Nontrivial magnetic orders in the inverse-perovskite manganese nitrides are theoretically studied by constructing a classical spin model describing the magnetic anisotropy and frustrated exchange interactions inherent in specific crystal and electronic structures of these materials. With a replica-exchange Monte Carlo technique, a theoretical analysis of this model reproduces the experimentally observed triangular ${\mathrm{\Gamma }}^{5g}$ and ${\mathrm{\Gamma }}^{4g}$ spin-ordered patterns and the systematic evolution of magnetic orders. Our Rapid Communication solves a 40-year-old problem of nontrivial magnetism for the inverse-perovskite manganese nitrides and provides a firm basis for clarifying the magnetism-driven negative thermal expansion phenomenon discovered in this class of materials.

We report high-pressure Raman, synchrotron x-ray diffraction, and electrical transport studies on Weyl semimetals NbP and TaP along with first-principles density functional theoretical (DFT) analysis. The frequencies of first-order Raman modes of NbP harden with increasing pressure and exhibit a slope change at $P_c\sim 9$ GPa. The pressure-dependent resistivity exhibits a minimum at $P_c$. The temperature coefficient of resistivity below $P_c$ is positive as expected for semimetals but changes significantly in the high-pressure phase. Using DFT calculations, we show that these anomalies are associated with a pressure-induced Lifshitz transition, which involves the appearance of electron and hole pockets in its electronic structure. In contrast, the results of Raman and synchrotron x-ray diffraction experiments on TaP and DFT calculations show that TaP is quite robust under pressure and does not undergo any phase transition.

Absorption of ultrashort laser pulses in a metallic grating deposited on a transparent sample launches coherent compression/dilatation acoustic pulses in directions of different orders of acoustic diffraction. Their propagation is detected by delayed laser pulses, which are also diffracted by the metallic grating, through the measurement of the transient intensity change of the first-order diffracted light. The obtained data contain multiple frequency components, which are interpreted by considering all possible angles for the Brillouin scattering of light achieved through multiplexing of the propagation directions of light and coherent sound by the metallic grating. The emitted acoustic field can be equivalently presented as a superposition of plane inhomogeneous acoustic waves, which constitute an acoustic diffraction grating for the probe light. Thus the obtained results can also be interpreted as a consequence of probe light diffraction by both metallic and acoustic gratings. The realized scheme of time-domain Brillouin scattering with metallic gratings operating in reflection mode provides access to wide range of acoustic frequencies from minimal to maximal possible values in a single experimental optical configuration for the directions of probe light incidence and scattered light detection. This is achieved by monitoring the backward and forward Brillouin scattering processes in parallel. Potential applications include measurements of the acoustic dispersion, simultaneous determination of sound velocity and optical refractive index, and evaluation of samples with a single direction of possible optical access.

Energy-efficient classical information processing and storage based on topological defects in magnetic systems have been studied over the past decade. In this work, we introduce a class of macroscopic quantum devices in which a quantum state is stored in a topological defect of a magnetic insulator. We propose noninvasive methods to coherently control and read out the quantum state using ac magnetic fields and magnetic force microscopy, respectively. This macroscopic quantum spintronic device realizes the magnetic analog of the three-level rf-SQUID qubit and is built fully out of electrical insulators with no mobile electrons, thus eliminating decoherence due to the coupling of the quantum variable to an electronic continuum and energy dissipation due to Joule heating. For a domain wall size of 10–100 nm and reasonable material parameters, we estimate qubit operating temperatures in the range of 0.1–1 K, a decoherence time of about 0.01–$1\mu \mathrm{s}$, and the number of Rabi flops within the coherence time scale in the range of ${10}^2-{10}^4$.

By varying both the carrier momentum and spin orientation separately, the anisotropic Berry-phase effect in MnAs films is explicitly investigated. Specifically, the Berry-phase effect is characterized by the intrinsic anomalous Hall conductivity γ, which is extracted via treating various scattering sources independently. More than 50% change of γ is observed when changing the electric current direction from MnAs [2-1-10] to [0001], while γ varies moderately for the cases of applying the current along two orthogonal directions in the close-packed crystal plane. In addition, the magnetization direction induced variation of γ is also relatively small. The above phenomena can be explained under the framework of anisotropic spin-orbit interaction, which provides a unified picture for understanding the critical role played by electric current and magnetization directions. Our results pave an avenue for getting deep insights into the anisotropic Berry-phase effect and its contribution to the anomalous Hall effect.

The discovery of Weyl semimetals represents a significant advance in topological band theory. They paradigmatically enlarged the classification of topological materials to gapless systems while simultaneously providing experimental evidence for the long-sought Weyl fermions. Beyond fundamental relevance, their high mobility, strong magnetoresistance, and the possible existence of even more exotic effects, such as the chiral anomaly, make Weyl semimetals a promising platform to develop radically new technology. Fully exploiting their potential requires going beyond the mere identification of materials and calls for a detailed characterization of their functional response, which is severely complicated by the coexistence of surface- and bulk-derived topologically protected quasiparticles, i.e., Fermi arcs and Weyl points, respectively. Here, we focus on the type-II Weyl semimetal class in which we find a stoichiometry-dependent phase transition from a trivial to a nontrivial regime. By exploring the two extreme cases of the phase diagram, we demonstrate the existence of a universal response of both surface and bulk states to perturbations. We show that quasiparticle interference patterns originate from scattering events among surface arcs. Analysis reveals that topologically nontrivial contributions are strongly suppressed by spin texture. We also show that scattering at localized impurities can generate defect-induced quasiparticles sitting close to the Weyl point energy. These give rise to strong peaks in the local density of states, which lift the Weyl node, significantly altering the pristine low-energy spectrum. Remarkably, by comparing the ${\mathrm{WTe}}_2$ and the ${\mathrm{MoTe}}_2$ cases we found that scattering response and topological transition are not directly linked. Visualizing the existence of a universal microscopic response to scattering has important consequences for understanding the unusual transport properties of this class of materials. Overall, our observations provide a unifying picture of the type-II Weyl phase diagram.

We use density functional theory plus dynamical mean-field theory (DFT+DMFT) to study multiple control parameters for tuning the metal-insulator transition (MIT) in ${\mathrm{CaVO}}_3$ thin films. We focus on separating the effects resulting from substrate-induced epitaxial strain from those related to the reduced thickness of the film. We show that tensile epitaxial strain of around 3%–4% is sufficient to induce a transition to a paramagnetic Mott-insulating phase. This corresponds to the level of strain that could be achieved on a ${\mathrm{SrTiO}}_3$ substrate. Using free-standing slab models, we then demonstrate that reduced film thickness can also cause a MIT in ${\mathrm{CaVO}}_3$, however, only for thicknesses of less than 4 perovskite units. Our calculations indicate that the MIT in such ultrathin films results mainly from a surface-induced crystal-field splitting between the $t_{2g}$ orbitals, favoring the formation of an orbitally polarized Mott insulator. This surface-induced crystal-field splitting is of the same type as the one resulting from tensile epitaxial strain, and thus the two effects can also cooperate. Furthermore, our calculations confirm an enhancement of correlation effects at the film surface, resulting in a reduced quasiparticle spectral weight in the outermost layer, whereas bulklike properties are recovered within only a few layers away from the surface.

A promising route to the realization of Majorana fermions is in noncentrosymmetric superconductors, in which spin-orbit coupling lifts the spin degeneracy of both bulk and surface bands. A detailed assessment of the electronic structure is critical to evaluate their suitability for this through establishing the topological properties of the electronic structure. This requires correct identification of the time-reversal-invariant momenta. One such material is BiPd, a recently rediscovered noncentrosymmetric superconductor which can be grown in large, high-quality single crystals and has been studied by several groups using angular resolved photoemission to establish its surface electronic structure. Many of the published electronic structure studies on this material are based on a reciprocal unit cell which is not the actual Brillouin zone of the material. We show here the consequences of this for the electronic structures and show how the inferred topological nature of the material is affected.

Recently it was suggested that transient excitonic instability can be realized in optically pumped two-dimensional (2D) Dirac materials (DMs), such as graphene and topological insulator surface states. Here we discuss the possibility of achieving a transient excitonic condensate in optically pumped three-dimensional (3D) DMs, such as Dirac and Weyl semimetals, described by nonequilibrium chemical potentials for photoexcited electrons and holes. Similar to the equilibrium case with long-range interactions, we find that for pumped 3D DMs with screened Coulomb potential two possible excitonic phases exist, an excitonic insulator phase and the charge density wave phase originating from intranodal and internodal interactions, respectively. In the pumped case, the critical coupling for excitonic instability vanishes; therefore the two phases coexist for arbitrarily weak coupling strengths. The excitonic gap in the charge density wave phase is always the largest one. The competition between screening effects and the increase of the density of states with optical pumping results in a rich phase diagram for the transient excitonic condensate. Based on the static theory of screening, we find that under certain conditions the value of the dimensionless coupling constant screening in 3D DMs can be weaker than in 2D DMs. Furthermore, we identify the signatures of the transient excitonic condensate that could be probed by scanning tunneling spectroscopy, photoemission, and optical conductivity measurements. Finally, we provide estimates of critical temperatures and excitonic gaps for existing and hypothetical 3D DMs.

Adsorption of 1/3 monolayer of Sn on a heavily doped $p$-type Si(111) substrate results in the formation of a hole-doped Mott insulator, with electronic properties that are remarkably similar to those of the high-$T_c$ copper oxide compounds. In this work, we show that the maximum hole-density of this system increases with decreasing domain size as the area of the Mott insulating domains approaches the nanoscale regime. Concomitantly, scanning tunneling spectroscopy (STS) data at 4.4 K reveal an increasingly prominent zero-bias anomaly (ZBA). We consider two different scenarios as potential mechanisms for this ZBA: chiral $d_{x^2-y^2}+\mathrm{i}{d}_{xy}$ wave superconductivity and a dynamical Coulomb blockade (DCB) effect. The latter arises due to the formation of a resistive depletion layer between the nanodomains and the substrate. Both models fit the tunneling spectra with weaker ZBAs, while the DCB model clearly fits better to spectra recorded at higher temperatures or from the smallest domains with the strongest ZBA. Consistently, STS spectra from the lightly doped substrates display oscillatory behavior that can be attributed to conventional Coulomb staircase behavior, which becomes stronger for smaller sized domains. We conclude that the ZBA is predominantly due to a DCB effect, while a superconducting instability is absent or a minor contributing factor.

We determine conditions for the formation of compressible stripes near the quantum Hall effect (QHE) edges of top-gated epitaxial graphene on Si-terminated SiC (G/SiC) and compare those to graphene exfoliated onto insulating substrate in the field-effect-transistor (GraFET) geometry. For G/SiC, a large density of localized surface states on SiC just underneath graphene layer and charge transfer between them lead both to doping of graphene and to screening of potential profile near its edge. This suppresses formation of compressible stripes near QHE edges in graphene, making them much narrower than the corresponding compressible stripes in GraFETs.

We calculate the lifetime of a surface plasmon on the surface of jellium metal from the process of the surface plasmon decaying into electron-hole pairs in the metal. We find that this process only occurs over a finite interval of wave vector, and at large values of wave vector. Our calculation contained two new features: (i) we used the Brillouin formula to normalize the electromagnetic field traveling along the surface and (ii) we included the spin rotation term in the electron current. The latter contribution has no effect at large wave vector of the surface plasmon.

We report the structural, vibrational, and electrical transport properties up to $\sim 16\mathrm{GPa}$ of $1T\text{−}\mathrm{TiT}{\mathrm{e}}_2$, a prominent layered 2D system. We clearly show signatures of two isostructural transitions at $\sim 2\mathrm{GPa}$ and $\sim 4\mathrm{GPa}$ obtained from the minima in $c/a$ ratio concomitant with the phonon linewidth anomalies of $E_g$ and $A_{1g}$ modes around the same pressures, providing a strong indication of unusual electron-phonon coupling associated with these transitions. Resistance measurements present nonlinear behavior over similar pressure ranges shedding light on the electronic origin of these pressure-driven isostructural transitions. These multiple indirect signatures of an electronic transition at $\sim 2\mathrm{GPa}$ and $\sim 4\mathrm{GPa}$ are discussed in connection with the recent theoretical proposal for $1T\text{−}\mathrm{TiT}{\mathrm{e}}_2$ and also the possibility of an electronic topological transition from our electronic Fermi surface calculations. Between 4 GPa and $\sim 8\mathrm{GPa}$, the $c/a$ ratio shows a plateau suggesting a transformation from an anisotropic 2D layer to a quasi-3D crystal network. First-principles calculations suggest that the 2D to quasi-3D evolution without any structural phase transitions is mainly due to the increased interlayer Te-Te interactions (bridging) via the charge density overlap. In addition, we observed a first-order structural phase transition from the trigonal ($P\overline{3}m1$) to monoclinic ($C2/m$) phase at higher pressure regions. We estimate the start of this structural phase transition to be $\sim 8\mathrm{GPa}$ and also the coexistence of two phases [trigonal ($P\overline{3}m1$) and monoclinic ($C2/m$)] was observed from $\sim 8\mathrm{GPa}$ to $\sim 16\mathrm{GPa}$.

Observation of interference in the quantum Hall regime may be hampered by a small edge state velocity due to finite phase coherence time. Therefore designing two quantum point contact (QPCs) interferometers having a high edge state velocity is desirable. Here we present a new simulation method for designing heterostructures with high edge state velocity by realistically modeling edge states near QPCs in the integer quantum Hall effect (IQHE) regime. Using this simulation method, we also predict the filling factor at the center of QPCs and their conductance at different gate voltages. The 3D Schrödinger equation is split into 1D and 2D parts. Quasi-1D Schrödinger and Poisson equations are solved self-consistently in the IQHE regime to obtain the potential profile, and quantum transport is used to solve for the edge state wave functions. The velocity of edge states is found to be $\langle E\rangle /B$, where $\langle E\rangle$ is the expectation value of the electric field for the edge state. Anisotropically etched trench gated heterostructures with double-sided delta doping have the highest edge state velocity among the structures considered.

We propose a model describing Seebeck effect on a weak link between two quantum systems with fine-tunable ground states of Fermi and non-Fermi liquid origin. The experimental realization of the model can be achieved by utilizing the quantum devices operating in the integer quantum Hall regime [Z. Iftikhar et al., Nature (London) 526, 233 (2015)] designed for detection of macroscopic quantum charged states in multichannel Kondo systems. We present a theory of thermoelectric transport through hybrid quantum devices constructed from quantum-dot–quantum-point-contact building blocks. We discuss pronounced effects in the temperature and gate voltage dependence of thermoelectric power associated with a competition between Fermi and non-Fermi liquid behaviors. High controllability of the device allows to fine tune the system to different regimes described by multichannel and multi-impurity Kondo models.

We investigate the spin-resolved transport properties, such as the linear conductance and the tunnel magnetoresistance, of a double quantum dot device attached to ferromagnetic leads and look for signatures of the $\mathrm{SU}\left(4\right)$ symmetry in the Kondo regime. We show that the transport behavior greatly depends on the magnetic configuration of the device, and the spin-SU(2) as well as the orbital and spin-SU(4) Kondo effects become generally suppressed when the magnetic configuration of the leads varies from the antiparallel to the parallel one. Furthermore, a finite spin polarization of the leads lifts the spin degeneracy and drives the system from the SU(4) to an orbital-SU(2) Kondo state. We analyze in detail the crossover and show that the Kondo temperature between the two fixed points has a nonmonotonic dependence on the degree of spin polarization of the leads. In terms of methods used, we characterize transport by using a combination of analytical and numerical renormalization group approaches.

We consider a periodically $\delta$-kicked Haldane type Chern insulator with the kicking applied in the $\widehat{z}$ direction. This is known to behave as an inversion symmetry breaking perturbation, since it introduces a time-dependent staggered sublattice potential. We study here the effects of such driving on the topological phase diagram of the original Haldane model of a Hall effect in the absence of a net magnetic field. The resultant Floquet band topology is again that of a Chern insulator with the driving parameters—frequency and amplitude— influencing the inversion breaking mass $M$ of the undriven Haldane model. A family of such periodically related “Semenoff masses” is observed to occur, which support a periodic repetition of Haldane like phase diagrams along the inversion breaking axis of the phase plots. Out of these it is possible to identify two in-equivalent masses in the reduced zone scheme of the Floquet quasienergies, which form the centers of two inequivalent phase diagrams. Further, variation in the driving amplitude's magnitude alone is shown to effect the topological properties by linearly shifting the phase diagram of the driven model about the position of the undriven case, a phenomenon that allows the study of Floquet topological phase transitions in the system. Finally, we also discuss some issues regarding the modifications to Haldane's condition for preventing band overlaps at the Dirac point touchings in the Brillouin zone, in the presence of kicking.