Using double quantum dots as the weak link of a Josephson junction, we realize the superconducting analog of the celebrated two-impurity Kondo model. The device shows a cusped current-voltage characteristic, which can be modeled by an overdamped circuit relating the observed cusp current to the Josephson critical current. The gate dependence of the cusp current and of the subgap spectra are used as complementary ground-state indicators to demonstrate gate-tuned changes of the ground state from an interdot singlet to independently screened Yu-Shiba-Rusinov (YSR) singlets. In contrast to the two-impurity Kondo effect in normal-state systems, the crossover between these two singlets is heralded by quantum phase boundaries to nearby doublet YSR phases in which only a single spin is screened.

Hydrodynamic phonon transport in solids exhibits unique thermal transport behaviors, such as second sound, the Poiseuille flow, and ultrahigh thermal conductivity. However, those have been limited up to the cryogenic temperature ($\sim 1$ K) for a few materials. In this work, by employing the phonon Boltzmann transport equation, we demonstrate hydrodynamic phonon transport in organic systems such as bulk crystalline polymers. Remarkable hydrodynamic phonon transport up to 50 K is demonstrated for both polyacene ${(-{\mathrm{C}}_4{\mathrm{H}}_2-)}_n$ and polyacetylene ${(-{\mathrm{C}}_2{\mathrm{H}}_2-)}_n$ crystals. More interestingly, a weak phonon hydrodynamic behavior takes place in crystalline polyethylene ${(-{\mathrm{C}}_2{\mathrm{H}}_4-)}_n$ in the intermediate-temperature range around 120 K, different from the observed hydrodynamics in most systems. The spectral phonon analysis reveals that the torsional motion of the $A_2$ mode causes this unique hydrodynamic behavior. A modified criterion for the emergence of hydrodynamic phonon transport is proposed which agrees quantitatively with the results from thermal conductivity and phonon drifting component calculations. This study provides physical insights into the understanding of phonon hydrodynamics in organic materials and also a reliable criterion to probe the hydrodynamic phonon transport in complex systems.

We study the transport properties of a hybrid junction made of a ferromagnetic lead in electrical connection with the helical edge modes of a two-dimensional topological insulator. In this system, the time-reversal symmetry, which characterizes the ballistic edge modes of the topological insulator, is explicitly broken inside the ferromagnetic region. This conflict situation generates unusual transport phenomena at the interface which are the manifestation of the interplay between the spin polarization of the injected current and the spin-momentum locking mechanism operating inside the topological insulator. We show that the spin-polarized current originated in the ferromagnetic region is asymmetrically divided in spatially separated branch currents sustained by edge channels with different helicity inside the topological insulator. The above findings provide the working principle of a topological current divider in which the relative intensity of the branch currents is determined by the polarization of the incoming current. We discuss the relevance of this effect in spintronics where, for instance, it offers an alternative way to measure the current polarization generated by a ferromagnetic electrode.

We demonstrate a regime in which matter-wave condensates of exciton-polaritons trapped in an elliptically shaped two-dimensional potential appear as a coherent mixture of ground and first-excited states of the quantum harmonic oscillator. This system resembles an optically controllable two-level system and produces near-terahertz harmonic oscillations of the condensate's center of mass along the major axis of the elliptical trapping potential. The population ratio between the two trap levels is tunable through the excitation laser power and is shown to follow Lotka-Volterra dynamics. Furthermore, we demonstrate coherence formation between two spatially displaced trapped condensate oscillators—the polaritonic analog of Huygen's clock synchronization for coupled condensate oscillators.

We study time-dependent electrical properties of the Su-Schrieffer-Heeger (SSH) chain and coupled SSH chains on a substrate. Focusing on the midgap edge state dynamics we consider the abrupt transition from the normal to the SSH chain and determine characteristic timescale needed for topological states to develop. We have found that the midgap state is formed from the inside peaks of the normal chain density of states. For a ladderlike system we show that the edge SSH state vanishes in time or oscillates between neighboring sites. Moreover, for nonadiabatical time-dependent perturbations the midgap state can partially leak to other sites leading to induced topological states inside the trivial chain. We also analyze the mean-field correlation effects between the coupled chains revealing the induced Friedel oscillations in nontrivial chains.

The binary oxides MgO, CaO, ZnO, and CdO have a variety of band gaps which enables band-gap engineering over a wide range. ZnO takes the wurtzite structure, while MgO, CaO, and CdO take the rocksalt structure. We examine the formal polarization in these materials and in particular the formal polarization difference at the interface between ZnO and each of the rocksalt oxides. We find that any rocksalt oxide (including alloys) strained to ZnO will exhibit a very large polarization charge discontinuity of $-0.907{\mathrm{Cm}}^{-2}$. Heterostructures between ZnO and the rocksalt oxides can therefore host a free-carrier gas with sheet densities up to $5.7\times {10}^{14}{\mathrm{cm}}^{-2}$. Such large polarization fields and carrier densities may allow applications in tunnel junctions or as contact layers for UV devices.

We demonstrate that anyons on wire networks have fundamentally different braiding properties than anyons in two dimensions (2D). Our analysis reveals an unexpectedly wide variety of possible non-Abelian braiding behaviors on networks. The character of braiding depends on the topological invariant called the connectedness of the network. As one of our most striking consequences, particles on modular networks can change their statistical properties when moving between different modules. However, sufficiently highly connected networks already reproduce the braiding properties of 2D systems. Our analysis is fully topological and independent on the physical model of anyons.

Motivated by the recent discovery of a large anomalous Nernst effect in ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Ga}$, ${\mathrm{Fe}}_{3}X$ ($X$=Al, Ga) and ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, we performed a first-principles study to clarify the origin of the enhancement of the transverse thermoelectric conductivity ${\alpha }_{ij}$ in these ferromagnets. The intrinsic contribution to ${\alpha }_{ij}$ can be understood in terms of the Berry curvature $\mathrm{\Omega }$ around the Fermi level, and $\mathrm{\Omega }$ is singularly large along nodal lines (which are gapless in the absence of the spin-orbit coupling) in the Brillouin zone. We find that not only the Weyl points but also stationary points in the energy dispersion of the nodal lines play a crucial role. The stationary points make sharp peaks in the density of states projected onto the nodal line, clearly identifying the characteristic Fermi energies at which ${\alpha }_{ij}$ is most dramatically enhanced. We also find that ${\alpha }_{ij}/T$ breaks the Mott relation and show a peculiar temperature dependence at these energies. The present results suggest that the stationary points will give us a useful guiding principle to design magnets showing a large anomalous Nernst effect.

Owing to its hexagonal symmetry, indirect band gap, and relatively large unit cell, the electronic band structure of $4H$-SiC is comprised of a complicated series of anisotropic valence and conduction band extrema even very near to the uppermost valence band maximum and lowest conduction band minimum. This has presented a difficult challenge to those experiments which have attempted to resolve the small energy separations between these band extrema. To overcome this challenge, we have measured the wavelength-modulated absorption (WMA) spectrum of $4H$-SiC over a broader wavelength range (3500–3800 Å) and at a higher resolution ($<0.1$ Å) than in previous work. By comparing these measurements with the low-temperature photoluminescence spectrum in ultrapure $4H$-SiC, we have identified several features, which we attribute to a $56\pm 3$ meV crystal-field splitting of the valence band maximum or a $136\pm 3$ meV separation between the two lowest conduction band minima. We also show that the spin-orbit split-off valence band, which has been observed in previous measurements of $4H$-SiC, contributes to nonparabolic dispersion near the valence band maximum, and this is responsible for several previously misidentified features in the WMA spectrum. Finally, we report the first experimental measurement of fine structure splittings in the free exciton ground state, which manifests as four small ($0.7\pm 0.1$ meV) splittings in the WMA spectrum due to mass anisotropy and electron-hole exchange interaction.

In this paper, we experimentally demonstrate an oscillating energy shift of center-of-mass quantum-confined exciton levels in a semiconductor quantum well after excitation into a superposition of two quantum confined exciton states of different parity. Oscillations are observed at frequencies corresponding to the quantum beats between these states. We show that the observed effect is a manifestation of the exciton density oscillations in the real space similar to the dynamics of an electron in a Hertzian dipole. The effect is caused by the exciton-exciton exchange interaction and appears only if the excitons are in a superposition quantum state. This effect exists until the Raman coherence in the interacting excitons has decayed. This effect may be harnessed for quantum technologies requiring the quantum coherence of states.

As electronic devices shrink down to their ultimate limit, the fundamental understanding of interfacial thermal transport becomes essential in thermal management. However, a comprehensive understanding of phonon transport mechanisms that drive interfacial thermal transport is still under development. The thermal transport across interfaces can be strongly affected by factors such as crystalline structure, surface roughness, chemical diffusion, etc. These complications lead to a significant quantitative uncertainty between experimentally measured thermal boundary conductance (TBC) across real material interfaces and theoretically calculated TBCs that are often predicted on structurally and/or chemically ideal interfaces. In this paper, we report on the thermal conductance across interfaces between various epitaxially grown metal films (Co, Ru, and Al) and $c$-plane sapphire substrates via time-domain thermoreflectance over the temperature range of ∼80 to ∼500 K. The room-temperature interface conductances of Al/sapphire, Co/sapphire, and Ru/sapphire are all $\sim 350\mathrm{MW}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ despite the phonon spectra differences among the metals. We compare our results to predictions of TBC using atomistic Green's function calculations and the modal nonequilibrium Landauer method with transmission from the diffuse mismatch model. We found a consistent quantitative agreement between the experimentally measured TBCs and the predictions using the modal nonequilibrium Landauer model for the $\mathrm{Al}/{\mathrm{Al}}_2{\mathrm{O}}_3$, $\mathrm{Co}/{\mathrm{Al}}_2{\mathrm{O}}_3$, and $\mathrm{Ru}/{\mathrm{Al}}_2{\mathrm{O}}_3$ interfaces. This result suggests that interfacial elastic phonon thermal transport dominates TBC for the various epitaxial metal/sapphire combinations of interest in this work, while other mechanisms are negligible.

The long-range Su-Schrieffer-Heeger (SSH) model, in which the second nearest-neighbor hopping is taken into account, exhibits a topological phase diagram that contains winding numbers $w=0,1$, and 2. In the clean system, the change in winding number stems from the band-touching phenomenon. In the presence of disorder, the renormalization of energy band and Fermi level results in the nonzero density of states in the energy gap. These midgap states cause the crossover phenomenon and the divergence of localization length at a critical disorder strength $U_c$ in the finite SSH system. In this study, we numerically computed the mean winding number and localization length for the disordered SSH system. We find that the disorder is able to drive phase transitions between different mean winding numbers: $w=0\to 1$, $0\to 2$, $1\to 2$, and $2\to 1$ in the weak disorder regime. By investigating the wave function distribution and the self-energy, the nonzero mean winding numbers correspond to the so-called topological Anderson insulating (TAI) phases. The finite size scaling for the mean winding number in the TAI phase is shown. For describing the phase transitions in the thermodynamic limit, we apply the criterion of band gap closure resulting from the broadening of energy band and Fermi level to determine the critical disorder strength. The critical disorder strength for self-consistent Born approximation (SCBA) $U_c^{\mathrm{SCBA}}$ and that for first Born approximation (FBA) $U_c^{\mathrm{FBA}}$ are numerically calculated. $U_c^{\mathrm{SCBA}}$ is found to match with $U_c$ qualitatively. Nonetheless, SCBA indicates the different roles of band shifts and Fermi level broadening near the topological phase transitions. Band shift/Fermi level broadening is more dominant for the transitions from low-to-high/high-to-low winding number. Interestingly, for the transition from bulk insulator to TAI, $U_c^{\mathrm{FBA}}$ is quantitatively closer to $U_c$ than $U_c^{\mathrm{SCBA}}$ as long as the renormalized band gap is zero within FBA.

We study the critical behavior of the Ising model in three dimensions on a lattice with site disorder by using Monte Carlo simulations. The disorder is either uncorrelated or long-range correlated with correlation function that decays according to a power law $r^{-a}$. We derive the critical exponent of the correlation length $\nu$ and the confluent correction exponent $\omega$ in dependence of $a$ by combining different concentrations of defects $0.05\le p_d\le 0.4$ into one global fit ansatz and applying finite-size scaling techniques. We simulate and study a wide range of different correlation exponents $1.5\le a\le 3.5$ as well as the uncorrelated case $a=\infty$ and are able to provide a global picture not yet known from previous works. Additionally, we perform a dedicated analysis of our long-range correlated disorder ensembles and provide estimates for the critical temperatures of the system in dependence of the correlation exponent $a$ and the concentration of defects $p_d$. We compare our results to known results from other works and to the conjecture of Weinrib and Halperin: $\nu =2/a$, and discuss the occurring deviations.

Backscattering Raman spectroscopic investigations were performed on an yttrium-iron garnet single crystal using linearly and circularly polarized light. A terahertz (THz) magnon of the Kaplan-Kittel (KK) exchange resonance was discovered, which had been regarded as unobservable via optical methods. The KK exchange resonance had a 7.8-THz frequency at 80 K, and the polarization selection rule led to an antisymmetric Raman tensor of the $A_2$ mode. Moreover, the assignment of all the Raman-active phonon modes, $3A_{1\mathrm{g}}, 8E_{\mathrm{g}}$, and $14T_{2\mathrm{g}}$, was proposed. This study will stimulate further investigation of the coupling of THz magnons and phonons and pave the way toward THz optomagnonics.

We show that the coupling between two ferromagnetic layers separated by a nonmagnetic spacer can be used to control the depinning of domain walls and induce unidirectional domain wall propagation. We investigated CoFeB/Ti/CoFeB trilayers where the easy axis of the magnetization of the top CoFeB layer is out of plane and that of the bottom layer is in plane. Using magneto-optic Kerr effect microscopy, we find that the depinning of a domain wall in the perpendicularly magnetized CoFeB layer is influenced by the orientation of the magnetization of the in-plane layer, which gives rise to a field-driven asymmetric domain expansion. This effect occurs due to the magnetic coupling between the internal magnetization of the domain wall and the magnetization of the in-plane CoFeB layer, which breaks the symmetry of up-down and down-up homochiral Néel domain walls in the perpendicular CoFeB layer. Micromagnetic simulations support these findings by showing that the interlayer coupling either opposes or favors the Dzyaloshinskii-Moriya interaction in the domain wall, thereby generating an imbalance in the depinning fields. This effect also allows for artificially controlling the chirality and dynamics of domain walls in magnetic layers lacking a strong Dzyaloshinskii-Moriya interaction.

We present a complete analysis of the Nernst signal due to superconducting fluctuations in a large variety of superconductors from conventional to unconventional ones. A closed analytical expression of the fluctuation contribution to the Nernst signal is obtained in a large range of temperature and magnetic field. We apply this expression directly to experimental measurements of the Nernst signal in ${\mathrm{Nb}}_x{\mathrm{Si}}_{1-x}$ thin films and ${\mathrm{URu}}_2{\mathrm{Si}}_2$ superconductors. Both magnetic field and temperature dependence of the available data are fitted with very good accuracy using only two fitting parameters, the superconducting temperature $T_{\mathrm{c}0}$ and the upper critical field $H_{\mathrm{c}2}\left(0\right)$. The obtained values agree very well with experimentally obtained values. We also extract the ghost lines (the maximum of the Nernst signal for constant temperature or magnetic field) from the complete expression and also compare it to several experimentally obtained curves. Our approach predicts a linear temperature dependence for the ghost critical field well above $T_{\mathrm{c}0}$. Within the errors of the experimental data, this linearity is indeed observed in many superconductors far from $T_{\mathrm{c}0}$.

In this paper, we propose a way to realize topological $s$-wave superconductivity with the application of circularly polarized laser light in two-dimensional bilayer transition metal dichalcogenides (TMDs). Using Floquet theory, we analyze a tight-binding model of bilayer TMDs with time-periodic electric fields. After deriving an effective Hamiltonian, we investigate topological properties of the $s$-wave superconducting state. The laser light induces valley-dependent layer polarization and makes the system a topologically nontrivial superconducting state characterized by the Chern number. We show topological phase diagrams in the absence and presence of the Kane-Mele spin-orbit coupling which causes hidden spin polarization in bilayer TMDs. Although the topological phase diagram is affected by the spin-orbit coupling, topological superconductivity can be realized without relying on the spin-orbit coupling in sharp contrast to a previous proposal of laser-induced topological superconductivity [Takasan et al., Phys. Rev. B 95, 134508 (2017)]. We also discuss experimental setups to detect the topological phase.

Polariton graphs were recently proposed and validated as a novel platform for solving hard optimization problems that can be mapped into the Ising or $XY$ models. Here, we elucidate a relationship between the energy spectrum of the $XY$ Hamiltonian, Hermitian and non-Hermitian, and the total number of condensed polariton particles. Using a hexagonal unit lattice we show that the lower-energy states of the $XY$ Hamiltonian are faithfully reproduced by mean-field simulations. We further confirm experimentally a possibility of finite spectral gaps near the condensation threshold for the triangular lattice configurations of polariton condensates. Our study paves the way to simulating the spectral gap of Hermitian and non-Hermitian $XY$ models using polariton graphs.

Thanks to the recent discovery of the magic-angle bilayer graphene, twistronics is quickly becoming a burgeoning field in condensed matter physics. This Rapid Communication expands the realm of twistronics to acoustics by introducing twisted bilayer phononic graphene, which remarkably also harbors the magic angle, evidenced by the associated ultraflat bands. Beyond mimicking quantum-mechanical behaviors of twisted bilayer graphene, we show that their acoustic counterpart offers a considerably more straightforward and robust way to alter the interlayer hopping strength, enabling us to unlock magic angles ($>3^{\circ }$) inaccessible in classical twisted bilayer graphene. This study not only establishes the acoustical analog of twisted (magic-angle) bilayer graphene, providing a test bed more easily accessible to probe the interaction and misalignment between stacked two-dimensional materials, but also points out the direction to a new phononic crystal design paradigm that could benefit applications such as enhanced acoustic emission and sensing.