We study a long topological Josephson junction with a ferromagnetic strip between two superconductors. The low-energy theory exhibits a nonlocal in time and space interaction between chiral Majorana fermions, mediated by the magnonic excitations in the ferromagnet. While short-ranged interactions turn out to be irrelevant by power counting, we show that sufficiently strong and long-ranged interactions may induce a ${\mathbb{Z}}_2$-symmetry breaking. This spontaneous breaking leads to a tilting of the magnetization perpendicular to the Majorana propagation direction and the opening of a fermionic gap (Majorana mass). It is analogous to the Peierls instability in the commensurate Fröhlich model and reflects the nontrivial interplay between Majorana modes and magnetization dynamics. Within a Gaussian fluctuation analysis, we estimate critical values for the temporal and spatial nonlocality of the interaction, beyond which the symmetry breaking is stable at zero temperature—despite the effective one dimensionality of the model. We conclude that nonlocality, i.e., the stiffness of the magnetization in space and time, stabilizes the symmetry breaking. In the stabilized regime, we expect the current-phase relation to exhibit an experimentally accessible discontinuous jump. At nonzero temperatures, as usual in the 1D Ising model, the long-range order is destroyed by solitonic excitations, which in our case carry each a Majorana zero mode. In order to estimate the correlation length, we investigate the solitons within a self-consistent mean-field approach.

We propose a refrigeration scheme in a mesoscopic superconductor–quantum dot hybrid device. The setup can significantly cool down a normal metal coupled to the device by applying a bias voltage across the system. We demonstrate that the cooling power can be as large as $0.05{\mathrm{\Delta }}_0^2/h$ where ${\mathrm{\Delta }}_0$ is the absolute value of the superconducting order parameter. In contrast to previous proposals, our device operates without any magnetic elements, such as ferromagnetic reservoirs or Zeeman splittings. The refrigeration scheme works over a broad parameter range and can be optimized by tuning system parameters, such as level position and bias voltage. Our theory self-consistently determines the temperature drop of the normal reservoir in the nonlinear transport regime including electron-electron interactions at the mean-field level. Finally, we evaluate the refrigeration performance and find efficiencies as large as half of the Carnot bound for realistic values of the coupling strength.

We report the dynamical magnetic properties of the $s=\frac{3}{2}$ kagome ferromagnet ${\mathrm{Li}}_9{\mathrm{Cr}}_3{\left({\mathrm{P}}_2{\mathrm{O}}_7\right)}_3{\left({\mathrm{PO}}_4\right)}_2$ using magnetic susceptibility, specific heat, electron spin resonance (ESR), and nuclear magnetic resonance techniques. The magnetic susceptibility and specific heat show ferromagnetic ordering at $T_{\mathrm{C}}=2.7$ K. The in-plane ferromagnetic correlation length ${\xi }_{ab}$ deduced from the magnetization curves reveals that ${\xi }_{ab}$ increases steeply from 18.2 to 41.3 Å as the temperature is lowered from 50 K to $T_{\mathrm{C}}$, indicative of the persistence of two-dimensional magnetic correlations up to temperatures as high as $\sim 18T_{\mathrm{C}}$. The ESR linewidth $\mathrm{\Delta }{H}_{\mathrm{pp}}$ and magnetic specific heat data indicate a change in spin dynamics at about $T^{*}\approx 4.4T_{\mathrm{C}}$. Intriguingly, for temperatures below 50 K, we observe an unexpected suppression of the ${}^7\mathrm{Li}$ spin-lattice relaxation rate $1/T_1$. The decrease in $1/T_1$, which occurs well above $T_{\mathrm{C}}$, is at odds with the critical slowing down of magnetic correlations typically observed in conventional ferromagnets. This anomalous thermal evolution of spin fluctuations is interpreted in terms of the concerted interplay of flat and Dirac magnon bands inherent in kagome ferromagnets.

We investigate the interaction between surface acoustic waves (SAWs) and spin waves (SWs) in a Pt/Co($2\mathrm{nm}$)/Ru($0.85\mathrm{nm}$)/Co($2\mathrm{nm}$)/Pt compensated synthetic antiferromagnet (SAF) composed of two ferromagnetic layers with equal thicknesses separated by a thin nonmagnetic Ru spacer layer. Because of the combined presence of interlayer dipolar coupling fields and interfacial Dzyaloshinskii–Moriya interaction (iDMI), the optical SW mode shows a large nondegenerate dispersion relation for counter-propagating SWs. Due to resonant SAW-SW interaction, we observe a nonreciprocal SAW transmission in the prepared piezoelectric/SAF hybrid device. We demonstrate that the nonreciprocity of the SAW transmission in symmetric SAFs with equal thicknesses of the magnetic layers can show a substantially different characteristic behavior in comparison to asymmetric SAFs or magnetic single layers with iDMI. For the prepared SAF, the nonreciprocal shift of the magnetoacoustic resonance fields and the magnetoacoustic SW excitation efficiency depend on the external magnetic field sweep direction. For one magnetic field sweep direction and angle of the magnetic field, the resonance fields of the waves propagating in one direction are larger than for the waves propagating in the opposite direction. In addition, the magnitude of the nonreciprocal field shift is at minimum if the external magnetic field is aligned perpendicular to the SW propagation direction. The experimental results are in agreement with a phenomenological SAW-SW interaction model.

The recent observation of the light-induced superconducting (SC)-like transient response in $c$-axis optical conductivity far above the SC transition temperature $T_{\mathrm{c}}$ in underdoped $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ (YBCO) has attracted a great deal of attention in the field of high-$T_{\mathrm{c}}$ superconductors. Since then, various theoretical and experimental studies have been devoted to elucidating its microscopic origin. One prominent fingerprint of light-induced superconductivity is the emergence of $1/\omega$-like spectral behavior in the imaginary part of the optical conductivity in the terahertz (THz) frequency range. However, the spectral profile can also be described by the Drude response of the quasiparticles (QPs) with a substantially low scattering rate. To circumvent this critical ambiguity, we investigated the light-induced nonequilibrium state in an underdoped YBCO sample with $T_{\mathrm{c}}$ of 61 K using the nonlinear THz optical response originating from the SC collective excitation of the ac-driven Josephson current. Upon the near-infrared (NIR) photoexcitation above $T_{\mathrm{c}}$ in the YBCO sample, the $1/\omega$-like spectral behavior in the imaginary part of the optical conductivity emerges, consistent with previous studies. However, the THz third-harmonic generation arising from the ac-driven Josephson current along the $c$-axis was absent in the NIR photoexcited state. These results indicate that the NIR pump-induced state exhibiting the $1/\omega$-like response above $T_{\mathrm{c}}$ is distinct from the long-range-ordered SC state in equilibrium. Based on these observations, the possible origins of the irregularly coherent charge-carrier response along the $c$-axis induced by photoexcitation are discussed.

We show that in contrast to the conventional view of a mean-field Landau-type behavior, the oxygen octahedral tilt $\left(R\right)$ and polarization $\left(P\right)$ in the $A_3{B}_2{\mathrm{O}}_7$ Ruddlesden-Popper hybrid improper ferroelectric ${(\mathrm{Ca},\mathrm{Sr})}_3{\mathrm{Ti}}_2{\mathrm{O}}_7$ exhibits the scaling behavior of a 2D Ising class near the para-to-ferroelectric transition temperature ($T_{\mathrm{c}}$). We also show that $P$ exhibits an additional transition from a 2D Ising to a 3D mean-field critical behavior at a temperature $T_{\mathrm{d}}<T_{\mathrm{c}}$, which is driven by enhanced interlayer dipolar correlations due to an increase in the ferroelectrically active $B$-site centered dipoles.

While the spin-liquid state in the frustrated pyrochlore ${\mathrm{Tb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7+y}$ has been studied both experimentally and theoretically for more than two decades, a definite description of this unconventional state still needs to be achieved. Using synchrotron-based THz spectroscopy in combination with quantum numerical simulations, we highlight a significant link between two features: the existence of a quadrupolar order following an ice rule and the presence of strong magnetoelastic coupling in the form of hybridized ${\mathrm{Tb}}^{3+}$ crystal-field and phonon modes. The magnitude of this so-called vibronic process, which involves quadrupolar degrees of freedom, is significantly dependent on small off-stoichiometry $x$ and favors all-in–all-out-like correlations between quadrupoles. This mechanism competes with the long-range ordered quadrupolar ice, and for slightly different stoichiometry, is able to destabilize it.

We report a study of the noncentrosymmetric TaReSi superconductor by means of the muon-spin rotation and relaxation $\left(\mu \mathrm{SR}\right)$ technique, complemented by electronic band-structure calculations. Its superconductivity, with $T_c=5.5\text{K}$ and upper critical field ${\mu }_0{H}_{\mathrm{c}2}\left(0\right)\sim 3.4\text{T}$, was characterized via electrical-resistivity and magnetic-susceptibility measurements. The temperature-dependent superfluid density, obtained from transverse-field $\mu \mathrm{SR}$, suggests a fully gapped superconducting state in TaReSi, with an energy gap ${\mathrm{\Delta }}_0=0.79\text{meV}$ and a magnetic penetration depth ${\lambda }_0=562\text{nm}$. The absence of a spontaneous magnetization below $T_c$, as confirmed by zero-field $\mu \mathrm{SR}$, indicates a preserved time-reversal symmetry in the superconducting state. The density of states near the Fermi level is dominated by the Ta- and Re-$5d$ orbitals, which account for the relatively large band splitting due to the antisymmetric spin-orbit coupling. In its normal state, TaReSi behaves as a three-dimensional Kramers nodal-line semimetal, characterized by an hourglass-shaped dispersion protected by glide reflection. By combining nontrivial electronic bands with intrinsic superconductivity, TaReSi is a promising material for investigating the topological aspects of noncentrosymmetric superconductors.

Single crystal of the binary equiatomic compound GdBi crystallizing in the rock-salt-type cubic crystal structure with the space group $Fm\overline{3}m$ has been grown by flux method. The electrical and magnetic measurements have been performed on well-oriented single crystals. The antiferromagnetic ordering of the Gd moments is confirmed at $T_{\mathrm{N}}=27.5$ K. The magnetization measurement performed at 2 K along the principal crystallographic direction [100] attained a value of $1.3{\mu }_B$/Gd in a field of 7 T, which is about 5 times smaller than its saturation value of $7{\mu }_B$. Zero-field electrical resistivity reveals a sudden drop at 27.5 K, suggesting a reduction in the spin disorder scattering due to the antiferromagnetic alignment of the Gd moments. The residual resistivity at 2 K is 390 $\mathrm{n}\mathrm{\Omega }\mathrm{cm}$, suggesting a good quality of the grown crystal. The magnetoresistance attains a value of ${10}^4\%$ with no signature of saturation, in a field of 14 T at $T=2$ K. Shubnikov–de Hass oscillations have been observed in the high field range of the magnetoresistance with three fundamental frequencies corresponding to the extremal areas of the Fermi surface. Our results suggest that the extremely large unsaturated magnetoresistance in this compound is due to near compensation of the charge carriers. We also performed angle-resolved photoelectron spectroscopy measurements with different photon energies and compared our results with the calculated band structure of GdBi.

We analyze possible ways to calculate magnetic exchange interactions within the density functional theory plus dynamical mean-field theory $(\mathrm{DFT}+\mathrm{DMFT})$ approach in the paramagnetic phase. Using the susceptibilities obtained within the ladder DMFT approach together with the random phase approximation result for the Heisenberg model, we obtain bilinear exchange interactions. We show that the earlier obtained result of Stepanov et al. [Phys. Rev. Lett. 121, 037204 (2018); Phys. Rev. B 105, 155151 (2022)] corresponds to considering individual magnetic moments in each orbital in the leading-order approximation in the nonlocal correlations. We consider a more general approach and apply it to evaluate the effective magnetic parameters of iron and nickel. We show that the analysis, based on the inverse orbital-summed susceptibilities, yields reasonable results for both weak and strong magnets. For iron, we find, in the low-temperature limit, the exchange interaction $J_0\simeq 0.20$ eV, while for nickel we obtain $J_0\simeq 1.2$ eV. The considered method also allows one to describe the spin-wave dispersion at temperatures $T\sim T_C$, which is in agreement with the experimental data.

We describe the existence of local distortion discovered in the synchrotron x-ray single-crystal structure analysis of layered ditelluride $1T\text{−}M{\mathrm{Te}}_2 (M=\mathrm{V}$, Nb, Ta). In $1T\text{−}{\mathrm{TaTe}}_2$, the double zigzag chain structure of Ta is deformed at about 170 K, and heptamer molecules are formed periodically at low temperatures. We found that some of the Ta atoms that compose the double zigzag chain structure appearing at high temperatures are locally displaced, resulting in local dimerization. This tendency weakens when Ta is replaced by V or Nb. Our results indicate that the local distortion persistently survives in these ditellurides, where the electronic degrees of freedom, including orbitals, are weakened. We further discuss the origin of local distortion in these ditellurides, which is different from many usual material systems where molecular formation occurs at low temperatures.

Recently, effective one-dimensional configuration coordinate diagrams have been utilized to calculate the line shapes of luminescence spectra and nonradiative carrier capture coefficients via point defects. Their calculations necessitate accurate total energies as a function of configuration coordinates. Although supercells under periodic boundary conditions are commonly employed, the spurious cell size effects have not been previously considered. In this study, we have proposed a correction energy formalism and verified its effectiveness by applying it to a nitrogen vacancy in GaN.

Perfect absorbers, which can achieve total absorption of all incoming energy, have been extensively studied in the last decades for various important technologies in general wave systems. Here, we show that perfect absorption (PA) is generically associated with topological spectral phase singularity (SPS), carrying conserved and tunable quantized topological invariants in spectral space. The order of topological invariant $\tilde{v}$ depends on the number of degenerate outgoing channels. Two commonly studied absorbers, mirror-backed and all-dielectric structures, are reexamined from a topological perspective to reveal the generation, evolution, and annihilation of SPSs with $\tilde{v}=\pm 1$ or even high orders (i.e., $\tilde{v}=\pm 2,\pm 4$). A strategy based on charge conservation of SPSs to design dual-band perfect absorbers has been established. Our findings establish the topological origin of the robust existence of PA. More broadly, in this letter, we highlight topology as the fundamental property of conventional scattering behaviors. This insight could lead to opportunities in applications such as biosensing, topological metasurfaces, and micro/nano thermal radiation.

A pair density wave (PDW) is a superconducting (SC) state with a spatially modulated order parameter. Although much is known about the properties of the PDW state, its realization in microscopic models with divergent susceptibility has been challenging. Here we report a density-matrix renormalization group study of a three-band Hubbard model (also known as the Emery model) for cuprates on long two-leg square cylinders. Upon light doping, we find that the ground state of the system is consistent with that of a PDW state with mutually commensurate and power-law SC, charge (CDW), and spin (SDW) density wave correlations. The SC correlations are dominant between neighboring Cu sites with $d$-wave pairing symmetry. Interestingly, we find that the near-neighbor interactions, especially the near-neighbor attractive $V_{pd}$ interaction between neighboring Cu and oxygen sites, can notably enhance the SC correlations while simultaneously suppressing the CDW correlations. For a modestly strong attractive $V_{pd}$, the SC correlations become quasi-long-ranged with a divergent PDW susceptibility.

We theoretically study the spectral properties of a one dimensional semiconductor-superconductor-ferromagnetic insulator (SE-SU-FMI) hybrid nanostructure, motivated by recent experiments where such devices have been fabricated using epitaxial growing techniques. We model the hybrid structure as a one-dimensional single-channel semiconductor nanowire under the simultaneous effect of two proximity-induced interactions: superconducting pairing and a (spatially inhomogeneous) Zeeman exchange field. The coexistence of these competing mechanisms generates a rich quantum phase diagram and a complex subgap Andreev bound state (ABS) spectrum. By exploiting the symmetries of the problem, we classify the solutions of the Bogoliubov-de Gennes equations into even and odd ABS with respect to the spatial inversion symmetry $x\to -x$. We find the ABS spectrum of the device as a function of the different parameters of the model: the length $L$ of the coexisting SU-FMI region, the induced Zeeman exchange field $h_0$, and the induced superconducting coherence length $\xi$. In particular we analyze the evolution of the subgap spectrum as a function of the length $L$. Interestingly, we generically find spin-polarized ABS emerging in the subgap region, which, depending on the ratio $h_0/\mathrm{\Delta }$, can eventually cross below the Fermi energy at certain critical values $L_c$, and induce spin- and fermion parity-changing quantum phase transitions. We argue that this type of device constitute a promising highly-tunable platform to engineer subgap ABS.

We study the surface state-dependent magneto-optical properties of an ultrathin Floquet topological insulator (FTI) under the influence of an external perpendicular magnetic field in the terahertz frequency regime. Under the Floquet picture, we treat the circularly polarized off-resonant light as an external perturbation that introduces a mass gap at the Dirac cone, thus, making the surface state Dirac fermions massive. By tuning the optical field energy in the FTI thin-film system, various electronic phase transitions can be driven between the trivial insulator state and the band insulator state. Using Kubo formalism, we derive the real and imaginary parts of the longitudinal and Hall conductivities and demonstrate that these conductivities are sensitively influenced by the strength of the off-resonant optical field, magnetic field, and chemical potentials. On the other hand, topological insulators exhibit strong magneto-optic effects. We further compute the Kerr and Faraday rotation angles and show that giant Kerr and Faraday rotations can be achieved in a FTI thin film by external tuning knobs, such as magnetic and off-resonant optical fields. The Kerr and Faraday rotations in symmetric (top) and antisymmetric (bottom) topological surface states can be controlled for interband and intraband transitions via gate bias voltage. Our results reveal the FTI as an intriguing versatile system whose magneto-optical properties can be effectively tuned optically, magnetically and electrically, thus, uncovering the strong photonics and optoelectronics device application potentials of the FTI.

Topological flat bands (TFBs) have been proposed in Chern-insulator lattice models. Some two-dimensional monolayer systems can host TFBs with the Chern number $\left|\mathcal{C}\right|=1$, and some TFBs with higher Chern numbers are achieved in multilayer or multicomponent systems. In this work, we construct a series of TFBs with higher Chern numbers in the monolayer star lattice by adding several distant-neighbor hopping terms with staggered magnetic fluxes. By considering the third and the fourth nearest-neighbor hopping terms, we find the $\left|\mathcal{C}\right|=2$ and $\left|\mathcal{C}\right|=3$ TFBs with large flatness ratios, respectively. We also obtain the $\left|\mathcal{C}\right|=4$ and $\left|\mathcal{C}\right|=5$ TFBs when the fifth and the sixth nearest-neighbor hopping terms are added. Intriguingly, we also observe multiple flat bands with various Chern numbers by tuning the distant-neighbor hopping amplitudes and the staggered-flux parameters. Furthermore, a series of fractional Chern insulator (FCI) states have been numerically observed when hard-core bosons are filled into these TFB models, with fractional fillings $\nu =1/3, 2/5, 3/7$ and $\nu =1/4, 2/7$ in the $\left|\mathcal{C}\right|=2$ and $\left|\mathcal{C}\right|=3$ TFBs, respectively. Our findings provide an approach to search for more TFBs with higher Chern numbers and further investigate more interesting FCIs.

Most materials exhibit a monotonically increasing lattice thermal conductivity with pressure. However, in this work, we report anomalous thermal transport properties of MgSe with a diamond phase under high pressure by first-principles calculations. Unlike other selenides, the lattice thermal conductivity of MgSe decreases with increasing pressure. We attribute this anomalous thermal transport to both harmonic and anharmonic effects. The harmonic effect is related to the anomalous downward shift of the transverse acoustic modes, which leads to decreasing phonon group velocities. This anomalous harmonic effect can also be confirmed by anomalous soft elastic properties. The anharmonic effect, on the other hand, is related to the stronger anharmonic scattering strength under pressure. Through bonding analysis, we found that the alternating bonding and antibonding states, combined with the unique lattice structure, are the main reason behind this anomaly. Our results reveal that the abnormal soft elastic properties under pressure can be a new way to judge whether lattice thermal conductivity decreases with pressure.