancies of the rare earth oxide nano-glass composite, where lattice strain related with NPs and their size plays a vital role. Such a material might be treated as a potential candidate to solve the problem of devices miniaturization.

We have theoretically studied intrinsic spin Hall effect in a quantum wire with spin-orbit interaction. Our numerical calculations show that the current-induced distribution of electronic spins has characteristic spatial dependence in a quasi-one-dimensional tight-binding model with spin-dependent hopping. The difference between the chemical potentials of electrons with up and down spins shows spatial oscillation in a direction perpendicular to the charge current and reaches the maximum around one edge and the minimum around the other edge, which suggests spin accumulation around edges of the quantum wire.

The conductance of a system containing two tunnel point-contacts and a single subsurface scatterer in the presence of a magnetic field is investigated theoretically. A general formula for the dependence of the conductance on the distance between contacts, the defect position, and the magnetic field is obtained. It is shown that in the presence of a magnetic field the conductance undergoes Aharonov-Bohm type oscillations. We find a simple relation between the period of the oscillations and the depth of the subsurface impurity. On the basis of this fact a new and easy method of determination the depth of the buried impurity is proposed.

We report experimental finding of current blockade in a quantum dot (QD), which effect originates from spin-filtering in a quantum point contact (QPC) with spin-orbit interaction (SOI). Our QPCs made of Ino.iGao.9 As showed clear G_q/2 plateaus (G_q ≡ 2e²/h), which suggest the spin-filtering. The effect has been further confirmed in the conduction of a QD defined by two such QPCs. The Kondo effect has been utilized for the clarification of the spin states in the QD and an ordinal Coulomb valley with spin 1/2 has been chosen for the stage to test the filtering. One of the bounding Coulomb peaks disappears with the applicaion of a finite source-drain voltage V_sd while the other grows in height. The sign reversal of V_sd transposes the heights of the two peaks. Further increase in |V_ad| recovers the dual peak configuration. Every aspect of the above characteristics behavior is explained under the hypothesis of spin-filtering in the QPC at the plateau of G_q/2. From the peak height ratio, the lower bound of the filtering efficiency is estimated to be above 80%.

We show that in three-dimensional (3D) topological insulator Bi₂Te₃, the surface nature is revealed as a quantization of motion in Dirac fermions due to their confinement at the surface. The consequence of this z-quantization is the oscillation in magnetoresistance (MR) with periodicity proportional to the magnetic field. We demonstrate that based on single Dirac theory at the surface, where disorder is properly taken into account, the thickness of the surface state or a fundamental length scale of topologically nontrivial state is extracted from the oscillating part of MR data. The same theoretical framework also explains the nonoscillating part of MR and Hall resistance at the low field region, showing that topological contribution is important.

Electrical conductivity of electrons on helium film covering a two dimensionally corrugated dielectric substrate was measured in a temperature interval between 0.4 and 1.64 K. As the temperature decreased from 1.5 K, the electrical conductivity decreases and approaches gradually to zero around 0.4 K. This temperature dependence is different from that measured in 2D electron system on bulk liquid helium. In order to understand the effect of the dielectric substrate, the dependences of electrical conductivity on magnetic field were measured by using a Corbino electrode and the mobility was evaluated in different temperatures. The results revealed that the mobility of electrons increases as the temperature decreases. Combining the data of electrical conductivity and the mobility, it is deduced that the decrease in conductivity as the temperature decreases is due to the decrease in number of mobile electrons.

We have observed the magneto-resistance (MR) enhancement due to the self-hole-doping (SHD) in LaMnO₃ (LMO) produced by low temperature heat treatment (LTHT). The polycrystalline LMO samples were prepared by solid-state-reaction. After the calcination at 1100°C for 18 hours under oxygen gas flow (OGF) and grinding, the powders were pressed into pellets and were sintered at 1100°C and 1300°C for 18 hours under the OGF Although the single crystal of LMO is an insulator, the polycrystalline LMO samples showed large electrical conduction due to the SHD and especially the sample sintered at 1100°C indicated the metallic conduction. We consider that by the sintering in sufficient oxygen gas atmosphere, excess oxygen was mainly introduced into the amorphous-like grain boundary regions (AGBRs) and the high SHD was caused. The LTHT generated large AGBRs in the LMO samples and the MR enhancement was observed.

We have observed large magneto-resistance (MR) intensified by rapid thermal annealing (RTA) in magnetite (Fe₃O₄) thin film (MTF) on SiO₂ glass (a-SiO₂) substrate. The MTF was produced by the RF magnetron sputtering method by using a magnetite target. The electrical resistivity (ER) of as-grown MTF (AG-MTF) showed the Mott's variable range hopping behavior, which implies that the AG-MTF is amorphous-like. Although the magneto-resistance (MR) ratio of bulk single crystal is very small except around the Verwey transition temperature (VTT), that of the AG-MTF showed moderately large below room temperature. Due to RTA of the AG-MTF by use of an IR image furnace, the MR ratio of MTFs was intensified, and especially by the annealing around the Curie temperature (585°C) of magnetite. Furthermore the ER of the rapid thermally annealed MTF (RTA-MTF) showed a slight kink at around the VTT, which indicates that the crystallinity of the RTA-MTF is higher than that of the AG-MTF The MTF produced by the RF magnetron sputtering method are composed of magnetite fine particles (MFPs). We consider that the directions of magnetic moments of MFPs in the MTF were spatially randomized by the RTA and the strong spin scattering of itinerant electrons transferring between adjacent MFPs caused the intensification of the MR ratio.

We theoretically examine a current drag in coupled double quantum dots (DQDs). When a current is driven through a DQD by a bias voltage, it drags the current through the other DQD by the Coulomb interaction between the DQDs. In the quantum mechanical regime of coherent transport, the drag current shows two resonant peaks, a sharp peak due to a cotunneling process of two electrons and a broad due to the single electron resonance. The emission of LA phonons suppresses the former resonance and broadens the latter. In the large limit of the phonon emission rate (classical regime), the drag current still flows in an inelastic way, showing a plateau structure.

A dynamical approach to ballistic transport in mesoscopic graphene samples of finite length Land contact potential difference with leads U is developed. It is shown that at ballistic times shorter than both relevant time scales, t_L = L/v_g (v_g - Fermi velocity) and t_u = ħ/(eU), the major effect of electric field is to creates the electron - hole pairs, namely causes interband transitions. At ballistic times lager than the two scales the mechanism is very different. The conductivity has its "nonrelativistic" or intraband value equal to the one obtained within the Landauer-Butticker approach for the barrier Uresulting from evanescent waves tunneling through the barrier.

The mathematical description of pure dephasing in a three-level system is not as straightforward as it is in a two-level system. Here we provide a detailed derivation of the Liouvillean for pure dephasing for a ladder three-level system, based on [J Li et al., Phys. Rev. B 84, 104527 (2011)]. Numerical calculations based on this model give good fittings to the spectral line and Autler-Townes splitting observed in a superconducting phase qubit.

Dissipative properties of the electromagnetic environment as well as on-chip RC filtering are shown to suppress random state switchings in the two-junction superconductor(S) - normal metal(N) electron trap. In our experiments, a local high-ohmic resistor increased the hold time of the trap by up to two orders of magnitude. A strong effect of on-chip noise filtering was observed for different on-chip geometries. The obtained results are promising for realization of the current standard on the basis of the S-N hybrid turnstile.

Structure inversion asymmetry (SIA) related circular photogalvanic effect (CPGE) has been investigated under near-infrared radiation at temperatures ranging from 80 K to 290 K in GaAs/Al_0.3Ga_0.7As heterostructures. The result shows nonmonotonic changes of CPGE with temperature variation; obviously larger signal at low temperature and sign inversions in the temperature range between 140 K and 170 K are observed. We suggest this result is not only related to the photoconductivity and Rashba spin splitting, but other factors superpose on them.

We calculate the energy band structure and the Fermi surface of PuS, PuSe and PuTe by using a self-consistent relativistic linear augmented-plane-wave method with the exchange and correlation potential in a local density approximation. It is found in common that the energy bands in the vicinity of the Fermi level are mainly due to the hybridization between Pu 5/ and monochalcogenide p electrons. The obtained main Fermi surfaces are composed of two hole sheets and one electron sheet, all of which are constructed from the band having the Pu 5/ state and the monochalcogenide p state.

We study the effect of annealing in high vacuum on the transport properties for In2O3-ZnO films. We prepared indium zinc oxide films by the DC-magnetron sputtering method using an In2O3-ZnO target (89.3 wt % In2O3 and 10.7 wt % ZnO). The annealing temperature is from 373 to 773K. From the XRD analysis, we find that all as deposited films are amorphous. In addition we find that amorphous films are crystallized by annealing at a temperature above 773 K over 2 hours. The temperature dependence of resistivity ρ of all amorphous films shows metallic behaviour. On the other hand, ρ(T) of poly In2O3-ZnO films shows semi-conducting behaviour. We carry out a detailed analysis of the temperature dependence of Hall mobility. The activation energy Ed has been obtained from the slope of the carrier concentration Ne vs. the inverse temperature plot at high temperatures. We found that the Ed takes values between 0.43 and 0.19 meV. Meanwhile, temperature dependence of Ne for poly-In2O3-ZnO films did not show activation-like behaviour. This behaviour is thought to be causally related to impurity conduction band.

The AuAl₂ intermetallic compounds are of substantial interest in view of their application potential. The investigated intermetallics AuAl₂+6%Cu were prepared from fine powders of AuAl2 and Cu by vacuum sputtering on a glass substrate and consisted of films with a thickness of about one micrometer. The films were annealed. The temperature and field dependence of the electroresistivity, the magnetoresistivity and the Hall effect of AuAl2+6%Cu films were measured in the temperature interval from 4.2 to 100 K and at magnetic fields of up to 15 T. We demonstrate that the temperature dependence of the electroresistivity has a minimum at T = 20 K and a metallic behavior above this temperature. The magnetoresistivity is very small (less then 1%), positive at low temperatures and negative above 12 K. The Hall coefficient is positive, which corresponds to the holes in a one zone model with a charge carrier concentration of about 1.6 10²⁰ cm⁻³.

The transverse magnetoresistivity of pure tungsten single crystals with a residual resistivity ratio ρ_293K/ρ_4.2K of about 75000 was measured from 4.2 to 20 K and in magnetic fields of up to 15 T. The size effect, i.e. the linear dependence of the magnetoconductivity on the inverse cross sample dimensions, was studied in detail at high fields. We show that the size effect can be used for the separation of the contributions from the electron-surface and the electron-phonon scattering mechanisms to the full conductivity. We demonstrate that the electron-phonon scattering leads to the exponential temperature dependence of the conductivity, and the interference between the electron-phonon and the electron-surface processes leads to a new scattering mechanism "electron-phonon-surface" with a quadratic temperature dependence of the magnetoconductivity.

We experimentally investigated the switching current I_sw of a superconducting single electron transistor (sSET), with competitive Josephson coupling energy E_J and charging energy E_c, in an in situ tunable dissipative environment. At low temperature, averaged I_sw shows a clear 1e periodic function of gate induced charge on the island. The relative contrast of this oscillation increases when E_J/Ec is lowered. Increasing dissipation reduce the quantum fluctuations of the phase across the sSET, leading to an increased I_sw.

We numerically study the behavior of σ_xy(ω) and σ_xx (ω) for graphene QHE system in the ac (frequency ω) domain. We interpret these conductivities with the dynamical scaling analysis. We also discuss the temperature flow of σ_xy(ω) — σ_xx(ω) diagram for graphene QHE system in the ac region.

We have succeeded in growing high purity single crystalline Ce₂PdGa₁₂ which crystallizes in the tetragonal system (space group P4/nbm). Ce₂PdGa₁₂ is a typical heavy-fermion antiferromagnet with the magnetic ordering temperature of 11K at ambient pressure. We investigate the pressure effects on the electrical resistivity for Ce₂PdGa₁₂ up to 8GPa. With increasing pressure, the ordering temperature is dramatically suppressed to 3K at 3GPa. The electronic state of Ce2 PdGa12 are tuned by pressure from the heavy-fermion antiferromagnetic state to the non-magnetic state through a critical pressure around P_c∼7GPa.

We theoretically study the transport through an ensemble of double quantum dots (DQDs) fabricated in a microwave cavity. The DQDs are coupled to a common eld of photon by coupling constant λ, whereas the photons escape from the cavity with the rate of phħ. When ph ≥ λ, a photon emission from the DQDs creates an entanglement among electrons in the DQDs, which enhances the rate of the subsequent emission (superradiance). We propose a transport experiment to observe the superradiance using a sequence of voltage pulses.

Interferometers are ubiquitous devices in optics, consisting of arrangements of totally reflective and semitransparent mirrors, fiber optics, detectors, etc. The smallest possible mirror consists of a single atom, and with recent advances in nanotechnology it is possible to fabricate them and couple them to transmission lines. This can be realized for example with superconducting qubits and superconducting coplanar waveguide resonators or with dipole emitters coupled to surface-plasmon nanowires. Based on a recent proposal [G S Paraoanu, Phys. Rev. A 82, 023802 (2010)], here we give a brief overview of the two-atom Mach-Zender interferometer. Here we show that both the phase and the amplitude of the output field can be used to extract information about the phase difference between the two arms of the interferometer. Also, we point out that this device can be used as well in the reflection mode.

Electron - hole pairs are copuously created by an applied electric field near the Dirac point in graphene or similar 2D electronic systems. It was shown recently that for sufficiently large electric fields E and ballistic times the I-V characteristics become strongly nonlinear due to Schwinger's pair creation rate, proportional to E^3/2. Since there is no energy gap the radiation from the pairs' annihilation is enhanced. The spectrum of radiation is calculated and exhibits a maximum at The angular and polarization dependence of the emitted photons with respect to the graphene sheet is quite distinctive. For very large currents the recombination rate becomes so large that it leads to the second Ohmic regime due to radiation friction.

We investigated the magneto-conductivity Δ in three dimensional indium zinc oxide films with different resistivity ρ prepared by postannealing in air. The weak localization theory was fitted to data of Δ H) at temperatures below 50K by the use of suitable inelastic scattering time τ_i(T) and spin-orbit(S-O) scattering time τ_i. We found the ρ dependences of both times τ and τ_i in a range 1.5 × 10⁻³Ω < ρ 300K) <4 × 10⁻⁶Ω. As ρ increases, the ratio τ_i/τ increases from ≍ .005 to ≍ .5 and the Δ ⁻ at low temperatures changes from positive to negative values. We suggest a picture that the annealing in air brings the change of the S-O scattering from light to heavy atoms, namely, oxygen to indium and/or zinc atoms.

Recently the condition for the existence of gapless modes in arbitrary topological defects is proposed [Jeffrey C. Y. Teo and C. L. Kane, Phys. Rev. B 82, 115120 (2010)]. It is suggested that the existence of gapless modes follows from topologically nontrivial Hamiltonian which varies with adiabatic material-parameters characterizing the defects. We show that such adiabatic approach fails in cases of the existence of Majorana edge modes at topological insulator-superconductor-ferromagnet junctions. It is pointed out that quantum corrections beyond adiabatic quasiclassical approximation are required for the evaluation of second Chern number associated with the line junction.

We introduce the diabatic and the adiabatic bases of a strongly driven superconducting two-level system. The multiphoton resonance conditions, the Rabi couplings, and the energy levels are calculated in both bases using the rotating wave approximation, and the results are compared with the corresponding numerical values. We show the basis dependence of the approximate energy levels and deduce the validity regions for both bases. We demonstrate a parameter region where neither of the bases is sufficient for calculations in the rotating wave approximation.

We theoretically present the theory of photoinduced spin due to the inverse Faraday effect in metals in the presence of Rashba system in the THz regime. We nd that the induced spin, s_μ, (μ = x, y, z) is proportional to frequency(Ω) and square of Rashba spin-orbit interaction(α), i.e., i Ω_μα_ν(E × E_*)_ν where E represents the electric eld. Its spin is derived from the perturbation of spin-orbit coupling and electromagnetic eld in THz light regime.

Traditionally impurity energy states fall in the band gap, which leads to statistical distribution of free carriers, contributing to spatial inhomogeneity, and small high-field Shubnikov-de Haas (SdH) oscillations in solid solutions of Bi₂Te₃ — Sb₂Te₃, the most common room temperature thermoelectrics. However, in these solid solutions with Sn impurity, the Sn states pin the Fermi level and tremendously improve the spatial homogeneity of carriers. This results in observation of high-amplitude SdH oscillations in lower magnetic field. The Fermi level was estimated to be at the top of the second valence band (heavy holes). However, the additional doping has not been studied. We chose Bi₂Te₃ doped with Sn and I impurities to shift the Fermi level and investigate the model that best fits the Sn states. The introduction of similar levels of concentration for the two dopants preserves the Sn impurity states but affects the filling factor. Our results of SdH effect show different frequencies around 4.2 K for samples with 0.05 % and 0.1 % of I and no feature in specific heat at low temperature. This indicates that the model for one-electron states in Bi₂Te₃ doped with Sn is that of two impurity bands with the Fermi level pinned in-between.

We study the stationary Josephson effect in a system of ballistic graphene in planar contact with two superconducting electrodes. Applying a quasi-classical Green's function approach, we derive a general expression of the Josephson current which is valid whenever the chemical potential is away from the Dirac point. In its derivation we treat the mono- and bi-layer cases in a parallel manner, and take account of the fact that the carrier density is higher in the region covered by the superconductors. The behavior of the Josephson critical current is investigated at zero temperature on the basis of the obtained formula.

We investigate theoretically the magnetic monopole in magnetic systems. The hedgehog monopole emerges in frustrated magnetic materials. In addition, a novel magnetic monopole is induced by magnetization dynamics in the presence of the spin-orbit interaction. To derive such a magnetic monopole, we calculate an electric current analytically based on the quantum many-body theory. From the result, we define effective electric and magnetic fields which drive the electric current and finally we obtain the Maxwell's equations with the magnetic monopole contribution which these effictive fields follow.

Electrons floating on the surface of superfluid helium have been suggested as promising mobile spin qubits. Three micron wide channels fabricated with standard silicon processing are filled with superfluid helium by capillary action. Photoemitted electrons are held by voltages applied to underlying gates. The gates are connected as a 3-phase charge-coupled device (CCD). Starting with approximately one electron per channel, no detectable transfer errors occur while clocking 10⁹ pixels. One channel with its associated gates is perpendicular to the other 120, providing a CCD which can transfer electrons between the others. This perpendicular channel has not only shown efficient electron transport but also serves as a way to measure the uniformity of the electron occupancy in the 120 parallel channels.

Though the Rashba-type spin-orbit interaction offers a possibility of manipulating electron's spin totally by electric field, it has been argued that the mere existence of spin-orbit interaction is insufficient to produce such spin-dependent transport. In the present work, we investigate how spin transport through a single-level dot is possible: Finite spin current appears as a result of spin-orbit interaction, strong electronic correlation and finite bias voltage. We show, by applying finite interaction slave-boson mean field theory, that Kondo physics is responsible for generating such spin-dependent transport.

Energy band structures of AnSn₃ (An = Th, U, Np, and Pu) are investigated by a relativistic linear augmented-plane-wave method with the exchange-correlation potential in a local density approximation. It is found in common that the energy bands in the vicinity of the Fermi level are mainly due to the hybridization between actinides 5f and Sn 5p electrons. The similarity is basically understood by the change of electron numbers inside the Fermi surfaces on the basis of a rigid-band picture.

The dynamical conductivity of double-wall carbon nanotubes for perpendicularly polarized light to the tube axis is studied by taking into account screening effects, exciton effects and depolarization effects within an effective-mass theory. The exciton peak of the semiconducting inner tube has an asymmetric line shape due to the coupling with a continuum state of the outer tube, indicating the Fano effect. The Fano coupling strength can be tuned by varying the inter-wall distance.

The electronic structure of hollandite ruthenate K₂Ru₈O₁₆ is calculated using the generalized gradient approximation in the density functional theory, where the Hubbard-type repulsive interaction is taken into account. We find that the band structure near the Fermi level consists only of a single band, which is highly quasi-one-dimensional, exactly at half filling, and has a pair of two nearly-parallel sheetlike Fermi surfaces separated by π/c. These results are consistent with observed quasi-one-dimensional transport properties of the material and thus establish that K₂Ru₈O₁₆ belongs to a class of Tomonaga-Luttinger-liquid systems.

We fabricated and examined the operation of graphene-based superconducting interference device (SQUID) consisting of two superconductor-single layer graphene-superconductor junctions connected in parallel on a superconducting loop made of aluminum. Current-voltage characteristic of the device exhibits supercurrent flowing through SGS junctions. Mean switching current can be modulated with the applied magnetic field periodically. Deduced oscillation period coincides well with that estimated from the device geometry, suggesting that our device works as a graphene-based SQUID.

Point-contact Andreev reflection spectroscopy is used to measure the spin polarization of metals but analysis of the spectra has encountered a number of serious challenges, one of which is the difficulty to distinguish the effects of spin polarization from those of the finite lifetime of Cooper pairs. We have recently confirmed the polarization-lifetime ambiguity for Nb-Co and Nb-Cu contacts and suggested to use Fermi surface mismatch, the normal reflection due to the difference of Fermi wave vectors of the two electrodes, to solve this dilemma. Here we present further experiments on contacts between superconducting Nb and the ferromagnets Fe and Ni as well as the noble metals Ag and Pt that support our previous results. Our data indicate that the Nb - normal metal interfaces have a transparency of up to about 80% and a small, if not negligible, spin polarization.

Electrons can be reflected at an interface between two metals because of a dielectric barrier or different properties of the Fermi surface. Andreev reflection allows to directly measure normal reflection when one of the metals is a superconductor. We have investigated normal reflection at interfaces between non-magnetic normal metals and superconducting Nb (T_c = 9.2K) and Al (T_c = 1.2K). The distribution of the values of the relative strength of the interface barrier, Z, for a number of contacts of a specific metal combination shows a well-defined peak which can be attributed to Fermi surface mismatch. Our reflection coefficients are generally smaller than those predicted theoretically or those derived from proximity-effect studies of normal-superconductor bi-layers.

Enhanced local conductance due to Andreev reflection is well known for high transparency Normal metal-Superconductor (NS) interface. For low transparency NS junctions, observation of two-electron tunneling contribution (enhanced Andreev reflection) to current was also reported previously. In our recent work [J Wei and V Chandrasekhar, Nat. Phys. 6, 494 (2010)], for a three-terminal Cooper pair splitter geometry, i.e., with two closely placed NS junctions sharing the same S terminal, we were able do a 2D scan of both local and nonlocal differential resistance, since for our ideal tunneling junctions there is little current redistribution (flow from one normal-metal lead to the other via the superconducting lead). In contrast to previous 1D nonlocal resistance measurements, 2D scans clearly show regime with pronounced contribution of the nonlocal processes to both local and nonlocal conductance enhancement. The enhanced local conductance and negative nonlocal resistance are consistent with enhanced Cooper pair splitting, and dynamical Coulomb blockade could be the origin of this enhancement.

The binding energy and effective mass of a polaron in a GaAs film deposited on the Al_0.3Ga_0.7As substrate are studied theoretically by using the fractional-dimensional space approach. Our calculations show that the polaron binding energy and mass shift decrease monotonously with increasing the film thickness. For the film thicknesses with L_w ≤ 70Å and the substrate thicknesses with L_b ≤ 200Å, the different values of the substrate thickness influence the polaron binding energy and mass shift in the GaAs film. The polaron binding energy and mass shift increase monotonously with increasing the substrate thickness. For the film thickness with L_w ≥ 70Å or the substrate thicknesses with L_b ≤ 200Å, the different values of the substrate thickness have no significant influence on the polaron binding energy and mass shift in the GaAs film deposited on the Al_0.3Ga_0.7As substrate.

We report that temperature dependence of Hall mobility of the strongly disordered films In₂O₃-ZnO. We made targets by mixing ZnO into In₂O₃ at the ratio 0.5 ∼ 2 wt%. Sputtering those targets on glass substrate by DC magnetron method, amorphous films with 25 nm thickness were obtained. By annealing at T = 150 ∼ 350 ^OC in the air, oxygen defect decreased and the conductance decreased. We obtained films with conductivity 0.2mS/m ∼ 300S/m. In the temperature range T = 90∼300K, we measured the Hall effect of these films. The density of electron was 4 × 10¹⁸ ∼ 1.2 × 10²² m⁻³ at the room temperature. The Hall mobility μ_H shows the thermal-activation-like temperature dependence μ_H = AT^−1/2 exp(E_B/k_BT), where E_B is activation energy. By fitting, we obtained E_B = 60 ∼ 86 meV.

Effects of disorder on a two-dimensional Z₂ topological insulator are studied numerically. We propose and study the phase diagram of a variant of Bernevig-Hughes-Zhang model, which takes account of the s_z non-conserving spin-orbit coupling. Using scaling analyses, we determine the phase boundary and the critical exponent characterizing the transition between metallic and the topologically insulating phases.

In this study we have investigated dopant effects on a typical charge-orbital ordered state in a single layered manganite La_0.5Sr_1.5MnO₄ using x-ray diffraction and magnetization measurement. We have studied how the orbital ordered states are affected by the 3% substitution of Cr, Fe and Ga ions for Mn ions. It is revealed that the orbital ordered states are suppressed by the magnetic impurities and the phase-separated state with charge-orbital ordering and ferromagnetic cluster and/or spin-cluster-glass state emerge in the all impurity doped compounds.

Transport equation of interacting fermions is re-considered in the light of the conservation law. We show that the exact equation for the particle-hole excitation is derived directly from the Ward identity, by simply transforming it. It is almost identical to that derived diagrammatically by Éliashberg, but the collision integral is written in the symmetric form. We further discuss the hidden ultraviolet divergence in the both treatments, claiming that the microscopic justification of the Landau-Silin equation has not yet been completed. The divergence is removed by incorporating the particle-particle and hole-hole pairs of excitations, without sacrificing the rigour. This requires re-identification of the distribution function, but does not alter the basic structure of the fermi liquid theory.

Transport process of non-interacting electrons is reformulated, in view of the exact treatment of interacting fermions reported by the present authors in this conference. It makes correspondence to the Boltzmann theory, covering the weakly localized regime, and also eliminates the hidden ultraviolet divergence by incorporating the particle-particle and hole-hole pairs. A mass renormalization of purely two-particle nature appears in the transport equation, giving rise to the conductivity of the Drude form. Taking the maximally crossed diagrams, the 2D conductivity is shown to vanish in the elastic scattering limit, while the distribition function preserves its sharp peak and the transport mass reduces to the free-electron value. The description differs from the self-consistent theory by Vollhardt and Wölfle, in which the Bethe-Salpeter structure of the scattering vertex is abandoned in favour of the time-reversal symmetry so the correspondence to the Boltzmann theory is lost.

We have grown single crystalline Ce₂Pt₆Ga₁₅ and investigated the transport and magnetic properties by means of electrical resistivity, magnetic susceptibility, and specific heat measurements. Our results suggest that Ce₂Pt₆Ga₁₅ is characterized as a heavy-fermion system with an electronic ground state of non-Fermi liquid character. The observed logarithmic temperature dependence of C₄f / T at lower temperature region is a distinctive feature of a non-Fermi liquid state mediated by a two-dimensional antiferromagnetic spin fluctuation. The C_4f/T reaches as much as about 0.7 J(mol-Ce)⁻¹K⁻² at 2K.

We propose a two-terminal spin filter utilizing a quantum dot with spin-orbit interaction and magnetic field. First we examine a quantum dot with two energy-levels, as a minimal model, and obtain an analytical expression for the spin-dependent conductance. When the spacing between the two levels is smaller than the level broadening due to the tunnel coupling to the leads, a largely spin-polarized current is generated around the current peaks of Coulomb oscillation. Next, we perform a numerical simulation using a realistic model for a quantum dot and tunnel barriers and evaluate the efficiency of our spin filter.

We theoretically investigate the Kondo effect in a quantum dot embedded in an Aharonov-Bohm (AB) ring. We obtain an analytical expression for the Kondo temperature T_K as a function of the AB phase ø for the magnetic flux penetrating the ring, by the scaling method. We also examine the decoherence due to the inelastic process in the quantum dot embedded in an AB ring. Solving the scattering problem, we obtain the elastic part of the conductance. The inelastic part of the conductance G_inel is obtained by the subtraction of elastic part from the total conductance. We show that G_inel = 0 at T = 0 and becomes significant when T ≥ T_K. We also show that the inelastic part of the conductance depends on the magnetic flux.

Transport properties of the junction composed of graphene nanoribbon with zigzag shaped edges attached to two normal metals are theoretically investigated. In order to clarify roles of zigzag edges on the transport properties, we consider the system where the hopping at the zigzag edges are weaker than the other bond. The results are compared with the case without modulation studied previously.

We conduct a systematic study on the disorder effect in two-dimensional (2D) topological insulators by calculating the Z₂ topological invariant. Starting from the trivial and nontrivial topological phases of the model describing HgTe/CdTe quantum wells (QWs), we introduce three different kinds of disorder into the system, including the fluctuations in the on-site potential, the hopping amplitude and the topological mass. These kinds of disorder commonly exist in HgTe/CdTe QWs grown experimentally. By explicit numerical calculations, we show that all three kinds of disorder have the similar effect: the topological phase in the system is not only robust to them, but also can be brought about by introducing them to the trivial insulator phase. These results make a further confirmation and extendability of the study on the interplay between the disorder and the topological phase.

A two-step transition model describing phase diffusion and switching process in underdamped Josephson junctions is discussed. The model takes into account the phase particle's escape rate out of the potential well and transition rate from phase diffusion to the running state. Using as examples the experimental switching current distributions of two Nb/AlO_x/Nb junctions of different sizes fabricated on the same chip, we extract the transition rate, which turns out to follow the predicted Arrhenius law in the thermal regime but is greatly enhanced when macroscopic quantum tunneling becomes the dominant escape mechanism. Our results show the validity of applying the model to obtain the transition rate out of the phase diffusion state into the running state from measured switching current distribution of underdamped Josephson junctions. The temperature and bias-current dependent data provide strong evidence for phase diffusion in both thermal and quantum regimes.