We theoretically study topological planar Josephson junctions (JJs) formed from spin-orbit-coupled two-dimensional electron gases (2DEGs) proximitized by two superconductors and subjected to an in-plane magnetic field $B_{\parallel }$. Compared to previous studies of topological superconductivity in these junctions, here we consider the case where the superconducting leads are narrower than the superconducting coherence length. In this limit the system may be viewed as a proximitized multiband wire, with an additional knob being the phase difference $\varphi$ between the superconducting leads. A combination of mirror and time-reversal symmetry may put the system into the class BDI. Breaking this symmetry changes the symmetry class to class D. The class D phase diagram depends strongly on $B_{\parallel }$ and chemical potential, with a weaker dependence on $\varphi$ for JJs with narrower superconducting leads. In contrast, the BDI phase diagram depends strongly on both $B_{\parallel }$ and $\varphi$. Interestingly, the BDI phase diagram has a “fan”-shaped region with phase boundaries which move away from $\varphi =\pi$ linearly with $B_{\parallel }$. The number of distinct phases in the fan increases with increasing chemical potential. We study the dependence of the JJ's critical current on $B_{\parallel }$, and find that minima in the critical current indicate first-order phase transitions in the junction only when the spin-orbit coupling strength is small. In contrast to the case of a JJ with wide leads, in the narrow case these transitions are not accompanied by a change in the JJ's topological index. Our results, calculated using realistic experimental parameters, provide guidelines for present and future searches for topological superconductivity in JJs with narrow leads, and are particularly relevant to recent experiments on InAs 2DEGs proximitized by narrow Al superconducting leads [A. Fornieri et al., Nature (London) 569, 89 (2019)].

We show how the pairing symmetry of superconducting states in twisted bilayer graphene can be experimentally identified by theoretically studying effects of externally applied in-plane magnetic field and strain. In the low-field regime, superconducting critical temperature $T_c$ is suppressed by in-plane magnetic field ${\mathit{B}}_{\parallel }$ in singlet channels, but is enhanced by weak ${\mathit{B}}_{\parallel }$ in triplet channels, providing an important distinction. The in-plane angular dependence of the critical ${\mathit{B}}_{\parallel ,c}$ has a sixfold rotational symmetry, which is broken when strain is present. We show that anisotropy in ${\mathit{B}}_{\parallel ,c}$ generated by strain can be similar for $s$- and $d$-wave channels in moiré superlattices. The $d$-wave state is pinned to be nematic by strain and consequently gapless, which is distinguishable from the fully gapped $s$-wave state by tunneling gap measurements.

Coexistence of metallicity and ferroelectricity has been a curiosity without a practical application, both because free-electron screening due to metallicity prevents the polarization from being switched by an electric field, diminishing the value of ferroelectricity, and because metallicity is usually achieved by doping, which leads to disorder and is often detrimental to other electronic properties. Here, we predict via first-principles calculation a switchable metallic ferroelectric barrier in ${\left({\mathrm{Co}}_2\right)}_9\text{−}{\mathrm{TiO}}_2\text{−}{\left(\mathrm{BaO}\text{−}{\mathrm{TiO}}_2\right)}_m\text{−}\mathrm{CoO}\text{−}\mathrm{Co}\left(m=4,5,6,7,8,9\right)$ multiferroic tunnel junction without doping. The metallic ferroelectricity is caused by an electrode proximity effect that is common to ionic ferroelectric materials and shifts the Fermi energy as a function of the termination layer at the interfaces. This effect is accentuated by the large polarization of the CoO layer relatively to that of ${\mathrm{BaTiO}}_3$ (BTO), leading to a larger electrostatic potential drop on the interface containing CoO, thus further pulling the conduction band bottom of the entire BTO region below the Fermi energy. Increasing the polarization of BTO relative to that of CoO, e.g., by applying strain, can remove the metallicity, allowing the polarization to be switched electrically. Switching between metallic and insulating states by controlling ferroelectric polarization leads to a large tunneling electroresistance.

Using nonlocal spin-transport devices, the tunnel spin polarization (TSP) of Fe/MgO tunnel contacts on Si is determined as a function of the thickness of the MgO. The TSP, extracted from the magnitude of the nonlocal spin-valve and Hanle signals, increases with MgO thickness and is shown to reach values of around 90% at low temperatures. Such a near-perfect spin polarization of the tunnel current indicates that symmetry-based spin filtering due to coherent tunneling occurs in Fe/MgO tunnel contacts on Si, despite the significant lattice mismatch. For MgO thicknesses below 1 nm, the TSP drops unexpectedly rapidly (to values below 25%), which is attributed to the lower crystalline quality of thin MgO layers. In fact, the data suggest that the first 0.7 nm of MgO on the Si contributes very little to the spin filtering.

The kagome-staircase compound ${\mathrm{Ni}}_3{\mathrm{V}}_2{\mathrm{O}}_8$ is an attractive multiferroic material exhibiting rich phase diagrams. However, the magnetic properties and magnetic transitions have been studied only above 1.3 K. In this work, we study the thermal conductivity $\kappa$ of ${\mathrm{Ni}}_3{\mathrm{V}}_2{\mathrm{O}}_8$ single crystals at low temperatures down to 0.3 K and in magnetic fields up to 14 T. In zero field, the magnetic transitions from the low-temperature incommensurate (LTI) phase to the commensurate phase (C) and then to a second commensurate phase $\left({\mathrm{C}}^{\prime }\right)$ yield anomalies in $\kappa \left(T\right)$ curves at $T_{\mathrm{LC}}=3.7$ K and $T_{{\mathrm{CC}}^{\prime }}=2.0$ K, respectively, which indicates a significant phonon scattering by the critical spin fluctuations. When the field is applied along the $a$ axis, the field dependence of $\kappa$ displays four anomalies associated with different magnetic transitions and reveals an undetected magnetic state at subkelvin temperatures. In addition, the $\kappa \left(B\right)$ curves are found to depend not only on the history but also on the magnitude of the applied field. When the field is applied along the $b$ axis, a high-field phase located above the LTI and high-temperature incommensurate phases is revealed.

Low-dimensional materials have an excellent prospect in thermoelectric applications. We have investigated the geometric structure, band structure, and electron transport properties of hydrogenated and pure multilayer silicene using first-principle calculation within density functional theory. The Boltzmann theory for electrons under relaxation time approximation was employed to obtain the Seebeck coefficient and electrical conductivity. The calculations of electron relaxation time were based on the deformation potential theory. Hydrogenation can effectively change the band structure of multilayer silicene. The simulation results reveal a big difference in the relaxation times between pure and hydrogenated structures. And, this difference decreases as the layer number $n$ increases. The anisotropy of hydrogenated structures leads to a high thermoelectric performance along the armchair direction. When the layer number $n$ is larger than 2, hydrogenation can greatly improve the electronic figure of merit $Z{T}_{\mathrm{e}}$ of multilayer silicene. The band structure can also be engineered by adjusting the hydrogenation ratio. A band of 0.33 eV can be achieved when the hydrogenation ratio is 66.7%. By combining the adjustment of the hydrogenation ratio with the method of changing the geometric structure, a high thermoelectric performance can be achieved in multilayer silicene. The results provide a viable strategy for thermoelectric optimization in multilayer silicene, and can be potentially extended to the thermoelectric optimization of other two-dimensional materials.

We study surface second-harmonic generation (SHG) from a singular plasmonic structure consisting of touching metallic wires. We use the technique of transformation optics and relate the structure to a rather simpler geometry, a slab waveguide. This allows us to obtain an analytical solution to the problem, revealing rich physical insights. We identify various conditions that govern the SHG efficiency. Importantly, our analysis demonstrates that apart from the mode-matching condition, the phase-matching condition is relevant even for this subwavelength structure. Furthermore, we identify a geometric factor which was not identified before. We support our analysis with numerical simulations.

$2H\text{−}\mathrm{Ta}{\mathrm{S}}_2$ undergoes a charge density wave (CDW) transition at $T_{\mathrm{CDW}}\sim 75$ K, however key questions regarding the onset of CDW order remain under debate. In this study, we explore the CDW transition through a combination of temperature and excitation-dependent Raman spectroscopy, angle resolved photoemission spectroscopy (ARPES), and density functional theory (DFT). Below $T_{\mathrm{CDW}}$ we identify two CDW amplitude modes that redshift and broaden with increasing temperature and one zone-folded mode that disappears above $T_{\mathrm{CDW}}$. Above $T_{\mathrm{CDW}}$, we observe a strong two-phonon mode that softens substantially upon cooling, which suggests the presence of substantial lattice distortions at temperatures as high as 250 K. This correlates with the ARPES observation of the persistence of a CDW energy gap above $T_{\mathrm{CDW}}$ and finite-temperature DFT calculations of the phonon band structure that indicate an instability occurring well above the CDW transition temperature. DFT also provides the atomic displacements of the CDW amplitude modes and reproduces their temperature dependence. From these observations we suggest that short range CDW order exists well above $T_{\mathrm{CDW}}$, which poses new questions regarding the interplay between electronic structure and vibrational modes in layered CDW materials.

The electronic structure and the anomalous Hall effect (AHE) at the recently grown $3d\text{−}5d$ superlattice structure ${\left({\mathrm{SrMnO}}_3\right)}_1/{\left({\mathrm{SrIrO}}_3\right)}_1$ are investigated theoretically using model and density-functional calculations. The observed ferromagnetism for the SMO layer, which becomes electron doped because of the charge transfer across the interface, is explained to be due to the Anderson-Hasegawa double exchange, while the SIO side becomes ferromagnetic due to the proximity effect and the Nagaoka physics of a hole-doped system. The broken time-reversal symmetry and the strong spin-orbit coupling in SIO provide the two essential ingredients for the AHE, which we describe within a square lattice model of $d$-orbitals relevant for the present structure. The model correctly predicts the order of magnitude of the anomalous Hall conductivity (AHC), which is computed using the Kubo formula. The density-functional results for the AHC are in good agreement with the experiments.

We study the extended Hubbard model on a two-dimensional half-filled square lattice using the dynamical cluster approximation. We present results on the phase boundaries between the paramagnetic metallic (normal) state and the insulating antiferromagnetic state, as well as between the antiferromagnetic and charge-order states. We find hysteresis along the antiferromagnet/charge order and normal/charge-order phase boundaries (at larger values of the on-site interaction), indicating first-order phase transitions. We show that nearest-neighbor interactions lower the critical temperature for the antiferromagnetic phase. We also present results for the effect of nearest-neighbor interactions on the antiferromagnetic phase boundary and for the evolution of spectral functions and energetics across the phase transitions.

We show how to compute the nonlinear optical functions (absorption, reflection, and transmission) for a medium with Rydberg excitons (REs), including the effect of the coherence between the electron-hole pair and the electromagnetic field. Using the real density-matrix approach the analytical expressions for nonlinear optical functions are obtained and numerical calculations are performed for ${\mathrm{Cu}}_2\mathrm{O}$ crystal where REs have been observed. We report a good agreement with recently published experimental data. Propagation of the electromagnetic waves in Rydberg exciton media with nonlinear effect is also discussed and the possibility of obtaining self-phase modulation due to Kerr nonlinearity is investigated.

Optical excited-state dynamics of bright and dark positively charged excitons (positive trions) in one-layer (1L-) $\mathrm{WS}{\mathrm{e}}_2$ were studied by a combination of time- and polarization-resolved photoluminescence and pump-probe spectroscopy. Bright positive trions showed ultrafast relaxation to dark positive trions (∼1 ps) and to ground state (10 ± 5 ps), while dark positive trions showed slow decay to the ground state (∼300 ps) at 10 K. Under valley selective excitation with circularly polarized light, the valley relaxation time of the bright trions was 1.5 ± 0.5 ns at 10 K, which is much longer than typical exciton valley relaxation times of 10 ps. This insight into the slower valley relaxation dynamics of positive trions suggests that the electron-hole exchange interactions that are dominant in neutral exciton valley relaxation, are suppressed by the presence of additional holes.

The role of electron-phonon scattering in finite-temperature anomalous Hall effect is still poorly understood. In this work we present a Boltzmann theory for the side-jump contribution from electron-phonon scattering, which is derived from the microscopic quantum mechanical theory. We show that the resulting phonon side-jump conductivity generally approaches different limiting values in the high and low temperature limits, and hence can exhibit strong temperature dependence in the intermediate temperature regime. Our theory is amenable to ab initio treatment, which makes quantitative comparison between theoretical and experimental results possible.

Hidden-order phases that occur in a number of correlated $f$-electron systems are among the most elusive states of electronic matter. Their investigations are hindered by the insensitivity of standard physical probes, such as neutron diffraction, to the order parameter that is usually associated with higher-order multipoles of the $f$ orbitals. The heavy-fermion compound ${\mathrm{Ce}}_3{\mathrm{Pd}}_{20}{\mathrm{Si}}_6$ exhibits magnetically hidden order at subkelvin temperatures, known as phase II. Additionally, for magnetic field applied along the [001] cubic axis, another phase ${\mathrm{II}}^{\prime }$ was detected, but the nature of the transition from phase II to phase ${\mathrm{II}}^{\prime }$ remained unclear. Here we use inelastic neutron scattering to argue that this transition is most likely associated with a change in the propagation vector of the antiferroquadrupolar order from (111) to (100). Despite the absence of magnetic Bragg scattering in phase ${\mathrm{II}}^{\prime }$, its ordering vector is revealed by the location of an intense magnetic soft mode at the (100) wave vector, that is orthogonal to the applied field. At the II-${\mathrm{II}}^{\prime }$ transition, this mode softens and transforms into quasielastic and nearly $\mathbf{Q}$-independent incoherent scattering, which is likely related to the non-Fermi-liquid behavior recently observed at this transition. Our experiment also reveals sharp collective excitations in the field-polarized paramagnetic phase, after phase ${\mathrm{II}}^{\prime }$ is suppressed in fields above 4 T.

We report the magnetic phase diagram of single-crystalline ${\mathrm{LiFePO}}_4$ in magnetic fields up to 58 T and present a detailed study of magnetoelastic coupling by means of high-resolution capacitance dilatometry. Large anomalies at $T_{\mathrm{N}}$ in the thermal-expansion coefficient $\alpha$ imply pronounced magnetoelastic coupling. Quantitative analysis yields the magnetic Grüneisen parameter ${\gamma }_{\mathrm{mag}}=6.7\left(5\right)\times {10}^{-7}$ mol/J. The positive hydrostatic pressure dependence $d{T}_{\mathrm{N}}/dp=1.46\left(11\right)$ K/GPa is dominated by uniaxial effects along the $a$ axis. Failure of Grüneisen scaling below $\approx 40\mathrm{K}$, i.e., below the peak temperature in the magnetoelectric coupling coefficient [7], implies several competing degrees of freedom. A broad and strongly magnetic field dependent anomaly in $\alpha$ in this temperature regime highlights the relevance of structure changes. Upon application of the magnetic field $B\left|\right|b$ axis, a pronounced jump in the magnetization implies spin reorientation at $B_{\mathrm{SF}}=32\mathrm{T}$ as well as a precursing phase at 29 T and $T=1.5\mathrm{K}$. In a two-sublattice mean-field model, the saturation field $B_{\mathrm{sat},\mathrm{b}}=64\left(2\right)\mathrm{T}$ enables assessing the effective antiferromagnetic exchange interaction $J_{\mathrm{af}}=2.68\left(5\right)\mathrm{meV}$ as well as anisotropies $D_{\mathrm{b}}=-0.53\left(4\right)\mathrm{meV}$ and $D_{\mathrm{c}}=0.44\left(8\right)\mathrm{meV}$.

We study the dynamic long range interaction induced topological photonic bands and edge modes in a one-dimensional (1D) array of strongly dispersive gyromagnetic resonant cylinders. We propose a 1D topological model for such a dispersive gyromagnetic system and demonstrate that the dynamic long-range interaction can lead to localized topological edge modes, while the quasistatic interaction alone does not. Different from the conventional Su-Schrieffer-Heeger model that has only nearest-neighbor interactions, we find that the normal modes of the system coupled strongly to the photon mode of the background medium and the dynamic effects create a different band gap. Our results indicate the importance of the dynamic long-range interactions on the band structures and topological edge modes in strongly dispersive gyromagnetic systems.

Analogue physics became very popular in recent decades. It allows simulating inaccessible physical phenomena, such as black holes, in the laboratory. The first success of analogue physics is in fact much older being due to Maxwell, who derived his equations for the electromagnetic field by analogy with fluid dynamics in presence of vortices. Here we propose to use vortices for analogue gravity. We implement an acoustic Kerr black hole with quantized angular momentum in a Bose-Einstein condensate. We show that the condensate's metric is equivalent to the Kerr's one, exhibiting a horizon and an ergosphere. We confirm that this metric is obeyed not only by weak density waves, but also by quantum vortices which behave as massive test particles. We use these topological defects to demonstrate a quantum Penrose effect, extracting the rotation energy of the black hole by quanta of angular momentum. The particle trajectories are well described by the timelike geodesics of the Kerr metric, confirming the potential of analogue quantum gravity.

Motivated by flat lensing effects, now commonplace utilizing negative refraction with finite slabs, we emulate negative refraction upon a graded line array. We do so in the setting of flexural waves on a structured Kirchhoff-Love elastic plate, as a paradigm in wave physics, that has direct extensions to electromagnetic and acoustic wave systems. These graded line arrays are geometrically simple and provide strong coupling from the array into the bulk. A thorough analysis of the dispersion curves, and associated mode structure, of these meta-arrays, supported by mode coupling theory, creates array-guided wave reversal and hybridization into the bulk that leads to striking wave control via generalized flat lensing.