We propose a setup for the experimental realization of anisotropic $0\text{−}\pi$ transitions of the Josephson current, in a junction whose link is made of irradiated Weyl semimetal (WSM), due to the presence of chiral nodes. The Josephson current through a time-reversal symmetric WSM has anisotropic (with respect to the orientation of the chiral nodes) periodic oscillations as a function of $k_{0}L$, where $k_0$ is the (relevant) separation of the chiral nodes and $L$ is the length of the sample. We then show that the effective value of $k_0$ can be tuned with precision by irradiating the sample with linearly polarized light, which does not break time-reversal invariance, resulting in $0\text{−}\pi$ transitions of the critical current. We also discuss the feasibility and robustness of our setup.

We investigate the nature of the quantum Hall liquid in a half-filled second Landau level ($n=1$) as a function of band mass anisotropy using numerical exact diagonalization and density matrix renormalization group methods. We find increasing the mass anisotropy induces a quantum phase transition from the Moore-Read state to a charge density wave state. By analyzing the energy spectrum, guiding center structure factors, and by adding weak pinning potentials, we show that this charge density wave is a unidirectional quantum Hall stripe, which has a periodicity of a few magnetic lengths and survives in the thermodynamic limit. We find smooth profiles for the guiding center occupation function that reveal the strong coupling nature of the array of chiral Luttinger liquids residing at the stripe edges.

Specific heat has had an important role in the study of superfluidity and superconductivity, and could provide important information about the fractional quantum Hall effect as well. However, traditional measurements of the specific heat of a two-dimensional electron gas are difficult due to the large background contribution of the phonon bath, even at very low temperatures. Here, we report measurements of the specific heat per electron in the second Landau level by measuring the thermalization time between the electrons and phonons. We observe the activated behavior of the specific heat of the $\frac{5}{2}$ and $\frac{7}{3}$ fractional quantum Hall states, and extract the entropy by integrating over temperature. Our results are in excellent agreement with previous measurements of the entropy via longitudinal thermopower. Extending the technique to lower temperatures could lead to the detection of the non-Abelian entropy predicted for bulk quasiparticles at $\frac{5}{2}$ filling.

We propose a generic construction of exactly soluble local bosonic models that realize various topological orders with gappable boundaries. In particular, we construct an exactly soluble bosonic model that realizes a (3+1)-dimensional [(3+1)D] $Z_2$-gauge theory with emergent fermionic Kramers doublet. We show that the emergence of such a fermion will cause the nucleation of certain topological excitations in space-time without ${\mathrm{pin}}^{+}$ structure. The exactly soluble model also leads to a statistical transmutation in (3+1)D. In addition, we construct exactly soluble bosonic models that realize 2 types of time-reversal symmetry-enriched $Z_2$ topological orders in 2+1 dimensions, and 20 types of simplest time-reversal symmetry-enriched topological (SET) orders which have only one nontrivial pointlike and stringlike topological excitation. Many physical properties of those topological states are calculated using the exactly soluble models. We find that some time-reversal SET orders have pointlike excitations that carry Kramers doublet, a fractionalized time-reversal symmetry. We also find that some $Z_2$ SET orders have stringlike excitations that carry anomalous (nononsite) $Z_2$ symmetry, which can be viewed as a fractionalization of $Z_2$ symmetry on strings. Our construction is based on cochains and cocycles in algebraic topology, which is very versatile. In principle, it can also realize emergent topological field theory beyond the twisted gauge theory.

Semiconductor photocatalysts that can produce hydrogen from water splitting are of great interest. Among the various possibilities, nitrogen (N)-doped $\mathrm{SrTi}{\mathrm{O}}_3$ is a promising candidate for hydrogen evolution under visible-light irradiation. In this study, hybrid density-functional calculations are employed to investigate the stability and impact of nitrogen impurities on the electronic and optical properties in $\mathrm{SrTi}{\mathrm{O}}_3$. We find that the substitutional N on O site (${\mathrm{N}}_{\mathrm{O}}$) is a deep acceptor in $\mathrm{SrTi}{\mathrm{O}}_3$. Moreover, ${\mathrm{N}}_{\mathrm{O}}$ predominates over other N-related defects under equilibrium growth conditions in $n$-type $\mathrm{SrTi}{\mathrm{O}}_3$. Our results reveal that ${\mathrm{N}}_{\mathrm{O}}$ gives rise to visible-light absorption in agreement with experimental observations. In addition, we find that hydrogen can bind to ${\mathrm{N}}_{\mathrm{O}}$, forming ${\mathrm{N}}_{\mathrm{O}}-{\mathrm{H}}_i$ complex which leads to a blueshift in the optical absorption. Other N configurations can also contribute to optical absorption in the visible range. The vibration frequencies of different N configurations are provided to support identification using vibrational spectroscopy techniques.

Dirac fermions have been actively investigated in recent years, both theoretically and experimentally. Some materials hosting Dirac fermions are natural platforms for interlayer coherence effects such as Coulomb drag and exciton condensation. Here we determine the role played by the anomalous Hall effect in Coulomb drag in doped massive Dirac fermion systems. We focus on topological insulator films with out-of plane magnetizations in both the active and passive layers. The transverse response of the active layer is dominated by a topological term arising from the Berry curvature. We show that the topological mechanism does not contribute to Coulomb drag, yet the longitudinal drag force in the passive layer gives rise to a transverse drag current. This anomalous Hall drag current is independent of the active-layer magnetization, a fact that can be verified experimentally. It depends nonmonotonically on the passive-layer magnetization, exhibiting a peak that becomes more pronounced at low densities. These findings are expected to stimulate new experiments and quantitative studies of anomalous Hall drag.

Spin states of photoelectrons excited from spin-polarized surface states of Bi(111) surfaces are theoretically studied. The electronic states with a mirror symmetry are focused on in particular. We present a method for calculating spin expectation values of photoelectrons using results of density-functional methods in the plane-wave basis. We show that the free-electron approximation for final states is not sufficient for explaining an experimental result obtained in spin- and angle-resolved photoelectron spectroscopy. We point out two effects of the time-reversal symmetry in photoelectron emission. One is that the out-of-plane component of spin orientation is reversed to that of the time-reversed state. This property may be used for distinguishing the orientation of samples by shedding light with polarizations that break the mirror symmetry. Another is the asymmetry in photoemission intensity between an initial state and its time-reversed state due to the breaking of the time-reversal symmetry intrinsic in the photoemission process.

We investigate the proximity effect in junctions between $N=3$ superconductors under commensurate voltage bias. The bias is chosen to highlight the role of transport processes that exchange multiple Cooper pairs coherently between more than two superconductors. Such nonlocal processes can be studied in the dc response, where local transport processes do not contribute. We focus on the proximity-induced normal density of states that we investigate in a wide parameter space. We reveal the presence of deep and highly tunable pseudogaps and other rich structures. These are due to a static proximity effect that is absent for $N=2$ and is sensitive to an emergent superconducting phase associated to nonlocal coherent transport. In comparison with results for $N=2$, we find similarities in the signature peaks of multiple Andreev reflections. We discuss the effect of electron-hole decoherence and of various types of junction asymmetries. Our predictions can be investigated experimentally using tunneling spectroscopy.

The in-plane thermal conductivity of silicon phononic membranes is investigated by micro time domain thermoreflectance and Monte Carlo simulations. Strong reduction of thermal conductivity is observed mainly due to phonon boundary scattering for both aligned and staggered lattices of holes. The measured and calculated thermal conductivities of the porous membranes with cylindrical holes are found to be in good quantitative agreement (at 4 K and 300 K). A significant difference between thermal conductivities of aligned and staggered lattice of identical porosities is observed. This difference is shown to arise from ballistic phonons that acquired directionality by propagating between the holes. The directionality effect strengthens when the temperature is decreased or when the diameter of the holes becomes close to the period. Finally, we propose a model, which quantifies and explains the difference between thermal conductivities of aligned and staggered lattices based on geometric considerations.

We study the conductance of a junction between the normal and superconducting segments of a nanowire, both of which are subjected to spin-orbit coupling and an external magnetic field. We directly compare the transport properties of the nanowire assuming two different models for the superconducting segment: one where we put superconductivity by hand into the wire and one where superconductivity is induced through a tunneling junction with a bulk $s$-wave superconductor. While these two models are equivalent at low energies and at weak coupling between the nanowire and the superconductor, we show that there are several interesting qualitative differences away from these two limits. In particular, the tunneling model introduces an additional conductance peak at the energy corresponding to the bulk gap of the parent superconductor. By employing a combination of analytical methods at zero temperature and numerical methods at finite temperature, we show that the tunneling model of the proximity effect reproduces many more of the qualitative features that are seen experimentally in such a nanowire system.

In this paper, we elucidate the changes of magnetoelectric coupling mechanism in different zones of the rich magnetic field-temperature $\left(B\text{−}T\right)$ phase diagram of the molecular multiferroic ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2\left[\mathrm{FeC}{\mathrm{l}}_5·{\mathrm{H}}_2\mathrm{O}\right]$, which represents one of the rare cases where improper ferroelectricity has been observed in a hybrid material. We have recently proposed a mechanism of multiferroicity in zero magnetic field in the deuterated form of this material from a detailed determination of its crystal and magnetic structures. The proposed magnetic structure at zero magnetic field corresponds to a cycloidal spin arrangement that gives rise to a ferroelectric polarization through the spin current mechanism induced via the inverse Dzyaloshinskii-Moriya interaction. In this paper, we present a single-crystal neutron diffraction study under external magnetic field, aimed at elucidating the evolution of the magnetic structure under applied magnetic field, and determine the mechanism of magnetoelectric coupling, which allows us to describe an unprecedented change from spin current to spin-dependent $p\text{−}d$ hybridization mechanism.

The magnetism in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ has been investigated using thermodynamic measurements, first-principles calculations, neutron diffraction, and diffuse neutron scattering on single crystals. These data confirm that ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ is a ferrimagnet below $T_{\mathrm{C}}$ $\approx 78$ K. The magnetism is anisotropic, with magnetization and neutron diffraction demonstrating that the moments lie within the basal plane of the trigonal structure. The saturation magnetization of $\approx 1.6{\mu }_B$/Mn at 5 K originates from the different multiplicities of the two antiferromagnetically aligned Mn sites. First-principles calculations reveal antiferromagnetic exchange for the three nearest Mn-Mn pairs, which leads to a competition between the ferrimagnetic ground state and three other magnetic configurations. The ferrimagnetic state results from the energy associated with the third-nearest-neighbor interaction, and thus long-range interactions are essential for the observed behavior. Diffuse magnetic scattering is observed around the 002 Bragg reflection at 120 K, which indicates the presence of strong spin correlations well above $T_{\mathrm{C}}$. These are promoted by the competing ground states that result in a relative suppression of $T_{\mathrm{C}}$ and may be associated with a small ferromagnetic component that produces anisotropic magnetism below $\approx 330\mathrm{K}$.

We discuss a possible link between the recently observed macroscopic ordering of ultracold dipolar excitons (MOES) and the phenomenon of supersolidity. In the dilute limit we predict a stable supersolid state for a quasi-one-dimensional system of bosonic dipoles characterized by two- and three-body contact repulsion. We phenomenologically extend our theory to the strongly-correlated regime and find a critical value of the contact interaction parameter at which the supersolid exhibits a quantum phase transition to a fragmented state. The wavelength of the fragmented-condensate solid is defined by the balance between the quantum pressure and the entropy due to fluctuations of the relative phases between the fragments. Our model appears to be in good agreement with the relevant experimental data, including the very recent results on commensurability effect and wavelength of the MOES.

Inelastic neutron scattering experiments have been conducted to investigate the spin dynamics and crystal-electric-field level scheme of the Ce $4f$ electrons in ${\mathrm{Ce}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$. This compound exhibits a large specific heat at low temperatures and anomalous semimetallic transport in the chiral crystallographic phase below 160 K. Distinctly observed magnetic excitations at approximately 6 and 29 meV are asymmetric in spectral shape and are reproduced by two inequivalent crystal-electric-field splitting schemes, which are deduced from the chiral structure. We have also observed the spin dynamics reflecting antiferromagnetic correlations below 1 meV, which is enhanced with an upturn in the electrical resistivity below 15 K and which yields a low-energy density of state relevant to the large specific heat. We discuss the possibility of a three-dimensional Weyl semimetal state, considering the chiral-lattice symmetry, electronic hybridization, and magnetic correlation.

We study the dynamics of phase relaxation between a pair of one-dimensional condensates created by a bi-directional, supersonic ‘unzipping’ of a finite single condensate. We find that the system fractures into different extensive chunks of space-time, within which correlations appear thermal but correspond to different effective temperatures. Coherences between different eigen-modes are crucial for understanding the development of such thermal correlations; at no point in time can our system be described by a generalized Gibbs' ensemble despite nearly always appearing locally thermal. We rationalize a picture of propagating fronts of hot and cold sound waves, populated at effective, relativistically red- and blue-shifted temperatures to intuitively explain our findings. The disparity between these hot and cold temperatures vanishes for the case of instantaneous splitting but diverges in the limit where the splitting velocity approaches the speed of sound; in this limit, a sonic boom occurs wherein the system is excited only along an infinitely narrow, and infinitely hot beam. We expect our findings to apply generally to the study of superluminal perturbations in systems with emergent Lorentz symmetry.

Based on the random phase approximation (RPA), RPA renormalization [J. E. Bates and F. Furche, J. Chem. Phys. 139, 171103 (2013)] is a robust many-body perturbation theory that works for molecules and materials because it does not diverge as the Kohn-Sham gap approaches zero. Additionally, RPA renormalization enables the simultaneous calculation of RPA and beyond-RPA correlation energies since the total correlation energy is the sum of a series of independent contributions. The first-order approximation (RPAr1) yields the dominant beyond-RPA contribution to the correlation energy for a given exchange-correlation kernel, but systematically underestimates the total beyond-RPA correction. For both the homogeneous electron gas model and real systems, we demonstrate numerically that RPA renormalization beyond first order converges monotonically to the infinite-order beyond-RPA correlation energy for several model exchange-correlation kernels and that the rate of convergence is principally determined by the choice of the kernel and spin polarization of the ground state. The monotonic convergence is rationalized from an analysis of the RPA renormalized correlation energy corrections, assuming the exchange-correlation kernel and response functions satisfy some reasonable conditions. For spin-unpolarized atoms, molecules, and bulk solids, we find that RPA renormalization is typically converged to 1 meV error or less by fourth order regardless of the band gap or dimensionality. Most spin-polarized systems converge at a slightly slower rate, with errors on the order of 10 meV at fourth order and typically requiring up to sixth order to reach 1 meV error or less. Slowest to converge, however, open-shell atoms present the most challenging case and require many higher orders to converge.

The interaction of point defects with extrinsic Frank loops in the photovoltaic absorber material $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ was studied by aberration-corrected scanning transmission electron microscopy in combination with electron energy-loss spectroscopy and calculations based on density-functional theory. We find that Cu accumulation occurs outside of the dislocation cores bounding the stacking fault due to strain-induced preferential formation of ${\mathrm{Cu}}_{\mathrm{In}}^{-2}$, which can be considered a harmful hole trap in $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$. In the core region of the cation-containing $\alpha$-core, Cu is found in excess. The calculations reveal that this is because Cu on In-sites is lowering the energy of this dislocation core. Within the Se-containing $\beta$-core, in contrast, only a small excess of Cu is observed, which is explained by the fact that ${\mathrm{Cu}}_{\mathrm{In}}$ and ${\mathrm{Cu}}_{\mathrm{i}}$ are the preferred defects inside this core, but their formation energies are positive. The decoration of both cores induces deep defect states, which enhance nonradiative recombination. Thus, the annihilation of Frank loops during the $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ growth is essential in order to obtain absorbers with high conversion efficiencies.

Spin-orbit coupling is key to all-electrical control of quantum-dot spin qubits, and is often much stronger for holes than for electrons. The recent development of high-quality hole nanostructures has generated considerable interest in hole-spin qubit architectures [C. Kloeffel and D. Loss, Annu. Rev. Condens. Matter Phys. 4, 51 (2013)]. Yet hole-spin quantum computing hinges on the ability to discriminate between competing Zeeman terms and on the understanding of the complex interplay between the Zeeman and spin-orbit interactions, which are probed via Pauli spin blockade. Here we investigate spin blockade for two heavy holes in a gated double quantum dot in an in-plane magnetic field $\mathbf{B}$. We find that the leakage period as a function of the field orientation is critically dependent on the relative magnitude of Zeeman interaction terms linear and cubic in $B$, exhibiting a beat pattern when the two are comparable in magnitude, and providing an effective way to discriminate between the two. Moreover, in certain materials the singlet-triplet exchange splitting is highly tunable by an appropriate choice of field direction, yielding a straightforward control variable for quantum information processing. These findings should stimulate new experiments on hole qubits.