We measured the local magnetic response of a niobium thin film by applying a millitesla-scale AC magnetic field using a micron-scale field coil and detecting the response with a micron-scale pickup loop in a scanning superconducting quantum interference device (SQUID) susceptometry measurement. Near the film's critical temperature, we observed a steplike nonlinear and dissipative magnetic response due to the dynamics of a small number of vortex-antivortex pairs induced in the film by the local applied AC field. We modeled the dynamics of the measurement using a combined two-dimensional London-Maxwell and time-dependent Ginzburg-Landau approach, allowing us to construct a detailed real-space picture of the vortex motion causing the observed dissipative response. This work pushes scanning SQUID susceptometry of two-dimensional superconductors beyond the regime of linear response and lays the foundation for microscopic studies of vortex dynamics and pinning in superconducting devices and more exotic materials systems.

We study a novel equal-spin pairing state with opposite center-of-mass momentum for each spin polarization. This state, dubbed a Kramers Fulde-Ferrell (KFF) state, respects time-reversal symmetry and can be realized in a one-dimensional system with spin-orbit coupling and nearest-neighbor attraction. We find that the KFF state supports nonreciprocal spin transport for both bulk superconductor and Josephson junctions. In addition to the spin Josephson diode effect, the charge transport is controlled by intriguing dynamics of bound states whose transitions can be manipulated by the length of the KFF superconductor. The KFF state is relevant for embedded quantum structures in monolayer Fe-based superconductors and dissipationless superconducting spintronics.

In this work, we show that a buckled honeycomb lattice can host a boundary-obstructed topological superconductor (BOTS) in the presence of $f$-wave spin-triplet pairing (fSTP). The underlying buckled structure allows for the manipulation of both chemical potential and sublattice potential using a double gate setup. Although a finite sublattice potential can stabilize the fSTP with a possible higher-order band topology, because it also breaks the relevant symmetry, the stability of the corner modes is not guaranteed. Here we show that the fSTP on the honeycomb lattice gives a BOTS under nonzero sublattice potential, thus the corner modes can survive as long as the boundary is gapped. Also, by examining the large sublattice potential limit where the honeycomb lattice can be decomposed into two triangular lattices, we show that the boundary modes in the normal state are the quintessential ingredient leading to the BOTS. Thus the effective boundary Hamiltonian becomes nothing but the Hamiltonian for Kitaev chains, which eventually gives the corner modes of the BOTS.

Having the potential for performing quantum computation, topological superconductors have been generalized to the second-order case. The hybridization of different orders of topological superconductors is attractive because it facilitates the simultaneous utilization of their respective advantages. However, previous studies found that they cannot coexist in one system due to the constraint of symmetry. We propose a Floquet engineering scheme to generate two-dimensional (2D) hybrid-order topological superconductors in an odd-parity superconductor system. Exotic hybrid-order phases exhibiting coexisting gapless chiral edge states and gapped Majorana corner states not only in two different quasienergy gaps but also in one single quasienergy gap are created by periodic driving. The generalization of this scheme to the 3D system allows us to discover a second-order Dirac superconductor featuring coexisting surface and hinge Majorana Fermi arcs. Our results break a fundamental limitation on topological phases and open a feasible avenue to realize topological superconductor phases without static analogs.

Photoluminescence and polarized reflection spectra of quantum well structures with symmetric ${\mathrm{Cd}}_{0.9}{\mathrm{Zn}}_{0.1}\mathrm{Te}/\mathrm{CdTe}/{\mathrm{Cd}}_{0.9}{\mathrm{Zn}}_{0.1}\mathrm{Te}$ and asymmetric ${\mathrm{Cd}}_{0.9}{\mathrm{Zn}}_{0.1}\mathrm{Te}/\mathrm{CdTe}/{\mathrm{Cd}}_{0.4}{\mathrm{Mg}}_{0.6}\mathrm{Te}$ barriers were studied. The Stokes parameters of the reflected light from these structures were measured. In structures with asymmetric barriers, in the region of exciton resonances, the phenomenon of light birefringence was detected, caused by a lower symmetry of the interfaces compared with the symmetry of bulk crystals. A discussion of this phenomenon is given.

Around 50 years ago, ${\mathrm{LiTi}}_2{\mathrm{O}}_4$ was reported to be the only spinel oxide to exhibit a superconducting transition with the highest $T_c\approx 13.7$ K. Recently, ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ has been found to be another spinel oxide to reveal a superconducting transition with $T_c\approx 3$ K, however, its superconducting state is realized only in thin film superlattices involving ${\mathrm{SrTiO}}_3$. We find that a V-doped ${\mathrm{Mg}}_{1-x}{\mathrm{Ti}}_2{\mathrm{O}}_4$ phase, which gets stabilized as a thin surface layer on top of a nearly stoichiometric and insulating V-doped ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ bulk sample, exhibits high-temperature superconductivity with $T_c\approx 16$ K. The superconducting transition is also confirmed through a concomitant sharp diamagnetic transition immediately below $T_c$. The spinel phase of the superconducting surface layer is conformed through grazing-incidence x-ray diffraction and micro-Raman spectroscopy. A small shift of the sharp superconducting transition temperature ($\sim 4$ K) with the application of a high magnetic field (up to 9 T) suggests a very high critical field for the system, $\sim 25$ T. Thus, V-doped ${\mathrm{Mg}}_{1-x}{\mathrm{Ti}}_2{\mathrm{O}}_4$ exhibits the maximum $T_c$ among spinel superconductors and also possesses a very high critical field.

Charge density wave (CDW) order is widely existing and fundamentally important in solid-state physics. However, several critical issues regarding the vibrational and electronic subsystems and their coupling still need to be better understood. Here, we tune the electrical transport and collective vibrational excitation, i.e., phonon and amplitude mode, by pressure in a prototype charge density wave material, $2H\text{−}{\mathrm{NbSe}}_2$. A complete pressure-temperature phase diagram is revisited. The anomaly in Hall and magnetoresistivity at CDW critical temperature, $T_{\mathrm{CDW}}$, was suppressed by the pressure. In the Raman spectroscopy measurements, the appearance of CDW amplitude mode is accompanied by the freezing of the two-phonon mode. The frequency of CDW amplitude mode under pressure follows modified mean-field theory with power-law scaling $\left(\beta =0.18\right)$. The renormalization of the Raman phonon across the CDW transition and the mean-field temperature dependence of CDW amplitude mode emphasized the importance of electron-phonon coupling in the formation of CDW state in $2H\text{−}{\mathrm{NbSe}}_2$. Our work clarifies the complex vibrational and electronic subsystems and sheds light on the mechanism of the charge density state in $2H\text{−}{\mathrm{NbSe}}_2$.

Bosonic condensates of spinless noninteracting particles confined on a ring cannot propagate circular periodic currents once rotation symmetry of the system is broken. However, a persistent current may appear due to interparticle interactions exceeding some critical strength. In this up-critical regime breaking of the symmetry between the clockwise and anticlockwise rotations takes place. We consider this symmetry-breaking scenario in the case of a spinor condensate of exciton polaritons trapped on a ring split by a potential barrier. Due to the intrinsic symmetry of the effective spin-orbit interaction, which stems from the linear splitting between transverse-electric and transverse-magnetic microcavity modes, the potential barrier blocks the circulating current and imposes linear polarization patterns. On the other hand, circularly polarized polaritons form circular currents propagating in opposite directions with equal absolute values of angular momentum. In the presence of interparticle interactions, the symmetry of clockwise and anticlockwise currents can be broken spontaneously. We describe several symmetry-breaking scenarios, which imply either restoration of the global condensate rotation or the onset of the circular polarization in the symmetry-broken state.

The formation energy of an arbitrary graphene edge or that of other 2D materials has been estimated as a summation of the armchair (AC) and zigzag (ZZ) edge sites. Such an estimation assumes that each site is independent from its neighboring sites, which is unlikely due to the overlap of electron densities. Here, we show that to accurately calculate the formation energy of graphene edges with various functional groups the energy of the junction between AC and ZZ sites, the “kink energy,” is essential. It is significant that the kink energies of graphene edges with different functional groups are all negative, namely, kink formation stabilizes the chiral graphene edges.

So far, the emergence of perfect absorption in a disordered medium at a given wavelength has been limited to specifically designed incoming waves, or the joint use of metasurfaces. By relying on the effects of cooperative scattering, we numerically demonstrate up to $95\%$ of absorption for versatile radiations incident on optimized mixtures of randomly distributed plasmonic and lossless particles. Our simulations show that the power entering the structure is absorbed on extremely short depths, allowing the numerical sample to be of subwavelength thickness. In addition, the design includes the property of spectral selectivity, which has implications for heat transfer technologies.

Lithium niobate is an electro-optic material with many applications in microwave and optical signal processing, communication, quantum sensing, and quantum computing. In this Letter, we present findings on evaluating the complex electromagnetic permittivity of lithium niobate at millikelvin temperatures. Measurements are carried out using a resonant-type method with a superconducting radio-frequency cavity operating at 7 GHz and designed to characterize anisotropic dielectrics. The relative permittivity tensor and loss tangent are measured at 50 mK with unprecedented accuracy.

We investigate the topological Hall effect (THE) in the monoaxial chiral crystal ${\mathrm{GdPt}}_2\mathrm{B}$, a recently discovered compound that exhibits putative helimagnetism below 87 K. The distinct THE was observed in ${\mathrm{GdPt}}_2\mathrm{B}$ in the magnetically ordered state. The scaling relations for anomalous and topological Hall conductivities differed from those of conventional models based on the scattering process. We further demonstrate the clear scaling behavior of the THE in a wide temperature range, which we attribute to the monoaxial Dzyaloshinskii-Moriya (DM) interaction under external magnetic fields perpendicular to the screw axis. The THE induced by the monoaxial DM interaction as well as the THE in a monoaxial chiral crystal of an $f$-electron system are demonstrated in this study.

Most of the H-rich superconductors have so far been discovered to be clathrate structures. Several clathrate hydrides, including ${\mathrm{H}}_3\mathrm{S}$, ${\mathrm{LaH}}_{10}$, and ${\mathrm{CaH}}_6$, were predicted to be excellent superconductors through structural searches and were later confirmed through experimental synthesis. Here, we conduct extensive crystal structure searches and first-principles calculations for ternary Na-P-H hydrides under high pressure. Two stoichiometries of ${\mathrm{NaPH}}_6$ and ${\mathrm{NaPH}}_8$ in ternary Na-P-H hydrides are found to be stable under 200 GPa. The $P\overline{6}m2$ phase of the ${\mathrm{NaPH}}_6$ hydride is identified as a novel ternary layered H-rich superconductor, exhibiting a high critical temperature $T_c$ of 112.2 K under 200 GPa. Our calculations indicate that the high $T_c$ of the ${\mathrm{NaPH}}_6$ hydride under high pressure is attributed to the high density of hydrogen $s$ states present at the Fermi level. Furthermore, a metastable $Pm\overline{3}$ phase of the clathrate ${\mathrm{NaPH}}_6$ hydride is uncovered under 350 GPa and displays an exceptional $T_c$ of 201.4 K. These results suggest that ternary alkali metal Na-based hydrides hold promise as high-$T_c$ superconductors under high pressure and offer critical insights into the design and synthesis of novel category high-temperature superconductors.

Superconductors lacking both inversion symmetry and time-reversal symmetry have been attracting much attention as a platform for exotic superconducting phases and anomalous phenomena, including the superconducting diode effect. Recent studies revealed intrinsic phases with this symmetry, named anapole superconductivity and monopole superconductivity, which are $PT$-symmetric superconducting states with and without Cooper pairs' total momentum, respectively. To explore characteristic phenomena in these states, we calculate and predict the superconducting piezoelectric effect and superconducting diode effect. A close relationship with the finite-$q$ pairing, asymmetric Bogoliubov spectrum, and quantum geometry is discussed. This study reveals the piezoelectric and diode effects as potential probes to elucidate exotic superconducting phases.

Anapole superconductivity recently proposed for multiband superconductors [S. Kanasugi and Y. Yanase, Commun. Phys. 5, 39 (2022)] is a key feature of time-reversal ($\mathcal{T}$) symmetry broken polar superconductors. The anapole moment was shown to arise from the asymmetric Bogoliubov spectrum, which induces finite center of mass momenta of Cooper pairs at the zero magnetic field. In this paper, we show an alternative mechanism of anapole superconductivity: the quantum geometry induces the anapole moment when the interband pairing and Berry connection are finite. Thus, the anapole superconductivity is a ubiquitous feature of $\mathcal{T}$-broken multiband polar superconductors. Applying the theory to a minimal model of ${\mathrm{UTe}}_2$, we demonstrate the quantum geometry induced anapole superconductivity. Furthermore, we show the Bogoliubov Fermi surfaces (BFS) in an anapole superconducting state and predict an unusual temperature dependence of BFS due to the quantum geometry. Experimental verification of these phenomena may clarify the superconducting state in ${\mathrm{UTe}}_2$ and reveal the ubiquitous importance of quantum geometry in exotic superconductors.

Motivated by recent experiments on pump-induced polar ordering in the quantum paraelectric $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$, we study a driven phonon system close to a second-order phase transition. Analyzing its classical dynamics, we find that sufficiently strong driving leads to transitions into polar phases whose structures, determined by the light polarization, are not all accessible in equilibrium. In addition, for certain intensity profiles, we demonstrate the possibility of two-step transitions as a function of fluence. For even stronger field intensities, the possibility of period-doubling and chaotic behavior is demonstrated. Finally we develop a generalized formalism that allows us to consider quantum corrections to the classical dynamics in a systematic fashion. We predict a shift in the critical pump fluence due to quantum fluctuations with a characteristic dependence on the fluence increase rate that should be observable in experiment.

We explore the magnetic properties of a two-dimensional Hubbard model on an inhomogeneous square lattice, which provides a platform for tuning the bandwidth of the flat band. In its limit, this inhomogeneous square lattice turns into a Lieb lattice, and it exhibits abundant properties due to the flat band structure at the Fermi level. By using the determinant quantum Monte Carlo simulation, we calculate the spin susceptibility, double occupancy, magnetization, spin structure factor, and effective pairing interaction of the system. It is found that the antiferromagnetic correlation is suppressed by the inhomogeneous strength and that the ferromagnetic correlation is enhanced. Both the antiferromagnetic correlation and ferromagnetic correlation are enhanced as the interaction increases. It is also found that the effective $d$-wave pairing interaction is suppressed by the increasing inhomogeneity. In addition, we also study the thermodynamic properties of the inhomogeneous square lattice, and the calculation of specific heat provides good support for our point. Our intensive numerical results provide a rich magnetic phase diagram over both the inhomogeneity and interaction.

How is two-party entanglement (TPE) distributed in many-body systems? This is a fundamental issue because the total TPE between one party and all the other parties, ${\mathcal{C}}^N$, is upper bounded by the Coffman, Kundu, and Wootters monogamy inequality, from which ${\mathcal{C}}^N\le \sqrt{N-1}$ can be proved by the geometric inequality. Here we explore the total entanglement ${\mathcal{C}}^{\infty }$ and the associated total tangle ${\tau }^{\infty }$ in a $p$-wave free-fermion model with long-range interaction, showing that ${\mathcal{C}}^{\infty }\sim O\left(1\right)$ and ${\tau }^{\infty }$ may become vanishingly small with increasing long-range interaction. However, we always find ${\mathcal{C}}^{\infty }\sim 2\xi {\tau }^{\infty }$, where $\xi$ is the truncation length of entanglement, beyond which the TPE quickly vanishes; hence ${\tau }^{\infty }\sim 1/\xi$. This relation is a direct consequence of the exponential decay of the TPE induced by the long-range interaction. When $\xi$ is comparable to the system size $N$, this relation is reduced to the Koashi, Bužek, and Imoto (KBI) upper bound in the fully connected Lipkin-Meshkov-Glick model and Dicke model. Thus this relation can be regarded as a generalization of the KBI bound in quantum models, which may have intriguing applications in the more complicated many-body models.

An analytically solvable time-dependent coupled light-matter problem is presented. An $n$-electron system is confined by a harmonic-oscillator potential and interacts with photons in a cavity. Both the electrons and the photons can interact with a time-dependent external field. The light-matter coupling is described by the Pauli-Fierz Hamiltonian. By separating the relative and center-of-mass motion, the Hamiltonian of the system can be simplified to a sum of a Hamiltonian of the relative and the center-of-mass motion. The Hamiltonian of the relative motion is time-independent, not coupled to light, and it can be solved by conventional approaches. The Hamiltonian of the center-of-mass motion reduces to that of a time-dependent harmonic oscillator and can be solved analytically. The analytical solution will be used to study excitations, the high-harmonic-generation spectrum, and nonlinear optical properties in a cavity.