Focus on Women's Perspectives in Quantum Materials

In a previous study (2017 Phys. Rev. B96 144501), a strong variation in the superconducting transition temperature T_c of YB₆ differing by a factor of two has been explained by a change in the density of yttrium and boron vacancies tuning the electron–phonon interaction. Here, by using an array of miniature Hall probes, we address the penetration of the magnetic field, pinning, and critical current density on a series of YB₆ single crystals with T_c variation between 4.25 and 7.35 K. The analysis of the superconducting and normal-state specific heat characteristics allowed us to determine T_c and the stoichiometry of our samples. We observed almost no pinning in the most stoichiometric YB₆ crystal with the lowest T_c. Upon increasing the number of vacancies weak pinning appears, and the critical current density is enhanced following the increased transition temperature in a linear variation. We argue that such an increase is, within weak collective pinning theory, consistent with the increasing number of vacancies that serve as pinning centers.

We consider two high symmetry stackings AA and AB and examine the changes induced in the electronic structure by considering small angles of rotation of 3.48° from both these stackings. In both cases we largely recover the low energy electronic structure of the untwisted limit. We additionally find flat bands emerging above the dispersing bands. Surprisingly, while the rotation from the AA end leads to one flat band above the highest occupied band at Γ, one finds two flat bands emerging for small rotations from the AB end. Examining the real space localisation of the flat bands allows us to discuss the origin of the flat bands in terms of quantum well states and qualitatively understand the dependence of the number of flat bands found on the twist angle.

Vacancies in atomically thin molybdenum disulphide play an essential role in controlling its optical and electronic properties, which are crucial for applications in sensorics, catalysis or electronics. For this reason, defect engineering employing thiol-terminated molecules is used to heal and/or functionalise defective nanosheets. In this work, chemical vapour deposition-grown MoS₂ with different defect densities was functionalised with three molecules: 4-aminothiophenol (ATP), biphenyl-4-thiol (BPT) and 4-nitrothiophenol (NTP). The molecules' efficacy in functionalising MoS₂ was probed by x-ray photoelectron, Raman and photoluminescence (PL) spectroscopy. The results show that exposing a defective single layer of MoS₂ to either ATP, BPT or NTP molecules heals the defects, however the chemical structure of these molecules affects the optical response and only for BPT the PL intensity increases.

Motivated by the discovery of superconductivity in KTaO₃-based heterostructures, we study a pairing mechanism based on spin-orbit assisted coupling between the conduction electrons and the ferroelectric (FE) modes present in the material. We use ab initio frozen-phonon computations to show a linear-in-momentum Rashba-like coupling with a strong angular dependence in momentum for the lower $j = 3/2$ manifold, deviating from the conventional isotropic Rashba model. This implies the Rashba-like interaction with the polar modes has substantial L = 3 cubic harmonic corrections, which we quantify for each electronic band. The strong anisotropy of the Rashba interaction is captured by a microscopic toy model for the $t_{2g}$ electrons. We find its origin to be the angular dependence in electronic momentum imposed by the kinetic term on the degenerate $j = 3/2$ manifold. A comparison between the toy model and ab initio results indicates that additional symmetry allowed terms beyond odd-parity spin-conserving inter-orbital hopping processes are needed to describe the Rashba-like polar interaction between the electrons and the soft FE mode.

The complex magnetic ordering of parent compounds of most unconventional superconductors is crucial for the understanding of high-temperature superconductivity (SC). Within this framework, we have performed temperature-dependent magnetotransport experiments on a single crystal of SmFeAsO, a parent compound of iron pnictide superconductors. We observe multiple features in the measured transport properties at temperatures below the antiferromagnetic (AFM) ordering of Sm, $T < {T_{{\text{NSm}}}}$, which evolve with in-plane magnetic field, suggesting a rich variety of metamagnetic transitions never before observed in this compound. Considering that transport mainly involves Fe d orbitals at the Fermi level, these findings suggest that the features originate from magnetic transitions of the Fe moments sublattice, which in turn may be induced by magnetic transitions of the Sm moments sublattice via the interaction between Fe and Sm moments. We outline a possible scenario in which the Fe moments, strongly affected by the Sm ordering below T_NSm, reorder to an in-plane canted AFM structure, which is washed out by the application of an in-plane magnetic field up to 9 T. Our work shows that transport properties are a valuable tool for investigating magnetic ordering in iron pnictide parent compounds, where the interplay of magnetism and SC is believed to be the origin of high-temperature SC.

The crystal structure, electron energy-loss spectroscopy (EELS), heat capacity, and anisotropic magnetic and resistivity measurements are reported for Sn flux grown single crystals of orthorhombic Pr₂Co₃Ge₅ (U₂Co₃Si₅-type, Ibam). Our findings show that o-Pr₂Co₃Ge₅ hosts nearly trivalent Pr ions, as evidenced by EELS and fits to temperature dependent magnetic susceptibility measurements. Complex magnetic ordering with a partially spin-polarized state emerges near T_sp = 32 K, with a spin reconfiguration transition near T_M = 15 K. Heat capacity measurements show that the phase transitions appear as broad peaks in the vicinity of T_sp and T_M. The magnetic entropy further reveals that crystal electric field splitting lifts the Hund's rule degeneracy at low temperatures. Taken together, these measurements show that Pr₂Co₃Ge₅ is an environment for complex f state magnetism with potential strongly correlated electron states.

The electronic properties of Fe-based superconductors are drastically affected by deformations on their crystal structure introduced by doping and pressure. Here we study single crystals of FeSe$_{1-x}$S_x and reveal that local crystal deformations such as atomic-scale defects impact the spectral shape of the electronic core level states of the material. By means of scanning tunneling microscopy we image S-doping induced defects as well as diluted dumbbell defects associated with Fe vacancies. We have access to the electronic structure of the samples by means of x-ray photoemission spectroscopy (XPS) and show that the spectral shape of the Se core levels can only be adequately described by considering a principal plus a minor component of the electronic states. We find this result for both pure and S-doped samples, irrespective that in the latter case the material presents extra crystal defects associated to doping with S atoms. We argue that the second component in our XPS spectra is associated with the ubiquitous dumbbell defects in FeSe that are known to entail a significant modification of the electronic clouds of surrounding atoms.

Chemical modifications such as intercalation can be used to modify surface properties or to further functionalize the surface states of topological insulators (TIs). Using ambient pressure x-ray photoelectron spectroscopy, we report copper migration in $\mathrm{Cu}_x\mathrm{Bi}_{2}\mathrm{Se}_{3}$, which occurs on a timescale of hours to days after initial surface cleaving. The increase in near-surface copper proceeds along with the oxidation of the sample surface and large changes in the selenium content. These complex changes are further modeled with core-level spectroscopy simulations, which suggest a composition gradient near the surface which develops with oxygen exposure. Our results shed light on a new phenomenon that must be considered for intercalated TIs—and intercalated materials in general—that surface chemical composition can change when specimens are exposed to ambient conditions.

Exploring two dimensional (2D) materials is important for further developing the field of quantum materials. However, progress in 2D material development is limited by difficulties with their production. Specifically, freestanding 2D materials with bulk non-layered structures remain particularly challenging to prepare. Traditionally, chemical or mechanical exfoliation is employed for obtaining freestanding 2D materials, but these methods typically require layered starting materials. Here we put forth a method for obtaining thin layers of β-Bi₂O₃, which has a three-dimensional covalent structure, by using chemical exfoliation. In this research, Na₃Ni₂BiO₆ was exfoliated with acid and water to obtain β-Bi₂O₃ nanosheets less than 10 nm in height and over 1 µm in lateral size. Our results open the possibility for further exploring β-Bi₂O₃ nanosheets to determine whether their properties change from the bulk to the nanoscale. Furthermore, this research may facilitate further progress in obtaining nanosheets of non-layered bulk materials using chemical exfoliation.

Dirac semimetals (DSM) host linear bulk bands and topologically protected surface states, giving rise to exotic and robust properties. Platinum ditelluride (PtTe₂) belongs to this interesting group of topological materials. Here, we employ scanning tunneling microscopy (STM) in combination with first-principles calculations to visualize and identify the native defects at the surface of a freshly cleaved PtTe₂ crystal. Around these defects, short-wavelength electron density oscillations are observed. Fourier transform analysis of the energy-dependent quasiparticle interference patterns is in good agreement with our calculated joint density of states, demonstrating the singular properties of the surface of this type-II DSM. Our results evidence the power of STM in understanding the surface of topological materials.

We study the magnon spectrum of stacked zig-zag chains of point magnetic dipoles with an easy axis. The anisotropy due to the dipolar interactions and the two-point basis of the zig-zag chain unit cell combine to give rise to topologically non-trivial magnon bands in 2D zig-zag lattices. Adjusting the distance between the two sublattice sites in the unit cell causes a band touching, which triggers the exchange of the Chern numbers of volume bands switching the sign of the thermal conductivity and the sense of motion of edges modes in zig-zag stripes. We show that these topological features survive when the range of the dipolar interactions is truncated up to the second nearest neighbors.

We propose a simple variational solution for calculating one-particle spectral functions in lattice models of spinless metals with strong electron-phonon coupling. It is based on a generalization of the Momentum Average variational approximation for single polarons, combined with the assumption that the other fermions in the system are locked into an inert Fermi sea. We expect the method to be accurate for fermion addition spectral functions in metals with a small Fermi energy (nearly empty band), and for fermion removal spectral functions in metals with a large Fermi energy (nearly full band), provided that the characteristic phonon frequency is not too small. Both these regions are far from the region where the Migdal theorem holds, thus our results offer new insights into polaronic behavior in a largely unexplored part of the parameter space. Here, we show results for the Holstein coupling in one-dimension and present ways to gauge their accuracy, but ultimately this will need to be verified against numerical calculations. This variational method can be extended straightforwardly to higher dimensions and other forms of electron-phonon coupling.

Spin-triplet bulk superconductors are a promising route to topological superconductivity, and UTe₂ is a recently discovered contender. The superconducting properties of UTe₂, however, vary substantially as a function of the synthetic route, and even nonsuperconducting single crystals have been reported. To understand the driving mechanism suppressing superconductivity, we investigate UTe₂ single crystals grown close to the nonsuperconducting boundary (growth temperature ∼710 ^∘C) through a combination of thermodynamic and x-ray diffraction measurements. Specific heat measurements reveal a sharp decrease in the superconducting volume and a concomitant increase in the residual specific heat coefficient close to the nonsuperconducting boundary. Notably, these crystals are inhomogeneous and show an apparent double transition in specific heat measurements, similar to samples grown at much higher temperatures (∼1000 ^∘C). Our single crystal x-ray diffraction measurements reveal that there are two important tuning parameters: uranium vacancies and the atomic displacement along the c axis, which shows a twofold increase in samples with a reduced superconducting volume. Our results highlight the key role of local disorder along the uranium-uranium dimers and suggest that the apparent double superconducting transition is more likely to emerge close to the superconducting limits of UTe₂.

The van der Waals (vdW) interaction plays a prominent role between neutral objects at separations where short ranged chemical forces are negligible. This type of dispersive coupling is determined by the interplay between geometry and response properties of the materials making up the objects. Here, we investigate the vdW interaction between 1D, 2D, and 3D standard and Dirac materials within the Random Phase Approximation, which takes into account collective excitations originating from the electronic Coulomb potential. A comprehensive understanding of characteristic functionalities and scaling laws are obtained for systems with parabolic energy dispersion (standard materials) and crossing linear bands (Dirac materials). By comparing the quantum mechanical and thermal limits the onset of thermal fluctuations in the vdW interaction is discussed showing that thermal effects are significantly pronounced at smaller scales in reduced dimensions.

We investigate the effect of spin–orbit interaction on the intra- and interdot particle dynamics of a double quantum dot (QD) under ac electric fields. The former is modeled as an effective ac magnetic field that produces electric-dipole spin resonance transitions, while the latter is introduced via spin-flip tunneling amplitudes. We observe the appearance of non-trivial spin-polarized dark states (DSs), arising from an ac-induced interference between photo-assisted spin-conserving and spin-flip tunneling processes. These DSs can be employed to precisely measure the spin–orbit coupling in QD systems. Furthermore, we show that the interplay between photo-assisted transitions and spin-flip tunneling enables the system to operate as a highly tunable spin filter. Finally, we investigate the operation of the system as a resonant flopping-mode qubit for arbitrary ac voltage amplitudes, allowing for high tunability and enhanced qubit control possibilities.