We report magnetic-field-induced rotation of the antiferromagnetic Néel vector in epitaxial CuMnAs thin films. First, using soft x-ray magnetic linear dichroism spectroscopy as well as magnetometry, we demonstrate spin-flop switching and continuous spin reorientation in films with uniaxial and biaxial magnetic anisotropies, respectively, for applied magnetic fields of the order of 2 T. The remnant antiferromagnetic domain configurations are determined using x-ray photoemission electron microscopy. Next, we show that the Néel vector reorientations are manifested in the longitudinal and transverse anisotropic magnetoresistance. Dependencies of the electrical resistance on the orientation of the Néel vector with respect to both the electrical current direction and the crystal symmetry are identified, including a weak fourth-order term evident at high magnetic fields. The results provide characterization of key parameters including the anisotropic magnetoresistance coefficients, magnetocrystalline anisotropy, and spin-flop field in epitaxial films of tetragonal CuMnAs, a candidate material for antiferromagnetic spintronics.

We present a theory of superconducting $p-n$ junctions. To this end, we consider a two band model of doped bulk semiconductors with attractive interactions between the charge carriers and derive the superconducting order parameter, the quasiparticle density of states, and the chemical potential as a function of the semiconductor gap ${\mathrm{\Delta }}_0$ and the doping level $\varepsilon$. We verify previous results for the quantum phase diagram for a system with constant density of states in the conduction and valence band, which show BCS-superconductor to Bose-Einstein-condensation (BEC) and BEC-to-insulator transitions as a function of doping level and the size of the band gap. Then, we extend this formalism to a density of states which is more realistic for 3D systems and derive the corresponding quantum phase diagram, where we find that a BEC phase can only exist for small band gaps ${\mathrm{\Delta }}_0<{\mathrm{\Delta }}_0^{*}$. For larger band gaps, we find rather a direct transition from an insulator to a BCS phase. Next, we apply this theory to study the properties of superconducting $p-n$ junctions. We derive the spatial variation of the superconducting order parameter along the $p-n$ junction. As the potential difference across the junction leads to energy band bending, we find a spatial crossover between a BCS and BEC condensate, as the density of charge carriers changes across the $p-n$ junction. For the two-dimensional system, we find two possible regimes, when the bulk is in a BCS phase, a BCS-BEC-BCS junction with a single BEC layer in the space charge region, and a BCS-BEC-I-BEC-BCS junction with two layers of BEC condensates separated by an insulating layer. In three dimensions we find that there can also be a conventional BCS-I-BCS junction for semiconductors with band gaps exceeding ${\mathrm{\Delta }}_0^{*}$. Thus, we find that there can be BEC layers in the well controlled setting of doped semiconductors, where the doping level can be varied to change and control the thickness of BEC and insulator layers, making Bose-Einstein condensates thereby possibly accessible to experimental transport and optical studies in solid-state materials.

We report measurements of the low-temperature $(T\le 15\mathrm{K})$ longitudinal spin Seebeck coefficient $\left(S_{LSSE}\right)$ in bulk single crystals of the helimagnetic insulator ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ with Pt contacts. Simultaneous measurement of both $S_{LSSE}$ and the magnon thermal conductivity (${\kappa }_m$) demonstrates their correlation and allows for quantitative and favorable comparison to bulk magnon spin current theory.

Kitaev quantum spin liquids host Majorana fermions via the fractionalization of spins. In a magnetic field, the Majorana fermions were predicted to comprise a topological state, which has attracted great attention by the discovery of the half-quantized thermal Hall conductivity. Nevertheless, a reliable theory remains elusive for the field effect, especially at finite temperature. Here we present unbiased large-scale numerical results for the Kitaev model in a wide range of magnetic field and temperature. We find that the unconventional paramagnetic region showing fractional spin dynamics extends at finite temperature, far beyond the field range where the topological state is expected at zero temperature. Our results show the confinement-deconfinement behavior between the fractional Majorana excitations and the conventional magnons.

Superconductors that possess both broken spatial inversion symmetry and spin-orbit interactions exhibit a mix of spin singlet and triplet pairing. Here, we report on measurements of the superconducting properties of electron-doped, strained $\mathrm{SrTi}{\mathrm{O}}_3$ films. These films have an enhanced superconducting transition temperature and were previously shown to undergo a transition to a polar phase prior to becoming superconducting. We show that some films show signatures of an unusual superconducting state, such as an in-plane critical field that is higher than both the paramagnetic and orbital pair breaking limits. Moreover, nonreciprocal transport, which reflects the ratio of odd versus even pairing interactions, is observed. Together, these characteristics indicate that these films provide a tunable platform for investigations of unconventional superconductivity.

The thermal transport properties of $M/\mathrm{F}{\mathrm{e}}_2\left(\mathrm{V},\mathrm{Ta}\right)\mathrm{Al}/M$ ($M=\mathrm{Mo}$, W) multilayer thin films, which were epitaxially grown on the [100] MgO single-crystal substrates by means of a radio-frequency magnetron sputtering system, were systematically investigated using a picosecond pulsed laser heating time-domain thermoreflectance method. Not only the thermal diffusivity but also the boundary thermal resistance was strongly affected by both the Ta substitution for V in $\mathrm{F}{\mathrm{e}}_2\mathrm{VAl}$ and the elements in the metallic layer of top and bottom sides. From the first-principles density-functional theory calculations, we found that such changes in heat transport properties were directly related to the intrinsic phonon dispersion. In the $\mathrm{F}{\mathrm{e}}_2\left(\mathrm{V},\mathrm{Ta}\right)\mathrm{Al}$ layer thickness region of less than 14 nm, the boundary thermal resistances of all thin-film series abruptly decreased with decreasing the thickness, which was due to the coherently ordered interfacial structure.

We exhaustively construct instanton solutions and elucidate their properties in one-dimensional antiferromagnetic chiral magnets based on the $O\left(3\right)$ nonlinear sigma model description of spin chains with the Dzyaloshinskii-Moriya (DM) interaction. By introducing an easy-axis potential and a staggered magnetic field, we obtain a phase diagram consisting of ground-state phases with two points (or one point) in the easy-axis dominant cases, a helical modulation at a fixed latitude of the sphere, and a tricritical point allowing helical modulations at an arbitrary latitude. We find that instantons (or skyrmions in two-dimensional Euclidean space) appear as composite solitons in different fashions in these phases: temporal domain walls or wall-antiwall pairs (bions) in the easy-axis dominant cases, dislocations (or phase slips called merons) with fractional instanton numbers in the helical state, and isolated instantons and calorons living on the top of the helical modulation at the tricritical point. We also show that the models with DM interaction and an easy-plane potential can be mapped into those without them, providing a useful tool to investigate the model with the DM interaction.

We present a study of the structural and electronic properties of ultrananocrystalline diamond films that were modified by adding nitrogen to the gas mixture during chemical vapor deposition growth. Hall bar devices were fabricated from the resulting films to investigate their electrical conduction as a function of both temperature and magnetic field. Through low-temperature magnetoresistance measurements, we present strong evidence that the dominant conduction mechanism in these films can be explained by a combination of three-dimensional weak localization (3DWL) and thermally activated hopping at higher temperatures. An anisotropic 3DWL model is then applied to extract the phase-coherence time as a function of temperature, which shows evidence of a power-law dependence in good agreement with theory.

Recent progresses in the growth and fabrication techniques for preparing crystalline two-dimensional (2D) superconductors have stimulated intense interest in the studies of the electronic properties of these systems. Here we investigate the superconducting transport properties based on chemical vapor deposition-grown thin NbC crystals consisting of network structures. The 2D character of the superconductivity in individual NbC crystals is revealed by examining the angular dependence of magnetotransport measurements. At low temperatures, the samples show nonmonotonic double-step superconducting transitions as a function of temperature and magnetic field. We demonstrate that the observed transport characteristics can be understood in terms of coupled Josephson junctions forming between isolated NbC crystals, including the effects of Josephson and quasiparticle tunneling. In particular, detailed analysis of the magnetic field-driven transition suggests the existence of quantum flux-creep regime at low temperatures and small magnetic fields in such thin NbC superconducting crystals. Our work underlines the importance of the morphology on the transport properties of 2D superconducting crystals, providing a comprehensive understanding of crystalline 2D superconductors.

The magnetically induced valley-spin filtering in transition metal dichalcogenide monolayers $(M{X}_2$, where $M$ = Mo, W and $X$ = S, Se, Te) promises a new paradigm in information processing. However, the detailed understanding of this effect is still limited, regarding its underlying transport processes. Herein it is suggested that the filtering mechanism can be greatly elucidated by the concept of metal-induced gap states (MIGS), appearing in the electrode-terminated $M{X}_2$ materials, i.e., the referential filter setup. In particular, the gap states are predicted here to mediate valley- and spin-resolved charge transport near the ideal electrode/$M{X}_2$ interface, and therefore to initiate filtering. It is also argued that the role of MIGS increases when the channel length is diminished, as they begin to govern the overall valley-spin transport in the tunneling regime. In what follows, the presented study yields fundamental scaling trends for the valley-spin selectivity with respect to the intrinsic physics of the filter materials. As a result, it facilitates insight into the analyzed effects and provides design guidelines toward efficient valley-spin filter devices that are based on the discussed materials or other hexagonal monolayers with a broken inversion symmetry.

Dirac-electronic tunneling and transport properties with both finite and zero energy band gap are investigated for graphene with an in-plane tilted potential barrier embedded with scatters. For a tilted barrier, by using Wentzel-Kramers-Brillouin approximation, an analytical solution is obtained first for transmission coefficient of Dirac electrons in gapped graphene in the absence of any scatters. In the presence of either a single or a continuous distribution of scatters embedded within a tilted barrier, however, a numerical scheme based on finite-difference approach is developed for accurately calculating both transmission coefficient and tunneling resistance of Dirac electrons. Here, the combination of a tilted barrier and a scatter potential can be viewed as an effective barrier-potential profile facilitated by a proper gate structure. Meanwhile, a full analysis and detailed comparisons are presented for the interplay between effects of both distributed scatters in a barrier and barrier tilting on tunneling transport of Dirac electrons in graphene. The barrier-tilting field and scatter position are found to play a key role in controlling a peak of tunneling resistance as well as in its switching to a cusp by a mid-barrier-embedded scatter as the incident energy reaches the Dirac point in a barrier. Different from a single scatter, a continuous distribution within a barrier can enhance the unimpeded incoherent tunneling for head-on collision while greatly suppressing skew ones with increasing barrier-tilting field. All these predicted attractive transport properties are expected to be extremely useful for designing both novel electronic and optical graphene-based devices and electronic lenses in ballistic-electron optics.

The half-Heusler rare-earth intermetallic GdPtBi has recently gained attention due to peculiar magnetotransport phenomena that have been associated with the possible existence of Weyl fermions, thought to arise from the crossings of spin-split conduction and valence bands. On the other hand, similar magnetotransport phenomena observed in other rare-earth intermetallics have often been attributed to the interaction of itinerant carriers with localized magnetic moments stemming from the $4f$ shell of the rare-earth element. In order to address the origin of the magnetotransport phenomena in GdPtBi, we performed a comprehensive study of the magnetization, electrical, and thermal magnetoresistivity on two single-crystalline GdPtBi samples. In addition, we performed an analysis of the Fermi surface via Shubnikov–de Haas oscillations in one of the samples and compared the results to ab initio band structure calculations. Our findings indicate that the electrical and thermal magnetotransport in GdPtBi cannot be solely explained by Weyl physics and is strongly influenced by the interaction of both itinerant charge carriers and phonons with localized magnetic Gd ions and possibly also paramagnetic impurities.

A critical point of the energy dispersion is the momentum where electron velocity vanishes. At the corresponding energy, the density of states (DOS) exhibits nonanalyticity such as divergence. Critical points can be first classified as ordinary and high-order ones, and the ordinary critical points have been studied thoroughly by Léon van Hove, whose DOS is particle-hole symmetric and logarithmically divergent. In this work, we describe and classify high-order critical points based on topology, scaling, and symmetry. We show that high-order critical points can have power-law-divergent DOS with particle-hole asymmetry, and can be realized at generic or symmetric momenta by tuning a few parameters such as twist angle, strain, pressure, and/or external fields.

The potential of zero charge (pzc), a fundamental concept in interfacial electrochemistry, is revisited using a jellium-Poisson-Boltzmann model. Under constant-potential description of the metal-solution interphase, this model is able to calculate the surface charging relation (surface free charge density as a function of the electrode potential) and then to determine the pzc therefrom. The potential corresponding to the minimum of differential double-layer capacitance curve is shown to be lower than the pzc determined from surface charging relation, which is caused by free metal electrons entering the solution phase. The model further reveals that the pzc decreases when the vacuum gap between the solution phase and the metal surface, $d$, becomes narrower. This is consistent with the common observation that the pzc of metal-solution interphase is lower than that calculated from the work function of metal-vacuum interphase (the latter corresponds to $d=\infty )$. Multifaceted roles played by the solvent, including electrostatic screening, polaron effect, and orthogonalizational repulsion, are analyzed. Also discussed are the effects of specific adsorption of ions and potential-dependent $d$ on the surface charging relation.

We discuss here the effect of band nesting and topology on the spectrum of excitons in a single layer of ${\mathrm{MoS}}_2$, a prototype transition metal dichalcogenide material. We solve for the single-particle states using the ab initio based tight-binding model containing metal $d$ and sulfur $p$ orbitals. The metal orbitals' contribution evolving from $K$ to $\mathrm{\Gamma }$ points results in conduction-valence band nesting and a set of second minima at $Q$ points in the conduction band. There are three $Q$ minima for each $K$ valley. We accurately solve the Bethe-Salpeter equation including both $K$ and $Q$ points and obtain ground and excited exciton states. We determine the effects of the electron-hole single-particle energies including band nesting, direct and exchange screened Coulomb electron-hole interactions, and resulting topological magnetic moments on the exciton spectrum. The ability to control different contributions combined with accurate calculations of the ground and excited exciton states allows for the determination of the importance of different contributions and a comparison with effective mass and $k·p$ massive Dirac fermion models.

We study the behavior of the magnetization and the magnetic susceptibility of molecular magnets with complex bridging structure. Our computations are based on a post-Hartree-Fock method accounting for the intricate network of interatomic bonds and an effective spinlike Hamiltonian that captures the essential magnetic features of magnetic molecules. The devised method and the constructed Hamiltonian are further employed to characterize the magnetic properties of the molecular magnet ${\mathrm{Ni}}_4{\mathrm{Mo}}_{12}$. The obtained results reproduce both quantitatively and qualitatively the main features of the magnetic spectrum. Furthermore, the computations for the magnetization and the low-field susceptibility are in very good agreement with their experimental counterparts. In this respect, they improve upon the results obtained with conventional Heisenberg models.

We applied the recently developed Generalized Langevin equation (GLE) approach for dielectric response of liquids and glasses to link the vibrational density of states (VDOS) to the dielectric response of a model orientational glass. The dielectric functions calculated based on the GLE, with VDOS obtained in experiments and simulations as input, are compared with experimental data for the paradigmatic case of 2-adamantanone at various temperatures. The memory function is related to the integral of the VDOS times a spectral coupling function $\gamma \left({\omega }_p\right)$, which tells the degree of dynamical coupling between molecular degrees of freedom at different eigenfrequencies. With respect to previous empirical fittings, the GLE-based fitting reveals a broader temperature range over which the secondary relaxation is active. Furthermore, the theoretical analysis provides clear evidence of secondary relaxation being localized within the THz ($0.5-1$ THz) range of eigenfrequencies, and thus not too far from the low-energy modes involved in $\alpha$ relaxation. In the same THz region, the same material displays a crowding of low-energy optical modes that may be related to the secondary relaxation.

Motivated by the rich phase diagrams recently unveiled in frustrated spin-1 chains with competing next-nearest-neighbor and four-spin interactions, we investigate the nature of the elementary excitations of spin-1 chains in the vicinity of phase transitions using the matrix-product state (MPS) representation of elementary excitations. First, we show that both spinons and magnons naturally arise in $\mathrm{SU}\left(2\right)$ invariant spin chains when describing ground states and elementary excitations using MPSs. Then we investigate the nature of the elementary excitations across the first-order transition between the Haldane phase and the topologically trivial next-nearest-neighbor Haldane phase. We show explicitly that spinons deconfine at the transition, and we calculate the dispersion of these deconfined spinons with MPSs. We also show that, immediately away from the transition line, spinons confine, forming dispersive spinon/antispinon bound states on both sides of the transition. Finally, we show that deconfined spinons also appear at the transition between the Haldane phase and the spontaneously dimerized phase when this transition is first order.

We study a theoretical model for synthetic anyons in a noninteracting quantum many-body system. Synthetic anyons can occur in a noninteracting system when it is perturbed with specially tailored localized probes, which supply the demanded nontrivial topology in the system. The model is represented by the Hamiltonian for noninteracting electrons in two dimensions, in a uniform magnetic field, pierced with solenoids with a magnetic flux that is a fraction of the flux quantum. In a potential experimental realization of the model, there should be a mechanism fixing the flux in all solenoid probes to an identical value for these perturbations to represent synthetic anyons. We find analytically and numerically the ground state of the model when only the lowest Landau-level states are occupied. We calculate the statistical parameter by using the Berry phase, and show that the ground state is anyonic in the coordinates of the probes. We show that these synthetic anyons cannot be considered as emergent quasiparticles. The fusion rules are discussed for different microscopic realizations of the fusion process.