The properties of III-nitride quantum wells (QWs) are significantly affected by polarization fields. We show that, in the case of wide QWs, there exist two fundamentally different transition types characterized by quantum-confinement effects or bulklike behavior—depending on the carrier density in the structure. At low carrier density, spontaneous and piezoelectric polarization lead to tight confinement and low spatial overlap between the electron and hole wave functions. However, if the carrier density is sufficient for screening of the polarization fields, the wave functions spread out, reach high overlap, and the energy states come close to each other, similar to a bulk semiconductor. Streak camera measurements in the subthreshold regime of nitride laser diodes and numerical simulations show consistent results and let us clearly differentiate between contributions of individual states for low carrier density and the emergence of a high density of energetically close states at high carrier density.

We study the optical absorption at graphene edges in a perpendicular magnetic field and the subsequent charge carrier multiplication via inelastic Auger scattering. For the latter, we identify exact and approximate selection rules and give estimates for the generated edge current. This permits an understanding of the experimentally observed photoresponse in suspended graphene.

Maxwell lattices are characterized by an equal number of degrees of freedom and constraints. A subset of them, dubbed topological lattices, are capable of localizing stress and deformation on opposing edges, displaying a polarized mechanical response protected by the reciprocal-space topology of their band structure. In two dimensions (2D), the opportunities for topological polarization have been largely restricted to the kagome and square lattice benchmark configurations, due to the nontriviality of generating arbitrary geometries that abide by Maxwell conditions. In this Letter, we introduce a generalized family of augmented topological lattices that display full in-plane topological polarization. We explore the robustness of such polarization upon selection of different augmentation criteria, with special emphasis on augmented configurations that display dichotomous behavior with respect to their primitive counterparts. We corroborate our results via intuitive table-top experiments conducted on a lattice prototype assembled from 3D-printed mechanical links.

Recently, Wang et al. [Phys. Rev. B 106, 195129 (2022)] challenged a widely held belief in the field of Weyl physics, demonstrating that single-pair-Weyl points (SP-WPs) can exist in nonmagnetic spinless systems, contrary to previous assumptions that they could only exist in magnetic systems. Wang et al. observed that the SP-WPs with opposite and even chiral charges (i.e., $\left|\mathcal{C}\right|$ = 2 or 4) could also exist in nonmagnetic spinless systems. In this Letter, we present a finding in which SP-WPs have a partner, namely a charged nodal surface, in nonmagnetic spinless systems. In contrast to previous observations, we show that the SP-WPs can have uneven chiral charges (i.e., $\left|\mathcal{C}\right|$ = 1). We identify six (out of 230) space groups (SGs) that contain such SP-WPs by searching the encyclopedia of emergent particles in three-dimensional crystals. Our finds were confirmed through the phonon spectra of two specific materials ${\mathrm{Zr}}_3\mathrm{O}$ (with SG 182) and ${\mathrm{NaPH}}_3{\mathrm{NO}}_3$ (with SG 173). This discovery broadens the range of materials that can host SP-WPs and applies to other nonmagnetic spinless crystals.

Superfluidity in Fermi systems is not destroyed by a flow exceeding the Landau velocity threshold. The overcritical state acquires a normal component even at zero temperature. We explore the peculiar hydrodynamics of this system and discover two sound modes, for which the explicit universal dispersion relations are provided.

The Johari-Goldstein secondary (β) relaxations are intrinsic features of supercooled liquids and glasses, and they play a crucial role in several properties of glassy materials. However, their structural origin remains elusive. Prevailing theories consider them as localized activation or “islands of mobility.” In this study, we utilize atomistic simulations and percolation analysis to demonstrate that the mobile clusters are actually percolated at the occurrence of β relaxation, in seven different glassy systems. The percolation transition shows the same temperature and time dependence with β relaxation. Percolation emerges universally when about 10% of the atoms are mobile, and the dimension of the system-spanning cluster is approximately 2. Our findings add an essential piece to the full picture for the understanding of β relaxation in glasses.

Magnetic skyrmions are spin topological textures of potential interest in spintronics-related data storage and processing devices. Here, we show the emergence of unconventional skyrmions in a geometrically frustrated triangular lattice on an inversion-symmetry-breaking two-dimensional electron gas substrate. Starting with a classical double-exchange mechanism, this generic interface induces exotic skyrmionic and unique noncoplanar magnetic states not observed in the equivalent square lattice interface. We study the model by deriving an effective spin Hamiltonian. Large scale classical Monte Carlo simulations provide a quantitative evidence for the emergence of these exotic magnetic states. We found that these chiral magnetic states exhibit a substantial and nonzero topological Hall conductivity. As potential material candidates, we propose ${\text{Cr/MoS}}_2$, ${\text{Fe/MoS}}_2$, and ${\text{Fe/WSe}}_2$ interfaces because they have the requisite underlying triangular lattice structures and large spin-orbit coupling.

Superconductivity in transition metal nitrides (TMNs) has been investigated for a long time, such as zirconium nitride (ZrN) with a superconducting transition temperature $T_c$ of 10 K. Recently, a phase diagram has been revealed in ${\mathrm{ZrN}}_x$ with different nitrogen concentrations, which is very similar to that of high-temperature copper oxide superconductors. Here, we study the TMNs with a face-centered cubic lattice where ZrN and HfN have been experimentally obtained and predict eight new stable superconductors by the first-principles calculations. We find that CuN has a high $T_c$ of 30 K with a very strong electron-phonon coupling (EPC) strength. In contrast to ZrN, CuN has softening acoustic phonons at the high-symmetry point $L$, which accounts for its much stronger EPC. In addition, the highly symmetrical structure leads to topological nodal points and lines, such as the hourglass Weyl loop on the $k_{x/y/z}=0$ plane and Weyl points on the $k_{x/y/z}=2\pi /a$ plane, as well as quadratic band touch at the $\mathrm{\Gamma }$ point. CuN could be a topological superconductor. Our results expand the transition metal nitrides superconductor family and would be helpful to guide the search for high-temperature topological superconductors.

Many seemingly contradictory experimental findings concerning the superconducting state in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ can be accounted for on the basis of a conjectured accidental degeneracy between two patterns of pairing that are unrelated to each other under the $\left(D_{4h}\right)$ symmetry of the crystal: a $d_{x^2-y^2}$-wave $\left(B_{1g}\right)$ and a $g_{xy(x^2-y^2)}$-wave $\left(A_{2g}\right)$ superconducting state. In this paper, we propose a generic multiband model in which the $g$-wave pairing involving the $xz$ and $yz$ orbitals arises from second-nearest-neighbor BCS channel effective interactions. Even if time-reversal symmetry is broken in a $d+ig$ state, such a superconductor remains gapless with a Bogoliubov Fermi surface that approximates a (vertical) line node. The model gives rise to a strain-dependent splitting between the critical temperature $T_c$ and the time-reversal symmetry-breaking temperature $T_{\text{TRSB}}$ that is qualitatively similar to some of the experimental observations in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

We elucidate the role that the dissipation in a bosonic channel plays in the prevalence and stability of time crystals (TCs) in a periodically driven spin-boson system described by the Dicke model. Here, the bosons are represented by photons, and they mediate the infinite-range interactions between the spin systems. For strong dissipation, we study the dynamics using an effective atom-only description and the closed Lipkin-Meshkov-Glick model. By mapping out the phase diagrams for varying dissipation strengths, ranging from zero to infinitely strong, we demonstrate that the area in the phase diagram, where a TC exists, grows with the dissipation strength but only up to an optimal point, beyond which most of the TCs become unstable. We find TCs in both closed-system and dissipative regimes, but dissipative TCs are shown to be more robust against random noise in the drive and are only weakly affected by the choice of initial state. We present the finite-sized behavior and the scaling of the lifetime of the TCs with respect to the number of spins and the interaction strength within a fully quantum mechanical description.

“Strange” correlators provide a tool to detect topological phases arising in many-body models by computing the matrix elements of suitably defined two-point correlations between the states under investigation and trivial reference states. Their effectiveness depends on the choice of the adopted operators. In this paper, we give a systematic procedure for this choice, discussing the advantages of choosing operators using the bulk-boundary correspondence of the systems under scrutiny. Via the scaling exponents, we directly relate the algebraic decay of the strange correlators with the scaling dimensions of gapless edge modes operators. We begin our analysis with lattice models hosting symmetry-protected topological phases and we analyze the sums of the strange correlators, pointing out that integrating their moduli substantially reduces cancellations and finite-size effects. We also analyze instances of systems hosting intrinsic topological order, as well as strange correlators between states with different nontrivial topologies. Our results for both translational and nontranslational invariant cases, and in the presence of on-site disorder and long-range couplings, extend the validity of the strange correlator approach for the diagnosis of topological phases of matter and indicate a general procedure for their optimal choice.

The magnetotropic susceptibility is the thermodynamic coefficient associated with the rotational anisotropy of the free energy in an external magnetic field and is closely related to the magnetic susceptibility. It emerges naturally in frequency-shift measurements of oscillating mechanical cantilevers, which are becoming an increasingly important tool in the quantitative study of the thermodynamics of modern condensed-matter systems. Here we discuss the basic properties of the magnetotropic susceptibility as they relate to the experimental aspects of frequency-shift measurements, as well as to the interpretation of those experiments in terms of the intrinsic properties of the system under study.

${\left({\text{TaSe}}_4\right)}_3\text{I}$, a compound belonging to the family of quasi-one-dimensional transition metal tetrachalcogenides, has drawn significant attention due to a recent report on the possible coexistence of two antagonistic phenomena, superconductivity and magnetism below 2.5 K (Bera et al., arXiv:2111.14525). Here, we report a structural phase transition of the trimerized phase at temperature $T\simeq 145$ K using Raman scattering, specific heat, and electrical transport measurements. The temperature-dependent single-crystal x-ray diffraction experiments establish the phase transition from a high-temperature centrosymmetric to a low-temperature noncentrosymmetric structure, belonging to the same tetragonal crystal family. The first-principles calculations find the aforementioned inversion symmetry-breaking structural transition to be driven by the hybridization energy gain due to the off-centric movement of the Ta atoms, which wins over the elastic energy loss.

This study performed first-principles calculations based on the time-dependent density functional theory to control the valley degree of freedom relating to the dynamical Franz-Keldysh effect (DFKE) in a monolayer of transition metal dichalcogenide. By mimicking the attosecond transient absorption spectroscopy, we performed numerical pump-probe experiments to observe DFKE around the $K$ or $K^{\prime }$ valley in ${\mathrm{WSe}}_2$ monolayer with a linearly-polarized pump field and a circularly-polarized probe pulse. We found that the circularly-polarized probe pulse with a given helicity can selectively observe the transient conductivity modulated by DFKE in each valley. The transient conductivity and excitation probability around each valley oscillate with the pump field frequency $\mathrm{\Omega }$. The phases of the $\mathrm{\Omega }$ oscillation for the $K$ and $K^{\prime }$ valleys are opposite to each other. Furthermore, the pump-driven DFKE alters the absorption rate of ${\mathrm{WSe}}_2$ monolayer and yields the valley-dependent $\mathrm{\Omega }$ oscillation of the electron excitation induced by the pump plus probe field. With a simplified two-band model, we identified the $\mathrm{\Omega }$ oscillation of the off-diagonal conductivity caused by the band asymmetry around the valleys as the physical mechanism responsible for the valley-selective DFKE.

The issue of surface sensitivity and its relationship with the interpretation of spectral features observed in angle-resolved photoemission spectroscopy experiments is investigated. Rather than attempt to make an explicit connection to bulk electronic structure calculations, we take the approach of exploring this issue within the natural context of a vacuum-terminated crystalline slab. Doing so, we reconcile the empirical reality of reliable $k_z$ fidelity with acute surface sensitivity of this technique. In addition, we identify several critical issues which impact the estimation of $k_F$, band velocity, and self-energy from photoemission experiments.

Given the considerable theoretical challenges in understanding strongly coupled metals and non-Fermi liquids, it is valuable to have a framework to understand properties of metals that are universal, in the sense that they must hold in any metal. It has previously been argued that an infinite-dimensional emergent symmetry group is such a property, at least for clean, compressible metals. In this paper, we will show that such an emergent symmetry group has very strong implications for the dynamics of the metal. Specifically, we show that consideration of the hydrodynamics of the associated infinitely many emergent conserved quantities automatically recovers the collisionless Boltzmann equation that governs the dynamics of a Fermi liquid. Therefore the hydrodynamic prediction is that in the low-temperature, collisionless regime where the emergent conservation laws hold, the dynamics and response to external fields of a general spinless metal will be identical to a Fermi liquid. We discuss some potential limitations to this general statement, including the possibility of nonhydrodynamic modes. We also report some interesting differences in the case of spinful metals.

The effect of hydrogen doping on the crystal structure and the electronic state of ${\mathrm{SmNiO}}_3$ is investigated by means of density functional theory with a combinatorial structure-generation approach. While 100% of hydrogen doping per Ni atom has been supposed to be responsible for the experimentally observed insulating phase, we found that 50% of hydrogen doping results in an outstandingly stable atomic structure showing the insulating property after the structural relaxation was performed while imposing a ferromagnetic configuration. The stable crystal structure shows the peculiar layered pattern of charge disproportionation of ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Ni}}^{3+}$ valences together with the strong Jahn-Teller distortion that causes the $e_g$ orbital state splitting and opens the band gap.

We study how bias and tunneling asymmetries affect nonlinear current through a quantum dot with $N$ discrete levels in the Fermi liquid regime, using an exact low-energy expansion of the current derived up to terms of order $V^3$ with respect to the bias voltage. The expansion coefficients are described in terms of the phase shift, the linear susceptibilities, and the three-body correlation functions, defined with respect to the equilibrium ground state of the Anderson impurity model. In particular, the three-body correlations play an essential role in the order $V^3$ term, and their coupling to the nonlinear current depends crucially on the bias and tunnel asymmetries. The number of independent components of the three-body correlation functions increases with $N$ the internal degrees of the quantum dots, and it gives a variety in the low-energy transport. We calculate the correlation functions over a wide range of electron fillings of the Anderson impurity model with the $\mathrm{SU}\left(N\right)$ internal symmetry, using the numerical renormalization group. We find that the order $V^3$ nonlinear current through the $\mathrm{SU}\left(N\right)$ Kondo state, which occurs at electron fillings of 1 and $N-1$ for strong Coulomb interactions, significantly varies with the three-body contributions as tunnel asymmetries increase. Furthermore, in the valence fluctuation regime toward the empty or fully occupied impurity state, a sharp peak emerges in the coefficient of $V^3$ current in the case at which bias and tunneling asymmetries cooperatively enhance the charge transfer from one of the electrodes.

The band gap of most III-V semiconductors is strongly reduced with the introduction of only a few percent of N, even if the III-N alloy has a much bigger band gap. N impurities in InAs introduce an impurity state around 1 eV above the conduction-band minimum, much deeper in the band than in other III-V materials. Topographic scanning tunneling spectroscopy measurements (STS) and areal spectroscopy measurements performed on N atoms up to two layers below the (110) surface of InAs show a reduction of the resonance energy of the N atom with increasing depth. This is attributed to tip induced band bending, pulling the N states up at positive bias and acting most strongly on surface N atoms. STS measurements obtained on undoped InAs and N-doped InAs show a band-gap reduction of <0.1 eV. Spacial imaging of features corresponding to N dopants up to two layers below the surface are also compared to density functional theory simulations and show excellent correspondence. Spectroscopy maps of N atoms up to two layers below the surface provide a high-resolution spatial and spectroscopic view of the N atoms. Here the characteristic shape of the N atoms in different layers below the surface is observed as an enhancement of the $dI/dV$ signal compared to the InAs background. At energies above the enhancement a reduction of the $dI/dV$ is observed, which has the same shape and size as the enhancement. This shows that the redistribution of density of states caused by the N impurities is mainly energetic in nature.

Bogoliubov quasiparticles are a coherent electron-hole quantum superposition which typically, for time-reversal symmetric superconductors, exhibit a spectral distribution with particle-hole symmetry. Here, we demonstrate that in two-dimensional noncentrosymmetric superconductors with multiorbital spin-triplet pairing the energy profile of the density of states at the edge can violate this paradigm. We show that the structure of Andreev reflections generally leads to pairing states made of configurations that are orbitally split due to the low degree of crystalline symmetry at the edge. The resulting pairing state has a mixed parity character in the orbital sector that, in the presence of reduced crystal symmetry at the edge, sets out a particle-hole asymmetric profile for the spectral function. These findings indicate a path to design asymmetric spectral functions at the edge of superconductors with orbital degrees of freedom and time-reversal symmetry. The emerging signatures can be exploited for the detection of spin-triplet pairing equipped with internal degrees of freedom.