In the linear regime, transport properties of ballistic two-terminal devices are generally considered to be independent of the direction of the current. This two-terminal reciprocity applies to both the electron transmission and reflection probabilities. However, it does not apply to the scattering delays of wave packets. Indeed, four different time delays describe the transmission and reflection processes of wave packets arriving from the two terminals, respectively. Unlike the probabilities, these delays are direction dependent if the channel exchange symmetry of the scattering matrix is broken.

van der Waals (vdW) interaction plays a significant role in controlling the physical properties of layered materials. Typically, the vdW interlayer interaction can be calculated by density functional theory or experimentally characterized by quantum capacitance measurement. Here, we report the probing of the interlayer interaction in layered honeycomb materials via chiral phonons. Through helicity resolved Raman measurements, we observed a reduced chirality of the Raman $G$ mode with increasing layer numbers. We introduced interlayer coupling terms into the traditional Raman $G$ mode tensor to simulate the reduced phonon chirality in Raman spectra. Our Raman tensor calculation results agree with the experiments well, suggesting that the interlayer interaction can significantly influence the lattice vibration. Our demonstration provides a perspective for characterizing the interlayer interactions in vdW layered materials with honeycomb lattice structure.

The paradigm of topological insulators asserts that an energy gap separates conduction and valence bands with opposite topological invariants. Here, we propose that equal-energy bands with opposite Chern invariants can be spatially separated, onto opposite facets of a finite crystalline Hopf insulator. On a single facet, the number of Berry-curvature quanta is in one-to-one correspondence with the bulk homotopy invariant of the Hopf insulator; this originates from a bulk-to-boundary flow of Berry curvature which is not a type of Callan-Harvey anomaly inflow. In the continuum perspective, such nontrivial boundary states arise as nonchiral, Schrödinger-type modes on the domain wall of a generalized Weyl equation, describing a pair of opposite-chirality Weyl fermions acting as a dipolar source of Berry curvature. A rotation-invariant lattice regularization of the generalized Weyl equation manifests a generalized Thouless pump, which translates charge by one lattice period over half an adiabatic cycle, but reverses the charge flow over the next half.

The recently discovered cuprate superconductor ${\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{O}}_{3+\delta }$ exhibits a high $T_c\simeq 73\mathrm{K}$ at $\delta \simeq 0.2$. The polycrystal grown under high pressure has a structure similar to ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_4$ but with dramatically different lattice parameters due to the $\mathrm{Cu}{\mathrm{O}}_6$ octahedron compression. The crystal field in the compressed ${\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{O}}_4$ leads to an inverted Cu $3d{e}_g$ complex with the $d_{x^2-y^2}$ orbital sitting below the $d_{3z^2-r^2}$ and an electronic structure highly unusual compared to the conventional cuprates. We construct a two-orbital Hubbard model for the Cu $d^9$ state at hole doping $x=2\delta$ and study the orbital-dependent strong correlation and superconductivity. For the undoped case at $x=0$, we found that strong correlation drives an orbital-polarized Mott-insulating state with the spin-$1/2$ moment of the localized $d_{3z^2-r^2}$ orbital. In contrast to the single-band cuprates where superconductivity is suppressed in the overdoped regime, hole doping the two-orbital Mott insulator leads to orbital-dependent correlations and the robust spin and orbital exchange interactions produce a high-$T_c$ antiphase $d$-wave superconductor even in the heavily doped regime at $x=0.4$. We conjecture that ${\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{O}}_{3+\delta }$ realizes mixtures of such heavily hole-doped superconducting ${\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{O}}_4$ and disordered ${\mathrm{Ba}}_2\mathrm{Cu}{\mathrm{O}}_3$ chains in a single-layer or predominately separated bilayer structure. Our findings suggest that unconventional cuprates with liberated orbitals as doped two-band Mott insulators can be a direction for realizing high-$T_c$ superconductivity with enhanced transition temperature $T_c$.

We present a study on magnetotransport in films of the topological Dirac semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ doped with Sb grown by molecular beam epitaxy. In our weak antilocalization analysis, we find a significant enhancement of the spin-orbit scattering rate, indicating that Sb doping leads to a strong increase of the pristine band-inversion energy. We discuss possible origins of this large enhancement by comparing Sb-doped ${\mathrm{Cd}}_3{\mathrm{As}}_2$ with other compound semiconductors. Sb-doped ${\mathrm{Cd}}_3{\mathrm{As}}_2$ will be a suitable system for further investigations and functionalization of topological Dirac semimetals.

We perform a theoretical study on the spatiotemporal correlation of two-dimensional (2D) nonequilibrium exciton-polariton (EP) systems driven by incoherent pumping. Based on the stochastic generalized Gross-Pitaevskii equation, we calculate the density fluctuation, coherence function, and scaling function. It is found that the correlations of short range agree well with the Bogoliubov linear theory, justifying the existence of an algebraic order. At large distances, both dynamic and static correlations are characterized by the nonlinear scaling behaviors of the Kardar-Parisi-Zhang universality class, especially when the interaction is weak. The coexistence of different types of correlations with their respective length scales is a distinctive feature of 2D EP systems. In addition, the interplay between vortices is modified in the regime of weak interaction and strong pumping, leading to complex nonequilibrium vortex patterns.

We have carried out a comprehensive investigation of the quasiparticle properties of a two-dimensional electron gas, interacting via the long-range Coulomb interaction in the presence of bare mass anisotropy (i.e., with an elliptic noninteracting Fermi surface) by calculating the self-energy, the spectral function, the scattering rate, and the effective mass within the leading-order dynamical self-energy approximation. Our theory is exact in the high-density limit. We find anisotropic features of quasiparticle properties that are not captured by the commonly used isotropic approximation where the anisotropic effective mass is replaced by the isotropic averaged density-of-states mass. Some of these interesting results are as follows: (1) The many-body renormalization of the quasiparticle spectrum becomes highly anisotropic as the quasiparticle energy increases away from the Fermi energy; (2) the interaction-induced inelastic-scattering rate features a strong anisotropy, exhibiting an abrupt jump at different injected energies depending on the momentum direction of the injected electron; (3) the effective-mass enhancement is larger (smaller) for the light (heavy) mass, showing that the anisotropy is reduced by interactions. Our results and analysis show that the unjustified neglect of the mass anisotropy can lead to an incorrect description of quasiparticle properties of the anisotropic system and inaccurate estimates of physical quantities of interest although the use of an equivalent isotropic approximation using the density-of-states effective mass as is commonly and uncritically performed in the literature, works as a reasonable approximation in many situations. In addition to the complete random phase approximation theory for the anisotropic quasiparticles, we also provide a theory using the simpler plasmon-pole approximation, commenting on its validity for anisotropic self-energy calculations. We comment also on the interaction effect on the Fermi-surface topology, finding that the elliptic shape of the bare Fermi surface is preserved with suppressed ellipticity in the interacting system to a high degree of accuracy except in the very strongly interacting limit (and for very high bare mass anisotropy). Our theory provides a complete generalization of the existing isotropic many-body theory of interacting electrons to the corresponding anisotropic systems.

The magnetic ratchet effect has been studied in phosphorene by the use of the Boltzmann kinetic equation, which is a semiclassical approach. The Hamiltonian of phosphorene in a steady parallel magnetic field is derived using the tight-binding model. We consider the effect of the magnetic field on nonlinear dynamics in the presence of an ac laser field and spatial inversion asymmetry. We have shown that for anisotropic two-dimensional materials and phosphorene, the ratchet current has a response to three different light polarizations: linearly polarized light, circularly polarized light, and unpolarized light.

The surface excitation spectra of the Pd(110) surface is analyzed in the framework of the time-dependent density functional theory. The ab initio electronic structure of this surface is included into the evaluation of the surface response function using a linear response approach. At small momentum transfer the extrapolated energy of the surface plasmon is close to electron-energy-loss-spectroscopy measurements and can be understood considering the bulk Pd dielectric function. However, upon momentum increase the evaluated dispersion of the surface plasmon presents significant discrepancies with the published experimental data. While the experimental surface plasmon dispersion initially presents a strong negative slope, our calculations indicate a weak positive dispersion at all finite momentum transfers where this collective excitation is observed. Interestingly, we do not observe any noticeable effect on the collective surface excitations induced by the large number of surface states in the valence band of Pd(110). These states do not produce any additional mode at energies below the surface plasmon.

We report a detailed investigation of the ${\mathrm{Ni}}_2\mathrm{Mn}\mathrm{Ga}$ shape memory alloy through magnetic, electronic, and thermal measurements. Our measurements of the anomalous Nernst effect (ANE) reveal that this technique is very sensitive to the onset of the premartensitic transition in sharp contrast to other transport measurements. With the ANE being sensitive to changes at the Fermi surface, we infer on the link between the structural modulations and the modulation of the Fermi surface via its nesting features, with the magnetic field induced strain being the mediating mechanism.

Manganese, in the $\alpha$-Mn structure, has been studied using synchrotron powder x-ray diffraction in a diamond anvil cell up to 220 GPa at room temperature combined with density functional calculations. The experiment reveals an extended pressure stability of the $\alpha$-Mn phase up to the highest pressure of this study, in contrast to previous experimental and theoretical studies. On the other hand, calculations reveal that the previously predicted hcp Mn phase becomes lower in enthalpy than the $\alpha$-Mn phase above 160 GPa. The apparent discrepancy is explained due to a substantial electron transfer between Mn ions, which stabilizes the $\alpha$-Mn phase through the formation of ionic bonding between monatomic ions under pressure.

Micromagnetic instabilities and nonuniform magnetization states play a significant role in spin-transfer induced switching of nanometer scale magnetic elements. Here we model domain wall mediated switching dynamics in perpendicularly magnetized magnetic tunnel junction nanopillars. We show that domain wall surface tension always leads to magnetization oscillations and instabilities associated with the disk shape of the junction. A collective coordinate model is developed that captures aspects of these instabilities and illustrates their physical origin. Model results are compared to those of micromagnetic simulations. The switching dynamics are found to be very sensitive to the domain wall position and phase, which characterizes the angle of the magnetization in the disk plane. This sensitivity is reduced in the presence of spin torques, and the spin current needed to displace a domain wall can be far less than the threshold current for switching from a uniformly magnetized state. A prediction of this model is conductance oscillations of increasing frequency during the switching process.

Due to the potential applications in low-power-consumption spintronic devices, the quantum anomalous Hall effect (QAHE) has attracted tremendous attention in past decades. However, up to now, the QAHE was only observed experimentally in topological insulators with Chern numbers $C=1$ and 2 at very low temperatures. Here, we propose three two-dimensional stable kagome ferromagnets ${\mathrm{Co}}_3{\mathrm{Pb}}_3{\mathrm{S}}_2, {\mathrm{Co}}_3{\mathrm{Pb}}_3{\mathrm{Se}}_2$, and ${\mathrm{Co}}_3{\mathrm{Sn}}_3{\mathrm{Se}}_2$ that can realize the QAHE with high Chern number of $\left|C\right|=3$. Monolayers ${\mathrm{Co}}_3{\mathrm{Pb}}_3{\mathrm{S}}_2, {\mathrm{Co}}_3{\mathrm{Pb}}_3{\mathrm{Se}}_2$, and ${\mathrm{Co}}_3{\mathrm{Sn}}_3{\mathrm{Se}}_2$ possess the large band gap of 70, 77, and 63 meV with Curie temperature $T_C$ of 51, 42, and 46 K, respectively. By constructing a heterostructure ${\mathrm{Co}}_3{\mathrm{Sn}}_3{\mathrm{Se}}_2/{\mathrm{MoS}}_2$, its $T_C$ is enhanced to 60 K and the band gap remains at about 60 meV due to the tensile strain of 2% at the interface. For the bilayer compound ${\mathrm{Co}}_6{\mathrm{Sn}}_5{\mathrm{Se}}_4$, it becomes a half metal, with a relatively flat plateau in its anomalous Hall conductivity corresponding to $\left|C\right|=3$ near the Fermi level. Our results provide topological nontrivial systems of kagome ferromagnetic monolayers and heterostructures possessing the QAHE with high Chern number $\left|C\right|=3$ and large band gaps.

M. Tinkham [Introduction to Superconductivity, 2nd ed. (Dover, Mineola, NY, 2004), Chap. 4, pp. 135–138] and P. G. de Gennes [Superconductivity of Metals and Alloys (Benjamin, New York, 1966), Chap. 6, pp. 199–201] described the existence of an intermediate type-I superconductor as a consequence of an external surface that affects the well-known classification of superconductors into type I and II. Here we consider the mesoscopic superconductor where the volume-to-area ratio is small and the effects of the external surface are enhanced. By means of the standard Ginzburg-Landau theory, the Tinkham–de Gennes scenario is extended to the mesoscopic type-I superconductor. We find additional features of the transition at the passage from the genuine to the intermediate type I. The latter has two distinct transitions, namely from a paramagnetic to diamagnetic response in descending field, and a quasi-type-II behavior as the critical coupling $1/\sqrt{2}$ is approached in ascending field. The intermediate type-I phase proposed here, and its corresponding transitions, reflect intrinsic features of the superconductor and not its geometrical properties.

We carry out a theoretical investigation of overpressurized superfluid phases of ${}^4\mathrm{He}$ by means of quantum Monte Carlo (QMC) simulations. As a function of density, we study structural and superfluid properties, and we estimate the energy of the roton excitation by inverting imaginary-time density correlation functions computed by QMC, using maximum entropy. We estimate the pressure at which the roton energy vanishes to be about 100 bars, which we identify with the spinodal density, i.e., the upper limit for the existence of a metastable superfluid phase.

We use record dynamics (RD), a coarse-grained description of the ubiquitous relaxation phenomenology known as “aging,” as a diagnostic tool to find universal features that distinguish between the energy landscapes of Ising spin models and the ferromagnet. According to RD, a nonequilibrium system after a quench relies on fluctuations that randomly generate a sequence of irreversible record-sized events (quakes or avalanches) that allow the system to escape ever-higher barriers of metastable states within a complex, hierarchical energy landscape. Once these record events allow the system to overcome such barriers, the system relaxes by tumbling into the following metastable state that is marginally more stable. Within this framework, a clear distinction can be drawn between the coarsening dynamics of an Ising ferromagnet and the aging of the spin glass, which are often put in the same category. To that end, we interpolate between the spin glass and ferromagnet by varying the admixture $p$ of ferromagnetic over antiferromagnetic bonds from the glassy state (at 50% each) to wherever clear ferromagnetic behavior emerges. The accumulation of record events grows logarithmically with time in the glassy regime, with a sharp transition at a specific admixture into the ferromagnetic regime where such activations saturate quickly. We show this effect for both the Edwards-Anderson model on a cubic lattice as well as the Sherrington-Kirkpatrick (mean-field) spin glass. While this transition coincides with a previously observed zero-temperature equilibrium transition in the former, that transition has not yet been described for the latter.

By combining two independent approaches, inelastic neutron-scattering measurements and density-functional-theory calculations, we study the spin waves in the collinear antiferromagnetic phase (AFM2) of ${\mathrm{Mn}}_5{\mathrm{Si}}_3$. We obtain its magnetic ground-state properties and electronic structure. This study allowed us to determine the dominant magnetic exchange interactions and magnetocrystalline anisotropy in the AFM2 phase of ${\mathrm{Mn}}_5{\mathrm{Si}}_3$. Moreover, the evolution of the spin excitation spectrum is investigated under the influence of an external magnetic field perpendicular to the anisotropy easy axis. The low-energy magnon modes show a different magnetic field dependence, which is a direct consequence of their different precessional nature. Finally, possible effects related to the Dzyaloshinskii-Moriya interaction are also considered.

We studied the magnetism in ${\mathrm{NdFe}}_2{\mathrm{Ga}}_8$ by magnetic susceptibility, specific heat, and resistivity measurements, which show ${\mathrm{Nd}}^{3+}$ ions forming one-dimensional chains along the $c$ axis. At zero field, two magnetic transitions are found at about 15 and 11 K. For field vertical to the $c$ axis, no significant effects on the magnetic orders are found. On the other hand, the two transitions are merged together at about 4 T for field parallel to the $c$ axis and completely suppressed at about 7 T. The suppression of the magnetic order results in quantum critical behaviors of the conventional three-dimensional spin-density-wave type, such as $\rho \sim T^{1.5}$ and $C/T\sim T^{-0.5}$. However, the temperature dependence of the resistivity is replaced by a $T^{0.5}$ behavior at very low temperatures for field larger than 3 T, which is attributed to the field-induced ferromagnetic moment. Our results suggest that there may be couplings between the antiferromagnetic quantum critical fluctuations and the field-induced ferromagnetic fluctuations.

We obtain the steady-state phase diagram of a transverse-field XY spin chain coupled at its ends to magnetic reservoirs held at different magnetic potentials. In the long-time limit, the magnetization bias across the system generates a current-carrying nonequilibrium steady state. We characterize the different nonequilibrium phases as functions of the chain's parameters and magnetic potentials, in terms of their correlation functions and entanglement content. The mixed-order transition, previously observed for the case of a transverse-field Ising chain, is established to emerge as a generic feature of a wider class of out-of-equilibrium problems. The critical exponents associated with this universality class are determined analytically. Results are also contrasted with those obtained in the limit of Markovian reservoirs. Our findings should prove helpful in establishing the properties of nonequilibrium phases and phase transitions of extended open quantum systems.

High-power and broad-tunable terahertz wave generating source on chip, which has broad application prospects, remains an unachieved goal of researchers after decades of pursuing. Here, we propose a concept of terahertz laser diode, which is a dc-biased self-excited microelectronic laser oscillator using collectively modulated free electrons emitted by field-emitter-arrays as the active medium and using an inherent Fabry-Pérot interferometer formed by port reflections as the frequency selector. It is sub-millimeter to centimeter in size and can generate continuous-wave coherent terahertz radiation with several watts of power per square millimeter. Its frequency can be tuned to cover a wide terahertz band by changing the cathode-to-anode distance and the bias voltage. It has the potential to outperform the conventional free-electron or solid-state terahertz devices in terms of portability, tunability, and output power, signifying a promising class of tunable and high-power terahertz source on chip.