We report on nonreciprocity observations in several configurations of graphene-based quantum Hall devices. Two distinct measurement configurations were adopted to verify the universality of the observations (i.e., two-terminal arrays and four-terminal devices). Our findings determine the extent to which epitaxial graphene anisotropies contribute to the observed asymmetric Hall responses. The presence of backscattering induces a device-dependent asymmetry rendering the Onsager-Casimir relations limited in their capacity to describe the behavior of such devices, except in the low-field classical regime and the fully quantized Hall state. The improved understanding of this quantum electrical process broadly limits the applicability of the reciprocity principle in the presence of quantum phase transitions and for anisotropic two-dimensional materials.

The interaction between a central qubit spin and a surrounding bath of spins is critical to spin-based solid-state quantum sensing and quantum information processing. Spin-bath interactions are typically strongly anisotropic, and rapid physical rotation has long been used in solid-state nuclear magnetic resonance to simulate motional averaging of anisotropic interactions, such as dipolar coupling between nuclear spins. Here, we show that the interaction between electron spins of nitrogen-vacancy centers and a bath of ${}^{13}\mathrm{C}$ nuclear spins in a diamond rotated at up to 300 000 rpm introduces decoherence into the system via frequency modulation of the nuclear spin Larmor precession. The presence of an off-axis magnetic field necessary for averaging of the dipolar coupling leads to a rotational dependence of the electron-nuclear hyperfine interaction, which cannot be averaged out with experimentally achievable rotation speeds. Our findings offer new insights into the use of physical rotation for quantum control with implications for quantum systems having motional and rotational degrees of freedom that are not fixed.

We build and probe a ${\mathbb{Z}}_2$ spin liquid in a programmable quantum device, the D-Wave DW-2000Q. Specifically, we observe the classical eight-vertex and six-vertex (spin ice) states and transitions between them. To realize this state of matter, we design a Hamiltonian with combinatorial gauge symmetry using only pairwise-qubit interactions and a transverse field, i.e., interactions which are accessible in this quantum device. The combinatorial gauge symmetry remains exact along the full quantum annealing path, landing the system onto the classical eight-vertex model at the endpoint of the path. The output configurations from the device allow us to directly observe the loop structure of the classical model. Moreover, we deform the Hamiltonian so as to vary the weights of the eight vertices and show that we can selectively attain the classical six-vertex (ice) model, or drive the system into a ferromagnetic state. We present studies of the classical phase diagram of the system as a function of the eight-vertex deformations and effective temperature, which we control by varying the relative strengths of the programmable couplings, and we show that the experimental results are consistent with theoretical analysis. Finally, we identify additional capabilities that, if added to these devices, would allow us to realize ${\mathbb{Z}}_2$ quantum spin liquids on which to build topological qubits.

We present x-ray spectral evidence for the evolution of valence- and spin-specific tetragonal distortions in single-layer cobaltates. Measurements of Co $L_3$-edge resonant inelastic x-ray scattering reveal the $t_{2g}$ electronic structure of Co in hole-doped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_4$ ($x=0.5$, 0.7, and 0.8). As the Sr-doping $x$ increases, the tetragonal splitting of the $t_{2g}$ states of high-spin ${\mathrm{Co}}^{2+}$ decreases, whereas that of low-spin ${\mathrm{Co}}^{3+}$ increases and the population fraction of high-spin ${\mathrm{Co}}^{3+}$ increases. The results enable us to clarify the origin of the change of magnetic anisotropy and in-plane resistivity in the mixed-valence cobaltates caused by the interplay of spin-orbit coupling and tetragonal distortion.

The mechanism behind the three-spin chiral interaction (TCI) included in the extended Heisenberg Hamiltonian and represented by an expression worked out recently [Phys. Rev. B 101, 174401 (2020)] is discussed. It is stressed that this approach provides a unique set of the multispin exchange parameters that are independent of each other either due to their different order of perturbation or due to different symmetry. This ensures in particular the specific properties of the TCI that were demonstrated previously via fully relativistic first-principles calculations, and that result from the common influence of several issues not explicitly seen from the expression for the TCI parameters. Therefore, an interpretation of the TCI is suggested, showing explicitly its dependence on the relativistic spin-orbit coupling and on the topological orbital susceptibility (TOS). This is based on an expression for the TOS that is worked out on the same footing as the expression for the TCI. Using first-principles calculations, we demonstrate in addition numerically the common topological properties of the TCI and TOS. To demonstrate the role of the relativistic spin-orbit coupling (SOC) for the TCI, a so-called “topological” spin susceptibility is introduced. This quantity characterizes the SOC-induced spin magnetic moment on the atom in the presence of noncollinear magnetic structure, giving a connection between the TOS and TCI. Numerical results again support our conclusions.

We consider the superconductor-semiconductor nanowire hybrid Majorana platform (“Majorana nanowire”) in the presence of a deterministic spatially slowly varying inhomogeneous chemical potential and a random spatial quenched potential disorder, both of which are known to produce nontopological almost-zero-energy modes mimicking the theoretically predicted topological Majorana zero modes. We study the crossover among these mechanisms by calculating the tunnel conductance while varying the relative strength between inhomogeneous potential and random disorder in a controlled manner. We find that the entire crossover region manifests abundant trivial zero modes, many of which showing the apparent “quantization” of the zero-bias conductance peak at $2e^2/h$, with occasional disorder-dominated peaks exceeding $2e^2/h$. We present animations of the simulated crossover behavior and discuss experimental implications. Additionally, in order to simulate the realistic disorder in experimental nanowires, we also study in depth the case of disorder arising from random individual static impurities along the wire, and consider crossover associated with such impurity effects. Our results, when compared qualitatively with existing Majorana nanowire experimental results, indicate the dominant role of random disorder in the experiments. It turns out that all three mechanisms may produce trivial zero-bias peaks in the tunnel conductance, and the crossover among these physical mechanisms (i.e., when more than one mechanism is present in the system) is smooth and continuous, making it difficult a priori to conclude which mechanism is dominant in a particular sample just by a casual inspection of the zero-bias conductance peaks.

We show that a collinear Heisenberg antiferromagnet, whose sublattice symmetry is broken at the Hamiltonian level, becomes a fluctuation-induced ferrimagnet at any finite temperature $T$ below the Néel temperature $T_{\mathrm{N}}$. We demonstrate this using a layered variant of a square-lattice $J_1\text{−}{J}_2$ model. Linear spin-wave theory is used to determine the low-temperature behavior of the uniform magnetization, and nonlinear corrections are argued to yield a temperature-induced qualitative change of the magnon spectrum. We then consider a layered Shastry-Sutherland model, describing a frustrated arrangement of orthogonal dimers. This model displays an antiferromagnetic phase for large intradimer couplings. A lattice distortion which breaks the glide symmetry between the two types of dimers corresponds to broken sublattice symmetry and hence gives rise to ferrimagnetism. Given indications that such a distortion is present in the material $\mathrm{Sr}{\mathrm{Cu}}_2{\left(\mathrm{B}{\mathrm{O}}_3\right)}_2$ under hydrostatic pressure, we suggest the existence of a fluctuation-induced ferrimagnetic phase in pressurized $\mathrm{Sr}{\mathrm{Cu}}_2{\left(\mathrm{B}{\mathrm{O}}_3\right)}_2$. We predict a nonmonotonic behavior of the uniform magnetization as function of temperature.

We study the parity-symmetry-breaking quantum phase transition (QPT) in a cavity magnonic system driven by a parametric field, where the magnons in a ferrimagnetic yttrium-iron-garnet sphere strongly couple to a microwave cavity. With appropriate parameters, this cavity magnonic system can exhibit a rich phase diagram, including the parity-symmetric phase, parity-symmetry-broken phase, and bistable phase. When increasing the drive strength beyond a critical threshold, the cavity magnonic system undergoes either a first- or second-order nonequilibrium QPT from the parity-symmetric phase with microscopic excitations to the parity-symmetry-broken phase with macroscopic excitations, depending on the parameters of the system. Our work provides an alternate way to engineer the QPT in a hybrid quantum system containing the spin ensemble in a ferri- or ferromagnetic material with strong exchange interactions.

We investigate the conduction-band structure and electron mobility in rocksalt ScN based on density functional theory. The first-principles band structure allows us to obtain band velocities and effective masses as a function of energy. Electron-phonon scattering is assessed by explicitly computing the $q$-dependent electron-phonon matrix elements, with the inclusion of the long-range electrostatic interaction. The influence of free-carrier screening on the electron transport is assessed using the random-phase approximation. We find a notable enhancement of electron mobility when the carrier concentration exceeds ${10}^{20}{\mathrm{cm}}^{-3}$. We calculate the room-temperature electron mobility in ScN to be 587 ${\mathrm{cm}}^2/\mathrm{Vs}$ at low carrier concentrations. When the carrier concentration is increased, the electron mobility starts to decrease significantly around $n={10}^{19}{\mathrm{cm}}^{-3}$ and drops to 240 ${\mathrm{cm}}^2/\mathrm{Vs}$ at $n={10}^{21}{\mathrm{cm}}^{-3}$. We also explore the influence of strain in (111)- and (100)-oriented ScN films. For (111) films, we find that a 1.0% compressive epitaxial strain increases the in-plane mobility by 72 ${\mathrm{cm}}^2/\mathrm{Vs}$ and the out-of-plane mobility by 50 ${\mathrm{cm}}^2/\mathrm{Vs}$. For (100) films, a 1.0% compressive epitaxial strain increases the out-of-plane mobility by as much as 172 ${\mathrm{cm}}^2/\mathrm{Vs}$, but has a weak impact on the in-plane mobility. Our study sheds light on electron transport in ScN at different electron concentrations and shows how strain engineering could increase the electron mobility.

We theoretically investigate Majorana zero modes emerging in an anisotropic XY spin chain with second-neighbor interactions. The spin chain is mathematically equivalent to the Kitaev chain comprised of spinless fermions if only nearest-neighbor interactions are considered. Recent studies on the Kitaev chain with long-ranged interactions, have presented coexistence of several Majorana zero modes. Investigating the topological phase diagram of the anisotropic XY spin chain with second-neighbor interactions, we find coexistence of several Majorana zero modes similar to the Kitaev chain. However, we confirm that one of the zero modes is restricted into a Hilbert subspace. The mode is regarded as a so-called weak zero mode that should exhibit thermodynamical properties different from a (strong) zero mode appearing in the whole Hilbert space. Since the quantum statistics of spin is the same as a hard-core boson, we expect an experimental realization of the weak mode in not only quantum spin materials but also optical lattices for cold atom systems.

Graphene bilayers with layer antisymmetric strains are studied using the Dirac-Harper model for a pair of single-layer Dirac Hamiltonians coupled by a one-dimensional moiré-periodic interlayer tunneling amplitude. This model hosts low-energy, nearly dispersionless bands near charge neutrality that support anomalous polarizations of its charge multipole distributions. These are analyzed introducing a generalized Berry curvature that encodes the field-induced dynamics of multipole fields allowed in a chiral medium with time-reversal symmetry. The formulation identifies a reciprocity relation between responses to layer-symmetric and layer-antisymmetric in-plane electric fields and reveals momentum-space quantum oscillations produced by a spatial pattern of band inversions on the moiré scale.

Fermi-arc surface states are the hallmark of Weyl semimetals, whose identification is usually challenged by their coexistence with gapless bulk states. Surface transport measurements by fabricating setups on the sample boundary provide a natural solution to this problem. Here, we study the Andreev reflection (AR) in a planar normal metal-superconductor junction on the Weyl semimetal surface with a pair of Fermi arcs. For a conserved transverse momentum, the occurrence of normal reflection depends on the relative orientation between the Fermi arcs and the normal of the junction, which is a direct result of the disconnected Fermi arcs. Consequently, a crossover from the suppressed to perfect AR occurs with varying the orientation of the planar junction, giving rise to a change from double-peak to plateau structure in conductance spectra. Moreover, such a crossover can be facilitated by imposing a magnetic field, making electrons slide along the Fermi arcs so as to switch between two regimes of the AR. Our results provide a decisive signature for the detection of Fermi arcs and open the possibilities of exploring novel phenomenology through their interplay with superconductivity.

Two-dimensional materials with valley-related multiple Hall effect are both fundamentally intriguing and practically appealing for exploring novel phenomena and applications, but have been largely overlooked to date. Here, using first-principles calculations, we show that the valley-related multiple Hall effect can exist in single-layer $\mathrm{V}{\mathrm{Si}}_2{\mathrm{P}}_4$. We identify single-layer $\mathrm{V}{\mathrm{Si}}_2{\mathrm{P}}_4$ as a ferromagnetic semiconductor with out of plane magnetization and valley physics. Arising from the joint effect of inversion symmetry breaking and time-reversal symmetry breaking, the exotic spontaneous valley polarization occurs in single-layer $\mathrm{V}{\mathrm{Si}}_2{\mathrm{P}}_4$, thus facilitating the observation of anomalous valley Hall effect. Moreover, under external strain, band inversion can occur at only one of the valleys of single-layer $\mathrm{V}{\mathrm{Si}}_2{\mathrm{P}}_4$, enabling the long-sought valley-polarized quantum anomalous Hall effect, and meanwhile the anomalous valley Hall effect is well preserved. Our work not only enriches the research on valley-related multiple Hall effect, but also provides an ideal platform for exploring valley-polarized quantum anomalous Hall effect.

We propose that Weyl triplons are expected to appear in the low energy magnetic excitations in the canonical Shastry-Sutherland compound, ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$, a quasi-2D quantum magnet. Our results show that, when a minimal, realistic interlayer coupling is added to the well-established microscopic model describing the excitation spectrum of the individual layers, the Dirac points that appear in the zero-field triplon spectrum of the 2D model split into two pairs of Weyl points along the $k_z$ direction. Varying the strength of the interlayer DM interaction and applying a small longitudinal magnetic field results in a range of band-topological transitions accompanied by changing numbers of Weyl points. We propose inelastic neutron scattering along with thermal Hall effect as the experimental techniques to detect the presence of Weyl node in the triplon spectrum of this material. We show that the logarithmic divergence in the second derivative in thermal Hall conductance near phase transition from regime Weyl points to a regime with topologically gapped bands as well as a finite slope in the thermal Hall conductance as a function of magnetic field at zero magnetic field are promising evidence for the presence of Weyl triplons.

We numerically study crossed Andreev reflection (CAR) in a topological insulator nanowire T junction where one lead is proximitized by a superconductor. We perform realistic simulations based on the three-dimensional (3D) Bernevig-Hughes-Zhang model and compare the results with those from an effective two-dimensional (2D) surface model, whose computational cost is much lower. Both approaches show that CAR should be clearly observable in a wide parameter range, including perfect CAR in a somewhat more restricted range. Furthermore, it can be controlled by a magnetic field and is robust to disorder. Our effective 2D implementation allows us to model systems of micron size, typical of experimental setups but computationally too heavy for 3D models.

The signatures of Majorana zero-energy mode leaking into a spin-charge entangled double quantum dot are investigated theoretically in the strong electron correlation regime. The considered setup consists of two capacitively coupled quantum dots attached to external contacts and side-attached to topological superconducting wire hosting Majorana quasiparticles. We show that the presence of Majorana mode gives rise to unique features in the local density of states in the $\text{SU}\left(4\right)$ Kondo regime. Moreover, it greatly modifies the gate voltage dependence of the linear conductance, leading to fractional values of the conductance. We also analyze the effect of a finite length of topological wire and demonstrate that nonzero overlap of Majorana modes at the ends of the wire is revealed in local extrema present in the local density of states of the dot coupled directly to the wire. The calculations are performed with the aid of the numerical renormalization group method.

Floquet engineering offers a path to optically controlled materials, but experimental implementations have frequently relied on femtosecond pulses to achieve the high peak fields required to maximize light-matter interactions and obtain a measurable response. The Floquet formalism, however, is based on a continuous, periodic drive. Here we use femtosecond laser pulses to drive the optical Stark effect, a simple realization of Floquet engineering, in monolayer $\mathrm{W}{\mathrm{S}}_2$. By monitoring the coherent evolution of the free-induction decay, we show that the system evolves adiabatically, and that the finite duration of the pulses does not introduce any effects beyond Floquet theory. Furthermore, we demonstrate that the induced energy shift follows a linear dependence on instantaneous intensity, even for ultrafast driving fields of fewer than 15 optical cycles.

Pairing symmetry which characterizes the superconducting pairing mechanism is normally determined by measuring the superconducting gap structure ($|{\mathrm{\Delta }}_k|$). Here, we report the measurement of a strain-induced gap modulation ($\partial |{\mathrm{\Delta }}_k|$) in uniaxially strained ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ utilizing angle-resolved photoemission spectroscopy and in situ strain tuning. We found that the uniaxial strain drives ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_2{\mathrm{As}}_2$ into a nematic superconducting state which breaks the fourfold rotational symmetry of the superconducting pairing. The superconducting gap increases on the $d_{yz}$ electron and hole pockets while it decreases on the $d_{xz}$ counterparts. Such orbital selectivity indicates that orbital-selective pairing exists intrinsically in non-nematic iron-based superconductors. The $d_{xz}$ and $d_{yz}$ pairing channels are balanced originally in the pristine superconducting state, but become imbalanced under uniaxial strain. Our results highlight the important role of intraorbital scattering in mediating the superconducting pairing in iron-based superconductors. It also highlights the measurement of $\partial |{\mathrm{\Delta }}_k|$ as an effective way to characterize the superconducting pairing from a perturbation perspective.