We have successfully synthesized single crystals of ${\mathrm{EuNi}}_5{\mathrm{As}}_3$ using a flux method, and we present a comprehensive study of the physical properties using magnetic susceptibility, specific heat, electrical resistivity, thermoelectric power, and x-ray absorption spectroscopy (XAS) measurements. ${\mathrm{EuNi}}_5{\mathrm{As}}_3$ undergoes two close antiferromagnetic transitions at respective temperatures of $T_{N1}=7.2$ K and $T_{N2}=6.4$ K, which are associated with the ${\mathrm{Eu}}^{2+}$ moments. Both transitions are suppressed upon applying a field, and we map the temperature-field phase diagrams for fields applied parallel and perpendicular to the easy $a$ axis. XAS measurements reveal that the Eu is strongly divalent, with very little temperature dependence, indicating the localized ${\mathrm{Eu}}^{2+}$ nature of ${\mathrm{EuNi}}_5{\mathrm{As}}_3$, with a lack of evidence for heavy-fermion behavior.

The concept of temperature in nonequilibrium thermodynamics is an outstanding theoretical issue. We propose an energy repartition principle that leads to a spectral (mode-dependent) temperature in steady-state nonequilibrium systems. The general concepts are illustrated by analytic solutions of the classical Heisenberg spin chain connected to Langevin heat reservoirs with arbitrary temperature profiles. Gradients of external magnetic fields are shown to localize spin waves in a Wannier-Zeemann fashion, while magnon interactions renormalize the spectral temperature. Our generic results are applicable to other thermodynamic systems such as Newtonian liquids, elastic solids, and Josephson junctions.

We discuss the nodal structure of odd-parity superconductors in the presence of nonsymmorphic crystal symmetries, both with and without spin-orbit coupling, and with and without time-reversal symmetry. We comment on the relation of our work to previous work in the literature, and also the implications for unconventional superconductors such as ${\mathrm{UPt}}_3$.

The classic composite fermion field theory [B. I. Halperin, P. A. Lee, and N. Read, Phys. Rev. B 47, 7312 (1993)] builds up an excellent framework to uniformly study important physical objects and globally explain anomalous experimental phenomena in fractional quantum Hall physics while there are also inherent weaknesses. We present a nonperturbative emergent Dirac fermion theory from this strongly correlated composite fermion field theory, which overcomes these serious long-standing shortcomings. The particle-hole symmetry of the Dirac equation resolves this particle-hole symmetry enigma in the composite fermion field theory. With the help of presented numerical data, we show that for main Jain's sequences of fractional quantum Hall effects, this emergent Dirac fermion theory in mean field approximation is most likely stable.

We report on numerically exact determinantal quantum Monte Carlo simulations of the onset of spin-density-wave (SDW) order in itinerant electron systems captured by a sign-problem-free two-dimensional lattice model. Extensive measurements of the SDW correlations in the vicinity of the phase transition reveal that the critical dynamics of the bosonic order parameter are well described by a dynamical critical exponent $z=2$, consistent with Hertz-Millis theory, but are found to follow a finite-temperature dependence that does not fit the predicted behavior of the same theory. The presence of critical SDW fluctuations is found to have a strong impact on the fermionic quasiparticles, giving rise to a dome-shaped superconducting phase near the quantum critical point. In the superconducting state we find a gap function that has an opposite sign between the two bands of the model and is nearly constant along the Fermi surface of each band. Above the superconducting $T_c$, our numerical simulations reveal a nearly temperature and frequency independent self-energy causing a strong suppression of the low-energy quasiparticle weight in the vicinity of the hot spots on the Fermi surface. This indicates a clear breakdown of Fermi liquid theory around these points.

We present a general picture of the exciton properties of layered materials in terms of the excitations of their single-layer building blocks. To this end, we derive a model excitonic Hamiltonian by drawing an analogy with molecular crystals, which are other prototypical van der Waals materials. We employ this simplified model to analyze in detail the excitation spectrum of hexagonal boron nitride (hBN) that we have obtained from the ab initio solution of the many-body Bethe-Salpeter equation as a function of momentum. In this way, we identify the character of the lowest-energy excitons in hBN, discuss the effects of the interlayer hopping and the electron-hole exchange interaction on the exciton dispersion, and illustrate the relation between exciton and plasmon excitations in layered materials.

We report a diagrammatic method to solve the general problem of calculating configurationally averaged Green's function correlators that appear in quantum transport theory for nanostructures containing disorder. The theory treats both equilibrium and nonequilibrium quantum statistics on an equal footing. Since random impurity scattering is a problem that cannot be solved exactly in a perturbative approach, we combine our diagrammatic method with the coherent potential approximation (CPA) so that a reliable closed-form solution can be obtained. Our theory not only ensures the internal consistency of the diagrams derived at different levels of the correlators but also satisfies a set of Ward-like identities that corroborate the conserving consistency of transport calculations within the formalism. The theory is applied to calculate the quantum transport properties such as average ac conductance and transmission moments of a disordered tight-binding model, and results are numerically verified to high precision by comparing to the exact solutions obtained from enumerating all possible disorder configurations. Our formalism can be employed to predict transport properties of a wide variety of physical systems where disorder scattering is important.

The conducting gas that forms at the interface between ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ has proven to be a fertile playground for a wide variety of physical phenomena. The bulk of previous research has focused on the (001) and (110) crystal orientations. Here we report detailed measurements of the low-temperature electrical properties of (111) LAO/STO interface samples. We find that the low-temperature electrical transport properties are highly anisotropic in that they differ significantly along two mutually orthogonal crystal orientations at the interface. While anisotropy in the resistivity has been reported in some (001) samples and in (110) samples, the anisotropy in the (111) samples reported here is much stronger and also manifests itself in the Hall coefficient as well as the capacitance. In addition, the anisotropy is not present at room temperature and at liquid nitrogen temperatures, but only at liquid helium temperatures and below. The anisotropy is accentuated by exposure to ultraviolet light, which disproportionately affects transport along one surface crystal direction. Furthermore, analysis of the low-temperature Hall coefficient and the capacitance as a function of back gate voltage indicates that in addition to electrons, holes contribute to the electrical transport.

Uniaxial compressive strain along the [001] direction strongly suppresses the spin relaxation in silicon. When the strain level is large enough so that electrons are redistributed only in the two valleys along the strain axis, the dominant scattering mechanisms are quenched and electrons mainly experience intra-axis scattering processes (intravalley or intervalley scattering within valleys on the same crystal axis). We first derive the spin-flip matrix elements due to intra-axis electron scattering off impurities, and then provide a comprehensive model of the spin relaxation time due to all possible interactions of conduction-band electrons with impurities and phonons. We predict a nearly three orders of magnitude improvement in the spin relaxation time of $\sim {10}^{19}{\text{cm}}^{-3}$ antimony-doped silicon (Si:Sb) at low temperatures.

We develop a microscopic large-$N$ theory of electron-electron interaction corrections to multilegged Feynman diagrams describing second- and third-order non-linear-response functions. Our theory, which reduces to the well-known random-phase approximation in the linear-response limit, is completely general and is useful to understand all second- and third-order nonlinear effects, including harmonic generation, wave mixing, and photon drag. We apply our theoretical framework to the case of graphene, by carrying out microscopic calculations of the second- and third-order non-linear-response functions of an interacting two-dimensional (2D) gas of massless Dirac fermions. We compare our results with recent measurements, where all-optical launching of graphene plasmons has been achieved by virtue of the finiteness of the quasihomogeneous second-order nonlinear response of this inversion-symmetric 2D material.

The core issue for the implementation of NV center qubit technology is a sensitive readout of the NV spin state. We present here a detailed theoretical and experimental study of NV center photoionization processes, used as a basis for the design of a dual-beam photoelectric method for the detection of NV magnetic resonances (PDMR). This scheme, based on NV one-photon ionization, is significantly more efficient than the previously reported single-beam excitation scheme. We demonstrate this technique on small ensembles of $\sim 10$ shallow NVs implanted in electronic grade diamond (a relevant material for quantum technology), on which we achieve a cw magnetic resonance contrast of 9%—three times enhanced compared to previous work. The dual-beam PDMR scheme allows independent control of the photoionization rate and spin magnetic resonance contrast. Under a similar excitation, we obtain a significantly higher photocurrent, and thus an improved signal-to-noise ratio, compared to single-beam PDMR. Finally, this scheme is predicted to enhance magnetic resonance contrast in the case of samples with a high proportion of substitutional nitrogen defects, and could therefore enable the photoelectric readout of single NV spins.

We construct a many-body theory of magnetoelasticity in one dimension and show that the dynamical correlation functions of the quantum magnet, connecting the spins with phonons, involve all energy scales. Accounting for all magnetic states nonperturbatively via the exact diagonalization techniques of Bethe ansatz, we find that the renormalization of the phonon velocity is a nonmonotonous function of the external magnetic field and identify a new mechanism for attenuation of phonons—via hybridization with the continuum of excitations at high energy. We conduct ultrasonic measurements on a high-quality single crystal of the frustrated spin-1/2 Heisenberg antiferromagnet ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ in its nearly one-dimensional regime and confirm the theoretical predictions, demonstrating that ultrasound can be used as a powerful probe of strong correlations in one dimension.

We investigate the phase transition between an ergodic and a many-body localized phase in infinite anisotropic spin-$1/2$ Heisenberg chains with binary disorder. Starting from the Néel state, we analyze the decay of antiferromagnetic order $m_s\left(t\right)$ and the growth of entanglement entropy $S_{\mathrm{ent}}\left(t\right)$ during unitary time evolution. Near the phase transition we find that $m_s\left(t\right)$ decays exponentially to its asymptotic value $m_s\left(\infty \right)\ne 0$ in the localized phase while the data are consistent with a power-law decay at long times in the ergodic phase. In the localized phase, $m_s\left(\infty \right)$ shows an exponential sensitivity on disorder with a critical exponent $\nu \sim 0.9$. The entanglement entropy in the ergodic phase grows subballistically, $S_{\mathrm{ent}}\left(t\right)\sim t^{\alpha }$, $\alpha \le 1$, with $\alpha$ varying continuously as a function of disorder. Exact diagonalizations for small systems, on the other hand, do not show a clear scaling with system size and attempts to determine the phase boundary from these data seem to overestimate the extent of the ergodic phase.

We examine normal-state quasiparticle excitations along the $\mathrm{\Gamma }\text{−}M$ cut in momentum space for the putative $p$-wave superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ on the basis of fluctuation exchange approximation calculations. We take as input first-principles derived parameters for the band structure and spin-orbit and electron-electron interactions. The numerical results are in excellent agreement with data from photoemission experiments and provide insight into the underlying quasiparticle properties. We find that, despite the correlation-induced effective mass increase near the Fermi surface, the full $\beta$ and $\gamma$ bandwidths are, if anything, increased by correlations. Furthermore, for the $\gamma$ band we find anomalous lifetime broadening and a significant temperature of variation of unoccupied state quasiparticle energies for temperatures between 25 and 100 K, both of which are accounted for by the momentum dependence of the electron self-energy. In addition to aiding our understanding of experimental data, these results point to the challenge of assigning appropriate Fermi-liquid parameters or momentum-independent self-energies for schemes that require such approximations in order to model ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

Achieving true bulk insulating behavior in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, the archetypal topological insulator with a simplistic one-band electronic structure and sizable band gap, has been prohibited by a well-known self-doping effect caused by selenium vacancies, whose extra electrons shift the chemical potential into the bulk conduction band. We report a synthesis method for achieving stoichiometric ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystals that exhibit nonmetallic behavior in electrical transport down to low temperatures. Hall-effect measurements indicate the presence of both electron- and holelike carriers, with the latter identified with surface state conduction and the achievement of ambipolar transport in bulk ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystals without gating techniques. With carrier mobilities surpassing the highest values yet reported for topological surface states in this material, the achievement of ambipolar transport via upward band bending is found to provide a key method to advancing the potential of this material for future study and applications.

We investigate Dirac-like conical dispersions in photonic crystals with complex unit cells. Comparing with photonic crystals with simple unit cells, the complex-unit-cell design can provide extra degrees of freedom to engineer the frequency of the Dirac-like point in a broad frequency regime. Interestingly, we find that many functionalities of double zero media associated with the Dirac-like point are well preserved in such complex photonic crystals, such as wave tunneling, cloaking, wave front control, etc. Different transmission behaviors, e.g., total reflection and negative refraction, can be achieved by shifting the frequency of the Dirac-like point.

We investigate the out-of-equilibrium dynamics after a local quench that connects two spin-1/2 XXZ chains prepared in the ground state of the Hamiltonian in different phases, one in the ferromagnetic phase and the other in the critical phase. We analyze the time evolution of the on-site magnetization and bipartite entanglement entropy via adaptive time-dependent density matrix renormalization group. In systems with short-range interactions, such as the one we consider, the velocity of information transfer is expected to be bounded, giving rise to a light-cone effect. Interestingly, our results show that, when the anisotropy parameter of the critical chain is sufficiently close to that of the isotropic ferromagnet, the light cone is determined by the velocity of spin-wave bound states that propagate faster than single-particle (“spinon”) excitations. Furthermore, we investigate how the system approaches equilibrium in the inhomogeneous ground state of the connected system, in which the ferromagnetic chain induces a nonzero magnetization in the critical chain in the vicinity of the interface.