Table of contents

By using the arc-melting method, we successfully synthesize the compound Sc_0.5Zr_0.5Co with the space group of Pm−3m. Both the resistivity and magnetic susceptibility measurements reveal a phase transition at about 86 K. This transition might be attributed to the establishment of an antiferromagnetic order. The magnetization hysteresis loop measurements in wide temperature region show a weak ferromagnetic feature, which suggests a possible canted arrangement of the magnetic moments. Bounded by the phase transition temperature, the resistivity at ambient pressure shows a change from Fermi liquid behavior to a super-linear behavior as temperature increases. By applying pressure up to 32.1 GPa, the transition temperature does not show a clear change and no superconductivity is observed above 2 K. The density functional theory calculations simulate the antiferromagnetic order and reveal a gap between the spin-up and spin-down d-orbital electrons. This kind of behavior may suggest that the antiferromagnetic order in this compound originates from the localized d-electrons which do not contribute to the electric conduction. Thus the itinerant and localized d-orbital electrons in the compound are decoupled.

A unified phenomenological description framework is proposed for the evaluation of some of the most important observables of the cuprate superconductors: the pseudogap (PG) Δ_PG, the local superconducting amplitudes Δ_SC(r_i), the critical temperature T_c and charge ordering (CO) parameters. Recent detailed measurements of CO structures and CO wavelengths λ_CO are faithfully reproduced by solutions of a Cahn–Hilliard differential equation with a free energy potential V_GL that produces alternating small charge modulations. The charge oscillations induce atomic fluctuations that mediate the SC pair interaction proportional to the V_GL amplitude. The local SC amplitude and phase θ_i are connected by Josephson coupling E_J(r_ij) and the SC long-range order transition occurs when $\left\langle {E}_{\text{J}}\right\rangle \sim {k}_{\text{B}}{T}_{\text{c}}$. The calculated results of the wavelength λ_CO, Δ_PG, $\left\langle {{\Delta}}_{\text{SC}}\right\rangle$ and T_c calculations are in good agreement with a variety of experiments.

The Kitaev spin liquid provides a rare example of well-established quantum spin liquids in more than one dimension. It is obtained as the exact ground state of the Kitaev spin model with bond-dependent anisotropic interactions. The peculiar interactions can be yielded by the synergy of spin-orbit coupling and electron correlations for specific electron configuration and lattice geometry, which is known as the Jackeli–Khaliullin mechanism. Based on this mechanism, there has been a fierce race for the materialization of the Kitaev spin liquid over the last decade, but the candidates have been still limited mostly to 4d- and 5d-electron compounds including cations with the low-spin d⁵ electron configuration, such as Ir⁴⁺ and Ru³⁺. Here we discuss recent efforts to extend the material perspective beyond the Jackeli–Khaliullin mechanism, by carefully reexamining the two requisites, formation of the j_eff = 1/2 doublet and quantum interference between the exchange processes, for not only d- but also f-electron systems. We present three examples: the systems including Co²⁺ and Ni³⁺ with the high-spin d⁷ electron configuration, Pr⁴⁺ with the f¹-electron configuration, and polar asymmetry in the lattice structure. In particular, the latter two are intriguing since they may realize the antiferromagnetic Kitaev interactions, in contrast to the ferromagnetic ones in the existing candidates. This partial overview would stimulate further material exploration of the Kitaev spin liquids and its topological properties due to fractional excitations.

Structural, magnetic and electromechanical changes resulting from spin crossover between the spin quintet and spin triplet forms of a mononuclear Mn³⁺ complex embedded in six lattices with different charge balancing counterions are reported. Isostructural ClO₄⁻ and BF₄⁻ salts (1) and (2) each have two unique Mn³⁺ sites which follow different thermal evolution pathways resulting in a crossover from the spin quintet form at room temperature to a 1:1 spin triplet:quintet ratio below 150 K. The PF₆⁻ (3) and NO₃⁻ (4) salts which each have one unique Mn³⁺ site show a complete conversion from spin quintet to spin triplet over the same temperature range. A complete two step spin crossover is observed in the CF₃SO₃⁻ lattice (5) with a 1:1 ratio of spin quintet and spin triplet forms at intermediate temperature, while the BPh₄⁻ lattice (6) stabilizes the spin triplet form over most of the temperature range with gradual and incomplete spin state switching above 250 K. An electromechanical piezoresponse was detected in NO₃⁻ complex 4 despite crystallization in a centrosymmetric space group. The role of deformations associated with stress-induced spin triplet-spin quintet switching in breaking the local symmetry are discussed and computational analysis is used to estimate the energy gap between the two spin states.

We employ an empirical coarse-grained model with a proposed Gaussian-like interfacial potential to describe proteins at curved fluid–fluid interfaces such as occurring in bubbles and droplets. We consider the air–water and oil–water interfaces. We study the mass distributions and the geometry of the aqueous proteins as a function of the radius of curvature for protein G and two lipid transfer proteins. At curved interfaces the distortion of the proteins is different than at flat interfaces. We find that the proteins come closer to the surface of a bubble than to the surface of similarly curved droplet. In addition, the bubbles adsorb more proteins. We identify the pinning residues. We demonstrate the existence of the second layer in the density profile for sufficiently dense solutions.

The control of magnetic materials and devices by voltages without electric currents holds the promise of power-saving nano-scale devices. Here we study the temperature-dependent voltage control of the magnetic anisotropy caused by rare-earth (RE) local moments at an interface between a magnetic metal and a non-magnetic insulator, such as Co|(RE)|MgO. Based on a Stevens operator representation of crystal and applied field effects, we find large dominantly quadrupolar intrinsic and field-induced interface anisotropies at room temperature. We suggest improved functionalities of transition metal tunnel junctions by dusting their interfaces with rare earths.

We demonstrate how image recognition and reinforcement learning combined may be used to determine the atomistic structure of reconstructed crystalline surfaces. A deep neural network represents a reinforcement learning agent that obtains training rewards by interacting with an environment. The environment contains a quantum mechanical potential energy evaluator in the form of a density functional theory program. The agent handles the 3D atomistic structure as a series of stacked 2D images and outputs the next atom type to place and the atomic site to occupy. Agents are seen to require 1000–10 000 single point density functional theory evaluations, to learn by themselves how to build the optimal surface reconstructions of anatase TiO₂(001)-(1 × 4) and rutile SnO₂(110)-(4 × 1).

We study the behaviour of the non-retarded van der Waals force between a planar substrate and a single-walled carbon nanotube, assuming that the system is immersed in a liquid medium which exerts hydrostatic pressure on the tube's surface, thereby altering its cross section profile. The shape of the latter is described as a continual structure characterized by its symmetry index n. Two principle mutual positions of the tube with respect to the substrate are studied: when one keeps constant the minimal separation between the surfaces of the interacting objects; when the distance from the tube's axis to the substrates bounding surface is fixed. Within these conditions, using the technique of the surface integration approach, we derive in integral form the expressions which give the dependance of the commented force on the applied pressure.

Non-associative neat liquids benzene, acetone and carbon tetrachloride have been examined via molecular dynamics simulations. Several models of each neat liquid have been simulated, and selected thermodynamic and structural results are presented. However, the models have been compared mainly in terms of their dynamic quantities. Since models are rarely parametrized with the dynamic properties in mind, the principal goal of this work is to present quantities such as the power spectra, rotational correlation functions and relaxation times, diffusion coefficients and self and distinct parts of the van Hove functions in relation to available experimental data. The general trends of the calculated data provide a benchmark for the behavior of neat simple liquids which will be built upon in the cases of mixtures with associative liquids.

Triangular lattice models for pattern formation by hard-core soft-shell particles at interfaces are introduced and studied in order to determine the effect of the shell thickness and structure. In model I, we consider particles with hard-cores covered by shells of cross-linked polymeric chains. In model II, such inner shell is covered by a much softer outer shell. In both models, the hard cores can occupy sites of the triangular lattice, and nearest-neighbor repulsion following from overlapping shells is assumed. The capillary force is represented by the second or the fifth neighbor attraction in model I or II, respectively. Ground states with fixed chemical potential μ or with fixed fraction of occupied sites c are thoroughly studied. For T > 0, the μ(c) isotherms, compressibility and specific heat are calculated by Monte Carlo simulations. In model II, 6 ordered periodic patterns occur in addition to 4 phases found in model I. These additional phases, however, are stable only at the phase coexistence lines at the (μ, T) diagram, which otherwise looks like the diagram of model I. In the canonical ensemble, these 6 phases and interfaces between them appear in model II for large intervals of c and the number of possible patterns is much larger than in model I. We calculated line tensions for different interfaces, and found that the favorable orientation of the interface corresponds to its smoothest shape in both models.

In this study, we performed density functional theory based calculations to determine the effect of the size of Cu_x (x = 1 (adatom), 3 (trimer), 7 (heptamer)) clusters supported on Cu(111) toward the adsorption of CO, O, and CO₂, and the dissociation of CO₂. CO adsorbs with comparable adsorption energies on the different cluster systems, which are influenced by the reactivity of the Cu atoms in the cluster and the interaction of CO with the Cu atoms in the terrace. The O atom, on the other hand, will always favor to adsorb on hollow sites and is more stable on the hollow sites of smaller clusters. CO₂ dissociates with lower activation energy on the cluster region than on flat Cu(111). We obtained the lowest activation energy on Cu₃ due to its more reactive Cu atoms than the Cu₇ case and due to the possibility of O to adsorb on the cluster region, which is not observed in the Cu₁ case. The presented results will provide insights on future studies on supported cluster systems and their possible use as catalysts for CO₂-related reactions.

The photocatalytic activity (PCA) of ZnO@Ag nanocomposites for different concentrations of Ag is reported. Dumbbells shaped zinc oxide (ZnO) nanostructures with silver (Ag) nanoparticles were synthesized by a simple chemical colloidal method. The as synthesized nanocomposites (without any heat treatment) were used for optical and photocatalytic studies. The FESEM analysis shows that the composite catalysts are composed of ZnO dumbbells coated with spherical Ag nanoparticles. UV-visible spectrum of ZnO@Ag photocatalysts shows a strong absorption band of ZnO at 380 nm with a plasmonic peak of Ag at 440 nm. The PL emission intensity of the composites varies with Ag concentration and has a minimum for the catalyst containing 13.7% of Ag. A possible growth mechanism of ZnO nanostructures with hexagonal cross-section has been proposed. Photocatalytic property of the as synthesized ZnO and ZnO@Ag catalysts was studied by investigating the degradation of methylene blue (MB) dye on exposure to UV-visible radiation. A relatively faster degradation of the dye was observed for ZnO@Ag composites as compared to pure ZnO, showing an improved photocatalytic behavior in the visible region. We proposed a possible mechanism for the enhancement in photocatalytic activity of Ag coated ZnO photocatalysts.

We study the transport properties of junctions of normal and superconducting Weyl semimetal with tilted dispersion, in the presence of magnetization induced by magnetic strips. The sub gap tunnelling conductance shows robust signatures in the presence of different orientation and strength of magnetization of the magnetic strips. We obtain the analytical results for the normal-magnetic-superconducting junction in the thin barrier limit and demonstrate that these results have no analogues to their conventional counterparts and junctions with Dirac electrons in two-dimensions. We discuss possible experimental setups to test our theoretical predictions.

The 2D triangle-shaped C₃-symmetric topological insulator with the chiral superconducting coupling on the triangular lattice is investigated. While such a system cannot provide the topologically protected corner excitations, we report the presence of the nontopological corner excitations with energy value to lie in the first-order edge spectrum gap. Though these excitations are not topologically protected, they appear for a rather wide range of the parameters values and are robust against the boundary defects and weak disorder. We reveal the presence of the Majorana corner states, which appear along the line in the parameter space.

Pyrrhotite, Fe₇S₈, is a commonly occurring carrier of magnetic remanence and has a low temperature transition, the Besnus transition, involving a change in spin state. Variations of the thermodynamic, magnetic and elastic properties through this transition at ∼33 K in a natural sample of 4C pyrrhotite have been tested against a group theoretical model for coupling between order parameters relating to Fe/vacancy ordering (irrep U₁(1/2,0,1/4)) and magnetic ordering (irreps m${{\Gamma}}_{4}^{+}$ and m${{\Gamma}}_{5}^{+}$). Magnetoelastic coupling is weak but the pre-existing microstructure of ferroelastic and magnetic domains, that develop as a consequence of Fe/vacancy and ferrimagnetic ordering during slow cooling in nature (P6₃/mmc → C2'/c'), causes subtle changes in the low temperature transition (C2'/c' → P$\overline{1}$). The Besnus transition involves a rotation of magnetic moments out of the a–c plane of the monoclinic structure, but it appears that the transition temperature might vary locally according to whether it is taking place within the pre-existing domain walls or in the domains that they separate. Evidence of metamagnetic transitions suggests that the magnetic field–temperature phase diagram will display some interesting diversity. Low temperature magnetic transitions in minerals of importance to the palaeomagnetism community have been used to identify the presence of magnetite and haematite in rocks and the Besnus transition is diagnostic of the existence of pyrrhotite, Fe₇S₈.

Current understanding of the origin of icosahedral clusters or icosahedral short-range ordering in undercooled metallic liquids or glasses is based on Frank's consideration of an isolated icosahedron whose core has lower potential energy than the shell. Using large scale atomistic simulations and statistical analysis of several bcc (body-centered-cubic) and fcc (face-centered-cubic) metals, here we show that the shells of icosahedrons spontaneously formed inside deeply undercooled metallic liquids or glasses in fact have lower (averaged) potential energy than the cores. The shell potential energy deficiency occurs only to the icosahedral clusters but not to the equilibrium-crystal clusters, and, for icosahedral clusters, this deficiency grows with decreasing temperature. Compared with fcc metals, bcc metals exhibit greater potential energy deficiency on the icosahedral shells and produce significantly more icosahedral clusters upon liquid quenching, which explains the higher tendency of bcc metals to be vitrified observed in ultrafast cooling experiments. Inspecting the potential energy deficiency on the icosahedral shells through computation provides a new avenue to the search for amorphous metals (i.e. metallic glasses) with high glass forming ability and processability.

We report results from visible and UV Raman spectroscopy studies of the phonon spectra of a polycrystalline sample of the prototypical perovskite type oxide BaZrO₃ and a 500 nm thick film of its Y-doped, proton conducting, counterpart BaZr_0.8Y_0.2O_2.9. Analysis of the Raman spectra measured using different excitation energies (between 3.44 eV and 5.17 eV) reveals the activation of strong resonance Raman effects involving all lattice vibrational modes. Specifically, two characteristic energies were identified for BaZrO₃, one around 5 eV and one at higher energy, respectively, and one for BaZr_0.8Y_0.2O_2.9, above 5 eV. Apart from the large difference in spectral intensity between the non-resonant and resonant conditions, the spectra are overall similar to each other, suggesting that the vibrational spectra of the perovskites are stable when investigated using an UV laser as excitation source. These results encourage further use of UV Raman spectroscopy as a novel approach for the study of lattice vibrational dynamics and local structure in proton conducting perovskites, and open up for, e.g., time-resolved experiments on thin films targeted at understanding the role of lattice vibrations in proton transport in these kinds of materials.

The structural stability of hydrogen C2/c phase from 0 K to 300 K is investigated by combining the first-principles molecular dynamics (MD) simulations and density functional perturbation theory. Without considering the temperature effect, the C2/c phase is stable from 150 GPa to 250 GPa based on the harmonic phonon dispersion relations. The hydrogen molecules at the solid lattice sites are sensitive to temperature. The structural stability to instability transition of the C2/c phase upon temperature is successfully captured by the radial distribution function and probability distribution of atomic displacements from first-principles MD simulations, confirmed by the phonon power spectrum analysis in the phase space. The existence of phonon quasiparticle for different normal modes is observed directly. The phonon power spectrum of specific normal modes corresponding to the Raman and infrared (IR) activations are depicted at different temperatures and pressures. The changes of frequency with temperature are in agreement with experimental results, supporting the C2/c as the hydrogen phase III. For the first time, the anharmonic phonon dispersion curves and density of states are predicted based on the phonon quasi-particle approach. Therefore, the temperature dependence of lattice vibrations can be observed directly, providing a more complete physical picture of phonon frequency distribution with respect to the Raman and IR spectra. It is found that the high-frequency regions adopt significant frequency shifts compared to the harmonic case.

A systematic study of electronic structure, mechanical and transport properties of RuV-based half-Heusler alloys (RuVZ, Z = As, P, Sb) have been presented using ab initio density functional and Boltzmann transport theory. The electronic structures are obtained using generalized gradient approximation with Perdew–Burke–Ernzerhof functional. All the compounds are crystallized in face centered cubic phase with space group #216. Our preliminary electronic structure simulations reveal that all the alloys are non-magnetic semiconductors. Additionally, the phonon dispersion and elastic constants (along with the related elastic moduli) also verify mechanical stability of the alloys. Due to strong dependence on the electronic bandgap in thermoelectric materials, we have estimated bandgap using more accurate hybrid functional i.e. Heyd–Scuseria–Ernzerhof. The transport coefficients (e.g. Seebeck, electrical conductivity, thermal conductivity due to electrons) are calculated by solving the Boltzmann transport equation for charge carriers as implemented in BoltzTraP software under constant relaxation time approximation. The lattice thermal conductivity due to phonons is calculated using more reliable shengBTE code based upon the Boltzmann transport equation for phonons. We have calculated the more reliable value of the thermoelectric figure of merit, ZT (related to the conversion efficiency) for all the compounds. The obtained ZT for RuVAs, RuVP and RuVSb is 0.41(0.32), 0.21(0.16) and 0.70(0.61) for p(n)-type behavior at 900 K. The corresponding carrier concentrations are also predicted. High value of ZT is obtained for RuVSb alloy due to low lattice thermal conductivity. Among these compounds, RuVSb emerged out as a most suitable candidate for thermoelectric power generation device. Minimum lattice thermal conductivity in theoretical limit along with the corresponding maximum value of ZT is also predicted in these alloys.

We investigated the specific electronic energy deposition by protons and He ions with keV energies in different transition metal nitrides of technological interest. Data were obtained from two different time-of-flight ion scattering setups and show excellent agreement. For protons interacting with light nitrides, i.e. TiN, VN and CrN, very similar stopping cross sections per atom were found, which coincide with literature data of N₂ gas for primary energies ⩽25 keV. In case of the chemically rather similar nitrides with metal constituents from the 5^th and 6^th period, i.e. ZrN and HfN, the electronic stopping cross sections were measured to exceed what has been observed for molecular N₂ gas. For He ions, electronic energy loss in all nitrides was found to be significantly higher compared to the equivalent data of N₂ gas. Additionally, deviations from velocity proportionality of the observed specific electronic energy loss are observed. A comparison with predictions from density functional theory for protons and He ions yields a high apparent efficiency of electronic excitations of the target for the latter projectile. These findings are considered to indicate the contributions of additional mechanisms besides electron hole pair excitations, such as electron capture and loss processes of the projectile or promotion of target electrons in atomic collisions.

Unconventional lattice fermions with high degeneracies that are not Weyl or Dirac fermions have attracted increased attention in recent years. In this paper, we consider pseudospin-1 Maxwell fermions and the (2 + 1)-dimensional parity anomaly, which are not constrained by the fermion doubling theorem. We derive the Hall conductivity of a single Maxwell fermion and explain how each Maxwell fermion has a quantized Hall conductance of e²/h. Parity is spontaneously broken in the effective theory of lattice Maxwell fermions interacting with an (auxiliary) U(1) gauge field, leading to an effective anomaly-induced Chern–Simons theory. An interesting observation about the parity anomaly is that the lattice Maxwell fermions are not constrained by the fermion doubling theorem, so a single Maxwell fermion can exist in a lattice. In addition, our work considers the quantum anomaly in odd-dimensional spinor space.

The research on nitridophosphate materials has gained significant attention in recent years due to the abundance of elements like Mg, Zn, P, and N. We present a detailed study of band gap and electronic structure of M₂PN₃ (M = Mg, Zn), using synchrotron-based soft x-ray spectroscopy measurements as well as density functional theory (DFT) calculations. The experimental N K-edge x-ray emission spectroscopy (XES) and x-ray absorption spectroscopy (XAS) spectra are used to estimate the band gaps, which are compared with our calculations along with the values available in literature. The band gap, which is essential for electronic device applications, is experimentally determined for the first time to be 5.3 ± 0.2 eV and 4.2 ± 0.2 eV for Mg₂PN₃ and Zn₂PN₃, respectively. The experimental band gaps agree well with our calculated band gaps of 5.4 eV for Mg₂PN₃ and 3.9 eV for Zn₂PN₃, using the modified Becke–Johnson (mBJ) exchange potential. The states that contribute to the band gap are investigated with the calculated density of states especially with respect to two non-equivalent N sites in the structure. The calculations and the measurements predict that both materials have an indirect band gap. The wide band gap of M₂PN₃ (M = Mg, Zn) could make it promising for the application in photovoltaic cells, high power RF applications, as well as power electronic devices.

In this work we present a new procedure to compute optical spectra including excitonic effects and approximated quasiparticle corrections with reduced computational effort. The excitonic effects on optical spectra are included by solving the Bethe–Salpeter equation, considering quasiparticle eigenenergies and respective wavefunctions obtained within DFT-1/2 method. The electron–hole ladder diagrams are approximated by the screened exchange. To prove the capability of the procedure, we compare the calculated imaginary part of the dielectric functions of Si, Ge, GaAs, GaP, GaSb, InAs, InP, and InSb with experimental data. The energy position of the absorption peaks are correctly described. The good agreement with experimental results together with the very significant reduction of computational effort makes the procedure suitable on the investigation of optical spectra of more complex systems.

A decomposition of the non-equilibrium stationary state of a quadratic Fermi system influenced by linear baths is obtained and used to establish a simulation protocol in terms of tensor states. The scheme is then applied to examine the occurrence of uncoupled Majorana fermions in Kitaev chains subject to baths on the ends. The resulting phase diagram is compared against the topological characterization of the equilibrium chain and the protocol efficiency is studied with respect to this model.

The concept of realization of Weyl points close to the Fermi level in materials with broken time-reversal symmetry has significant theoretical and technological ramifications. Here, we review the investigation of magneto-transport measurements in single crystals of magnetic Weyl semimetal Co₃Sn₂S₂. We see a turn-on like behaviour followed by saturation in resistivity under magnetic field in the low temperature region which is allocated to the topological surface states. A non-saturating magnetoresistance, linear at high fields, is observed at low temperatures where applied magnetic field is transverse to the current direction. The linear negative magnetoresistance at low magnetic fields (B < 0.1 T) provides evidence for time reversal symmetry breaking in Co₃Sn₂S₂. Chiral anomaly in Weyl metallic state in Co₃Sn₂S₂ is confirmed from the breakdown of Ohm's law in the electronic transport. Shubnikov de Haas (SdH) oscillation measurement has unveiled the multiple sub-bands on the Fermi surface that corresponds to a non-trivial Berry phase. The non-linear behaviour in Hall resistivity validates the existence of two type of charge carriers with equal electron and hole densities. Strong temperature dependence of carrier mobilities reflects the systematic violation of Kohler's rule in Co₃Sn₂S₂. Our findings open avenues to study kagome-lattice based magnetic Weyl semimetals that unfurl the basic topological aspects leading to significant ramification for spintronics.

We investigate the extent to which the class of Dirac materials in two-dimensions provides general statements about the behavior of both fermionic and bosonic Dirac quasiparticles in the interacting regime. For both quasiparticle types, we find common features for the interaction induced renormalization of the conical Dirac spectrum. We perform the perturbative renormalization analysis and compute the self-energy for both quasiparticle types with different interactions and collate previous results from the literature whenever necessary. Guided by the systematic presentation of our results in table 1, we conclude that long-range interactions generically lead to an increase of the slope of the single-particle Dirac cone, whereas short-range interactions lead to a decrease. The quasiparticle statistics does not qualitatively impact the self-energy correction for long-range repulsion but does affect the behavior of short-range coupled systems, giving rise to different thermal power-law contributions. The possibility of a universal description of the Dirac materials based on these features is also mentioned.

We report a quantum phase transition in Pauli limited d-wave superconductors and give the mean field estimates of the associated quantum critical point. For a population imbalanced d-wave superconductor a stable ground state phase viz quantum breached pair phase has been identified which comprises of spatial coexistence of gapless superconductivity and nonzero magnetization. Based on the thermodynamic and quasiparticle indicators we for the first time analyze this phase, discuss the thermal behavior of Pauli limited d-wave superconductor, give accurate estimates of the thermal scales associated with such systems and map out the pseudogap regime. Our work shows that while the Pauli limited superconductors are known to exhibit exotic modulated superconducting phase at large imbalance of fermion populations; in the regime of weak imbalance an intriguing phase of competing orders is realized. We have established that rather than the superconducting pairing field, it is the average magnetization of the system that quantifies this quantum phase transition. Given that the existing Pauli limited superconductors possess unconventional pairing state symmetry of the superconducting order, our work promises to open up new avenues in the experimental research of these materials. We have also demonstrated an alternate scenario wherein the quantum breached pair phase is a natural outcome for a d-wave superconductor with unequal effective masses of the fermion species.

Temperature and field-dependent magnetization M(T, H ) measurements and neutron scattering study of a single crystal CeSb₂ are presented. Several anomalies in magnetization curves have been confirmed, i.e., at 15.6 K, 12 K, and 9.8 K, respectively. These three transitions are all metamagnetic transitions, which shift to lower temperatures as the magnetic field increases. In contrast to the previous studies that the anomaly at 15.6 K has been suggested as paramagnetic to ferromagnetic phase transition, in our measurement no hysteresis loop around zero field with either H ∥ c or H  ⊥  c has been observed. The anomaly located at around 12 K is antiferromagnetic-like transition, and this turning point will clearly split into two when the magnetic field H ⩾ 2 kOe. A neutron scattering study reveals that the low temperature ground state of CeSb₂ orders magnetically with commensurate propagation wave vectors k = (−1, ±1/6, 0) and k = (±1/6, −1, 0), with phase transition temperature T_C ∼ 9.8 K.

Here we report the magnetic, electronic and thermal transport properties of the heavy-fermion semimetal Pr₃Os₄Ge₁₃ with a cage-like structure by means of magnetic susceptibility, χ(T), isothermal magnetization, M(B, T), electrical resistivity, ρ(B, T), Hall coefficient, R_H(T), specific heat, C_p(T), thermal conductivity, κ(T) and thermoelectric power, S(T). ρ(T) and R_H(T) show semimetallic features in a manner that mimics a thermal activated behaviour with an activation energy, Δ/k_B = 6.5 K indicating the opening of a small energy gap in the material. At low temperatures, a Sommerfeld coefficient, γ = 128 mJ (mol⁻¹ K⁻²) observed indicates a mass enhancement of the quasiparticles at low temperatures which bears witness to a heavy fermion state in Pr₃Os₄Ge₁₃. A Wilson–Sommerfeld ratio, W_R and the dimensionless ratio, S/γT of 1.01 and 0.62 ± 0.018 observed, respectively, are in good agreement with the Fermi liquid scenario. S(T) and R_H(T) reveal hole dominated transport (p-type material) with a relatively large room temperature value of S(T) = 32.85 ± 0.98 μV K⁻¹. A room temperature low value of κ(T) = 1.61 W K⁻¹ m⁻¹ leads to a thermoelectric figure of merit, ZT = 0.03 ± 0.001 which is comparable to values achieved in several clathrates around the same temperature. Features from S(T) and ρ(T) favour the realization of a higher ZT value at elevated temperatures.

NiO thin films with various strains were grown on SrTiO₃ (STO) and MgO substrates using a pulsed laser deposition technique. The films were characterized using an x-ray diffraction, atomic force microscopy, and infrared reflectance spectroscopy. The films grown on STO (001) substrate show a compressive in-plane strain which increases as the film thickness is reduced resulting in an increase of the NiO phonon frequency. On the other hand, a tensile strain was detected in the NiO film grown on MgO (001) substrate which induces a softening of the phonon frequency. Overall, the variation of in-plane strain from −0.36% (compressive) to 0.48% (tensile) yields the decrease of the phonon frequency from 409.6 cm⁻¹ to 377.5 cm⁻¹ which occurs due to the ∼1% change of interatomic distances. The magnetic exchange-driven phonon splitting Δω in three different samples, with relaxed (i.e. zero) strain, 0.36% compressive strain and 0.48% tensile strain, was measured as a function of temperature. The Δω increases on cooling in NiO relaxed film as in the previously published work on a bulk crystal. The splitting increases on cooling also in 0.48% tensile strained film, but Δω is systematically 3–4 cm⁻¹ smaller than in relaxed film. Since the phonon splitting is proportional to the non-dominant magnetic exchange interaction J₁, the reduction of phonon splitting in tensile-strained film was explained by a diminishing of J₁ with lattice expansion. Increase of Δω on cooling can be also explained by rising of J₁ with reduced temperature.

The spin wave resonances of BiFeO₃ ceramics have been followed at low temperature through far-infrared reflectance measurements. Following the scheme of Fishman et al (2015 Phys. Rev. B 92 094422) we have been able to assign all the spin wave modes observed. A complete lifting of the degeneracies of all these modes is seen at 250 K concomitant with the increase in single-ion anisotropy. For the first time, all the spin wave modes have been observed in the infrared spectra of BiFeO₃. Correlated changes in the strength and frequencies of spin wave excitations with the reported magnetic transitions at low temperature are observed. A simultaneous increase in anharmonicity of the magnetic cycloid and single-ion anisotropy with decreasing temperature results in a partial suppression of the spin wave excitations. An increase in the magnetoelectric coupling is also observed below 150 K.

Superconducting coplanar-waveguide (CPW) resonators are one of the key devices in circuit quantum electrodynamics (cQED). Their performance can be limited by dielectric losses in the substrate and in the material interfaces. Reliable modeling is required to aid in the design of low-loss CPW structures for cQED. We analyze the geometric dependence of the dielectric losses in CPW structures using finite-element modeling of the participation ratios of the lossy regions. In a practical scenario, uncertainties in the the dielectric constants and loss tangents of these regions introduce uncertainties in the theoretically predicted participation ratios. We present a method for combining loss simulations with measurements of two-level-system-limited quality factors and resonance frequencies of CPW resonators. Namely, we solve an inverse problem to find model parameters producing the measured values. High quality factors are obtainable by properly designing the cross-sectional geometries of the CPW structures, but more accurate modeling and design methods for low-loss CPW resonators are called for major future improvements. Our nonlinear optimization methodology for solving the aforementioned inverse problem is a step in this direction.

High pressure study on ultra-hard transition-metal boride Os₂B₃ was carried out in a diamond anvil cell under isothermal and non-hydrostatic compression with platinum as an x-ray pressure standard. The ambient-pressure hexagonal phase of Os₂B₃ is found to be stable with a volume compression V/V₀ = 0.670 ± 0.009 at the maximum pressure of 358 ± 7 GPa. Anisotropic compression behavior is observed in Os₂B₃ to the highest pressure, with the c-axis being the least compressible. The measured equation of state using the 3rd-order Birch–Murnaghan fit reveals a bulk modulus K₀= 397 GPa and its first pressure derivative ${K}_{0}^{\prime }$= 4.0. The experimental lattice parameters and bulk modulus at ambient conditions also agree well with our density-functional-theory (DFT) calculations within an error margin of ∼1%. DFT results indicate that Os₂B₃ becomes more ductile under compression, with a strong anisotropy in the axial bulk modulus persisting to the highest pressure. DFT further enables the studies of charge distribution and electronic structure at high pressure. The pressure-enhanced electron density and repulsion along the Os and B bonds result in a high incompressibility along the crystal c-axis. Our work helps to elucidate the fundamental properties of Os₂B₃ under ultrahigh pressure for potential applications in extreme environments.

Exotic surface states of topological insulators have long attracted the attention of researchers. Recently, surface-dominant electrical transport in topological insulators has been observed; however, surface conduction in topological insulators is still not fully understood. To address this knowledge gap, we measured the transport properties of a thin flake of a highly bulk-resistive topological insulator, Sn_0.02Bi_1.08Sb_0.9Te₂S (Sn-BSTS), whose carrier density was controlled with the field effect. Single crystals of Sn-BSTS were synthesized by the Bridgman method, and Hall devices were fabricated with exfoliated flakes. The bottom gate structure was used to control the bottom surface of a Sn-BSTS flake. The measured Hall resistance was analyzed using the two-band model, which quantitatively showed that ambipolar conduction was achieved. In addition, the carriers on the top surface were controlled by the formation of an electrical double layer by an ionic liquid. With a top-gate voltage of −1.5 V, a massive number of p-type carriers were induced on the top surface of the Sn-BSTS flake, as also confirmed with the two-band model. The longitudinal resistance was also found to be affected by the carrier density. The magnetoresistance was enhanced when n- and p-type carriers coexisted on the top and bottom surfaces. In particular, the magnetoresistance was quantitatively shown to increase when the densities of n- and p-type carriers were similar. This study is the first to quantitatively analyze the conduction in Sn-BSTS in the presence of multiple types of carriers. Our findings pave the way for a quantitative understanding of transport phenomena in topological insulators.

To gain fundamental understanding of the high-temperature optical gas-sensing and light-energy conversion materials, we comparatively investigate the temperature effects on the band gap and optical properties of rutile and anatase TiO₂ experimentally and theoretically. Given that the electronic structures of rutile and anatase are fundamentally different, i.e. direct band gap in rutile and indirect gap in anatase, it is not clear whether these materials exhibit different electronic structure renormalizations with temperature. Using ab initio methods, we show that the electron–phonon interaction is the dominant factor for temperature band gap renormalization compared to the thermal expansion. As a result of different contributions from the acoustic and optical phonons, the band gap is found to widen with temperature up to 300 K, and to narrow at higher temperatures. Our calculations suggest that the band gap is narrowed by about 147 meV and 128 meV at 1000 K for rutile and anatase, respectively. Experimentally, for rutile and anatase TiO₂ thin films we conducted UV–Vis transmission measurements at different temperatures, and analyzed band gaps from the Tauc plots. For both TiO₂ phases, the band gap is found to decrease for temperature above 300 K quantitatively, agreeing with our theoretical results. The temperature effects on the dielectric functions, the refractive index, the extinction coefficient as well as the optical conductivity are also investigated. Rutile and anatase show generally similar optical properties, but differences exist in the long wavelength regime above 600 nm, where we found that the dielectric function of rutile decreases while that of anatase increases with temperature increase.

TmFe₂O₄ is one kind of multiferroic material in which equivalent amounts of Fe²⁺ and Fe³⁺ occupy a two-dimensional triangular lattice, leading to charge and spin frustrations. The spin frustration is expected to be increased as the fraction of Fe²⁺ (Fe³⁺) becomes larger than that of Fe³⁺ (Fe²⁺). We have grown single-crystalline TmFe₂O_4−δ with oxygen vacancies by using floating zone melting method and examined its magnetic properties. On cooling the compound, a long-range magnetic ordering develops around ∼240 K. With further cooling, a maximum of zero-field-cooled (ZFC) magnetization is observed at 186.2 K. The ac magnetic susceptibility obtained by ZFC process also manifests a maximum in its temperature dependence, and the variation of spin-freezing temperature with frequency of ac magnetic field is explainable in terms of the dynamic scaling law with the critical component of 8.68(8). This value suggests that the spin glass transition occurs at 186.2 K. The effect of external dc magnetic field on the irreversible transition temperature is coincident with the de Almeida–Thouless (AT) line. Aging-memory and rejuvenation effect is also observed below the spin-freezing temperature. These facts support the idea that TmFe₂O_4−δ undergoes spin glass transition below the ferrimagnetic transition temperature. In other words, TmFe₂O_4−δ can be regarded as a reentrance spin glass. It is thought that the oxygen vacancies bring about unequal number of Fe²⁺ and Fe³⁺ ions and thereby strengthen the magnetic frustration among the iron ions coupled with antiferromagnetic interactions, leading to the spin glass behavior.

We present first principles calculations of the electrostatic properties of Ba₂NaOsO₆ (BNOO), a 5d¹ Mott insulator with strong spin orbit coupling (SOC) in its low temperature quantum phases. In light of recent NMR experiments showing that BNOO develops a local octahedral distortion that is accompanied by the emergence of an electric field gradient (EFG) and precedes the formation of long range magnetic order (Lu et al 2017 Nat. Commun.8 14407, Liu et al 2018 Phys. Rev. B97 224103; Liu et al 2018 Physica B536 863), we calculated BNOO's EFG tensor for several different model distortions. The local orthorhombic distortion that we identified as most strongly agreeing with experiment corresponds to a Q2 distortion mode of the Na–O octahedra, in agreement with conclusions given in (Liu et al 2018 Phys. Rev. B97 224103). Furthermore, we found that the EFG is insensitive to the type of underlying magnetic order. By combining NMR results with first principles modeling, we have thus forged a more complete understanding of BNOO's structural and magnetic properties, which could not be achieved based upon experiment or theory alone.

The needs of high speed performance electronic devices for various applications require novel materials and new physical phenomena. For these purposes we propose to study new physical effects based on electron scattering on magnetic skyrmions and vortices distributed in a 2D ferromagnetic material. We show that the topological spin Hall effect can be efficiently employed for the filtering, switching, and separation of spin currents. For some values of the parameters (conduction electron concentrations, and skyrmion/vortex sizes) it is possible to separate Hall currents for different electron spin projections as it is like for different carrier charges (electrons and holes) in the normal Hall effect. The calculations are performed using the Boltzmann kinetic equation for the nonequilibrium distribution function and the Lippmann–Schwinger equation for the transition matrix in the whole range of the adiabaticity parameter. The spin filtering due to the skyrmion/vortex scattering can be several orders of magnitude more efficient in the narrow range of the electron concentrations than that of the ordinary ferromagnetic spin polarization in spintronics.

The magnetic phase diagram of the two-dimensional van der Waals magnet CrPS₄ and the exchange bias effect of CrPS₄ in contact with NiFe film have been investigated. Based on the magnetic measurements, we figure out the relatively low spin-flop field and spin-flip field for CrPS₄, both of the spin transition phenomena are strongly affected by the temperature. The perpendicular exchange bias effect is studied in CrPS₄ single-crystal flake covered with 5 nm NiFe. Meanwhile, the variation of the cooling field has a great influence on the exchange bias field and coercivity, which is mainly attributed to the competition between the Zeeman energy and the exchange coupling at the interface as well as the formation of the multi-domain state.

We have systematically reported the magnetic and magneto-transport properties of two-dimensional itinerant ferromagnetic compound Fe₃GeTe₂ at high magnetic fields of 58 T and demonstrated the correlation between its transport and magnetism. Anomalous two-steps magnetic ordering and antiferromagnetic-like transitions in zero field-cooling (ZFC) curves for H ∥ ab-plane are observed. Additionally, we find that intrinsic negative magnetoresistances in bulk Fe₃GeTe₂ single crystal are mainly attributed to the suppression of spin-fluctuations in low magnetic fields. Complex evolutions of temperature dependent high field magnetoresistances are detected under different magnetic field and current configurations, which can be explained as a result of the competition between spin-fluctuations, the magnon-scatterings and classical cyclotronic effects.

Exchange-coupled nanocomposites are considered as the most promising materials for production of high-energy performance permanent magnets, which can exceed neodymium ones in terms of energy product. In this work, micromagnetic simulations of L1₀-FeNi/SmCo₅ composites based on the initially anisotropic structure of nanorods array were performed. Texturing effect on magnetic properties was investigated. It was revealed that even 30% of anisotropy axes misalignment of grains in L1₀-FeNi phase would lead to only ≈10% drop of coercivity. To maximize magnetic properties of the composites, parameters of microstructure were optimized for 120 × 120 array of interacting nanorods and were found to be 40 nm nanorod diameter and 12–20 nm interrod distance. The estimated diameter of nanorods and the packing density of the array provide energy product values of 149 kJ m⁻³. Influence of interrod distance on energy product values was explored. Approaches for production of exchange-coupled composites based on anisotropic nanostructures were proposed.

In this work, we have presented a solid-solution of Sm_0.6Dy_0.4FeO₃ in the form of nano-particles having spin reorientation transition (SRT) at a temperature interval of 220–260 K. The lattice dynamics of Sm_0.6Dy_0.4FeO₃ have investigated by temperature-dependent x-ray diffraction and Raman spectroscopy. A negative thermal expansion at low temperatures has observed, which might be due to the interaction between Sm³⁺ and Fe³⁺ sublattice. Anomalous behavior in lattice parameters, octahedral tilt angle, and bond lengths have observed in the vicinity of SRT, which confirms the existence of magneto-elastic coupling in the system. The strong anomaly has observed in linewidth and phonon frequencies of Raman modes around SRT, which may be related to the spin–phonon coupling in Sm_0.6Dy_0.4FeO₃. The contribution of SRT in lattice change and the presence of spin–phonon coupling can help to understand the correlation between the magnetic and structural properties of orthoferrite.