Recently, it was shown [H.-Y. Deng and E. A. Muljarov, Phys. Rev. B 106, 195301 (2022)] that the so-called additional boundary conditions (ABCs) had significant issues when describing the surface properties of spatially dispersive media and an alternative ABC-free theory was developed introducing the surface scattering amplitude (SSA), which determines how polarization waves are scattered by a surface. Here we analytically calculate the SSA for a spatially dispersive model dielectric using wave-scattering theory and discuss some emerging generic properties independent of the model. The model consists of a lattice of interacting harmonic oscillators which had historically been used to mimic local excitons. It was shown that for short-range interactions the SSA is a constant independent of where the polarization waves were originated, which is not the case for long-range interaction. The calculated optical properties of a slab verify the general ABC-free theory.

Ising superconductivity is present due to the combined effect of broken-inversion symmetry and spin-orbit coupling that locks the spins out of plane, features that are associated with two-dimensional materials. We show that bulk misfit superconductors, ${\left(\mathrm{LaSe}\right)}_{1.14}\left(\mathrm{Nb}{\mathrm{Se}}_2\right)$ and ${\left(\mathrm{LaSe}\right)}_{1.14}{\left(\mathrm{Nb}{\mathrm{Se}}_2\right)}_2$, comprising monolayers and bilayers of $\mathrm{Nb}{\mathrm{Se}}_2$, exhibit unexpectedly strong Ising protection with a Pauli-limit violation comparable to monolayer $\mathrm{Nb}{\mathrm{Se}}_2$. We establish these misfit compounds as Ising superconductors using complementary experimental methods in combination with first-principles calculations. A concerted effect of charge-transfer, defects, reduction of interlayer hopping, and stacking enables Ising superconductivity in these compounds and therefore provides a possible pathway to design of bulk superconductors that are resilient to magnetic fields.

Non-Abelian phases are among the most highly sought states of matter, with those whose anyons permit universal quantum gates constituting the ultimate prize. The most promising candidate of such a phase is the fractional quantum Hall plateau at filling factors $\nu =\frac{12}{5}$, which putatively facilitates Fibonacci anyons. Experimental validation of this assertion poses a major challenge and remains elusive. We present a measurement protocol that could achieve this goal with already-demonstrated experimental techniques. Interfacing the $\nu =\frac{12}{5}$ state with any readily available Abelian state yields a binary outcome of upstream noise or no noise. Judicious choices of the Abelian states can produce a sequence of yes–no outcomes that fingerprint the possible non-Abelian phase by ruling out its competitors. Crucially, this identification is insensitive to the precise value of the measured noise and can uniquely identify the anyon type at filling factors $\nu =\frac{12}{5}$. In addition, it can distinguish any non-Abelian candidates at half-filling in graphene and semiconductor heterostructures.

The electron-phonon coupling is the primary mechanism responsible for material relaxation after ultrafast laser irradiation. However, it remains an elusive variable that is extremely challenging to extract experimentally, especially at high electron temperatures. Various previous theoretical approaches to determine electron-phonon coupling demonstrated large degree of inconsistency. In this paper, we present a first-principles framework for simulating the electron-phonon coupling parameter based on the electron-phonon spectral function, going beyond the approximation introduced by Wang et al. [Phys. Rev. B 50, 8016 (1994)]. Our simulations provide electron-temperature-dependent electron-phonon coupling values for transition metals Ru, Pd, and Au. Our findings reveal significant differences between the values obtained from the exact and approximated spectral functions, thus highlighting the limitations of Wang's approximation at elevated electron temperatures.

We report on investigations of the complex magnetostructural and spin-state transitions in the breathing pyrochlore ${\mathrm{LiFeCr}}_4{\mathrm{O}}_8$ by means of magnetization, Mössbauer spectroscopy, and density functional theory (DFT) calculations. Three transitions corresponding to the ferrimagnetic transition at $T_N\sim 94$ K, the spin-gap transition at $T_{\text{SG}}\sim 50$ K, and the magnetostructural transition at $T_{\text{MS}}\sim 19$ K were observed from the $\chi$($T$) curve, whereas only $T_N$ and $T_{\text{MS}}$ were evidenced for the Fe site from our Mössbauer measurements, suggesting that the spin-gap transition is absent at the Fe site. This indicates that the spin-gap transition is an effect of the breathing ${\mathrm{Cr}}_4$ lattice, in agreement with our DFT calculations from which we see nearly decoupled electronic states for the ${\mathrm{FeO}}_4$ and ${\mathrm{CrO}}_6$ units. From the temperature dependence of the hyperfine magnetic field, we also observed a spin-state transition for the Fe spins at $T_{\text{MS}}$ consistent with earlier neutron diffraction measurements. These local characteristics are believed to be important for a complete understanding of the complex magnetostructural coupling effects observed in similar systems.

Superconductor–ferromagnetic-insulator heterostructures are paradigmatic systems for studying the mutual influence of superconductivity and magnetism via proximity effects. In particular, spin-split superconductivity is realized in such structures. Recent experiments and theories demonstrate a rich variety of transport phenomena occurring in devices based on such heterostructures that suggest direct applications in thermoelectricity, low-dissipative spintronics, radiation detection, and sensing. In this work we investigate the influence of the electron-magnon interaction at the superconductor–ferromagnetic-insulator interface on the spin-split superconductivity. It is predicted that due to the magnon-mediated electron spin-flip processes the spin-split quasiparticle branches are partially mixed and reconstructed, and the BCS-like spin-split shape of the superconducting density of states, which is typical for superconductors in the effective exchange field, is strongly modified. An odd-frequency superconducting order parameter admixture to the leading singlet order parameter is also found. These findings expand the physical picture of spin-split superconductivity beyond the mean-field description of the ferromagnet exchange field.

Ubiquitous in most superconducting materials and a common result of nanofabrication processes, weak links are known for their limiting effects on the transport of electric currents. Still, they are at the root of key features of superconducting technology. By performing quantitative magneto-optical imaging experiments and thermomagnetic model simulations, we correlate the existence of local maxima in the magnetization loops of focused ion beam (FIB)-patterned Nb films to a magnetic field induced weak-to-strong-link transformation increasing their critical current. This phenomenon arises from the nanoscale interaction between quantized magnetic flux lines and FIB-induced modifications of the device microstructure. Under an ac drive field, this leads to a rectified vortex motion along the weak link. The reported tunable effect can be exploited in the development of new superconducting electronic devices, such as flux pumps and valves, to attenuate or amplify the supercurrent through a circuit element and as a strategy to enhance the critical current in weak-link-bearing devices.

Most theoretical treatments of inhomogeneous superconductivity/fermionic superfluidity have been based on the Bogoliubov-deGennes equations (or, else, on their various simplified forms), which implement a standard mean-field decoupling in the presence of spatial inhomogeneities. This approach is reliable even at finite temperature for weak interparticle attraction, when the Cooper pair size is much larger than the average interparticle distance (corresponding to the BCS limit of the BCS-BEC crossover). However, it loses accuracy for increasing attraction when the Cooper pair size becomes comparable or even smaller than the average interparticle distance (corresponding to the BEC limit of the BCS-BEC crossover), in particular when finite-temperature effects are considered. In these cases, inclusion of pairing fluctuations beyond mean field is required, a task that turns out to be especially difficult in the presence of inhomogeneities. Here, we implement the inclusion of pairing fluctuations directly on a coarse-graining version of the Bogoliubov-deGennes equations, which makes it simpler and faster to obtain a solution over the whole sector of the temperature-coupling phase diagram of the BCS-BEC crossover in the broken-symmetry phase. We apply this method in the presence of a supercurrent flow, such that problems related to the Josephson effect throughout the BCS-BEC crossover can be addressed under a variety of circumstances. This is relevant in the view of recent experimental data with ultracold Fermi atoms, to which the results of the present approach are shown to favorably compare in the companion paper.

A realistic description of the Josephson effect at finite temperature with ultracold Fermi gases embedded in nontrivial geometrical constraints (typically, a trap plus a barrier) requires appropriate consideration of pairing fluctuations that arise in inhomogeneous environments. Here, we apply the theoretical approach developed in a companion paper [Pisani et al., Phys. Rev. B 108, 214503 (2023)], where the inclusion of pairing fluctuations beyond mean field across the BCS–Bose-Einstein-condensate (BEC) crossover at finite temperature is combined with a detailed description of the gap parameter in a nontrivial geometry. In this way, we are able to account for the experimental results on the Josephson critical current, reported both at low temperature for various couplings across the BCS-BEC crossover and as a function of temperature at unitarity. Besides validating the theoretical approach of the companion paper, our numerical results reveal generic features of the Josephson effect which may not readily emerge from an analysis of corresponding experiments with condensed-matter samples owing to the unique intrinsic flexibility of experiments with ultracold gases.

To solve the long-standing controversy over the number of ferroelectric phase transitions in ${\left({\mathrm{CH}}_3{\mathrm{NHCH}}_2\mathrm{COOH}\right)}_3{\mathrm{CaCl}}_2$, we performed a comprehensive series of new dielectric, second-harmonic generation (SHG), calorimetric, infrared, and Raman measurements down to 10 K. Detailed analysis of the data excludes other ferroelectric transitions than the one at $T_{\mathrm{C}}=130$ K from the $Pnma$ to $Pn{2}_{1}a$ orthorhombic space groups. The observed pseudoproper ferroelectric (possibly ferrielectric) phase transition is driven by a terahertz soft mode in the $E\parallel b$ spectra, which couples to a critical microwave relaxation (central mode). Below $T_{\mathrm{C}}$, additional phonons are activated in the infrared and Raman spectra due to the change of selection rules, but all the observed modes can be assigned to orthorhombic $Pn{2}_{1}a$ structure without further reduction of the crystal symmetry. We found a weak and strongly frequency-dependent dielectric anomaly in the radiofrequency permittivity ${\varepsilon }_b^{\prime }(T$) close to 65 K, where the second phase transition was previously suggested. This behavior is assigned to the domain wall dynamics. Temperature dependences of the spontaneous polarization, heat capacity, and SHG signal show no indication of any further phase transition below $T_{\mathrm{C}}$.

Left-handed (LH) metamaterials are generally structured materials possessing abnormal electromagnetic properties due to both negative permittivity and permeability. One of these properties is a backward wave (BW) propagation, in which the phase and group velocities are opposite to each other. Here we investigate the electrodynamic (dispersion and energy) characteristics of the BW existing in a magnetic LH metamaterial that is controlled by the external uniform magnetic field. Such a metamaterial is a host made from a $\mu$-negative (ferromagnetic) nonconductive medium that contains a two-dimensional periodic structure of thin and isolated wires placed in the bias field directed either transversely or longitudinally to the electromagnetic wave propagation. A finite-difference time-domain–Landau-Lifshits-Gilbert (LLG) electromagnetic solver MaxLLG is used for the BW numerical simulations. This solver is based on the simultaneous usage of the Maxwell and LLG equations. By operating this software, the authors validate the existence of the BWs in the investigated LH metamaterial for two bias field orientations for various values of magnetic LH layer thickness and wire conductivity as well as for two connection types of wires with metallic planes that are placed on both sides of the metamaterial layer.

We report on processes of generation of spin current and conversion into charge current in $\mathrm{Co}{\mathrm{Fe}}_2{\mathrm{O}}_4$/Pt bilayers by means of spin Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) experiments. Specifically, we explore (001) full-textured $\mathrm{Co}{\mathrm{Fe}}_2{\mathrm{O}}_4$ (CFO) thin films grown onto (001)-oriented $\mathrm{SrTi}{\mathrm{O}}_3$ substrates, covered with Pt layers deposited under two different conditions: one at room temperature and another at high temperature $\left(400{}^{\circ }\mathrm{C}\right)$. The x-ray absorption spectroscopy measurements indicate that the Pt layer deposited at high temperature induces an interfacial magneticlike phase (Fe,Co)-Pt alloy, which influences the magnetic behavior of the structure and is responsible for the enhancement of the spin transmission at the interface. By analyzing the SMR data, we conclude that collinear and noncollinear magnetic domains coexist at the CFO-(Fe,Co)-Pt interface. By combining the data from the SMR and SSE measurements, we obtain the ratios between the values of the spin Hall angle $({\theta }_{\mathrm{SH}}$) and between the ones of the spin-mixing conductance ($g_{\mathrm{eff}}^{\uparrow \downarrow }$) in the two samples. We demonstrate that while the value of ${\theta }_{\mathrm{SH}}$ decreases by one-half with the heat treatment, the value of $g_{\mathrm{eff}}^{\uparrow \downarrow }$ increases by more than one order of magnitude. We interpret the increase of $g_{\mathrm{eff}}^{\uparrow \downarrow }$ in terms of unexpected magnetic reconstructions, which produce an enhancement of the magnetic moment arisen at the interface. Since the spin-mixing conductance determines the efficiency of the spin current transmission through the interface, the spinel ferrite cobalt in contact with a normal metal with a suitable heat treatment becomes a promising material for spintronics device applications.

We report the high-pressure optical floating-zone growth of ${\mathrm{GdInO}}_3$ single crystals and show its magnetic phase diagram down to the millikelvin regime as determined by magnetization measurements. The centered-honeycomb lattice structure shows considerable magnetic frustration ($\left|\mathrm{\Theta }\right|/T_{\mathrm{N}}\simeq 5$) and develops long-range magnetic order below $T_{\mathrm{N}}=2.1$ K from a short-range-ordered paramagnetic phase. Concomitantly, a small net magnetic moment evolves at $T_{\mathrm{N}}$ which points along the crystallographic $c$ direction. Upon cooling, the net moment reorients at $T^{**}\simeq 1.7$ K and $T^{*}\simeq 1$ K. A broad 1/3 plateau indicative of the up-up-down ($uud$) spin configuration appears for magnetic fields $B\parallel c$ but is absent for $B\parallel ab$, thereby suggesting easy-axis anisotropy. At $T=0.4$ K, a jump in magnetization at $\simeq$3/5 of the saturation magnetization signals a discontinuous transition to a high field phase and we find evidence for a possible tricritical point. Small energy and field scales in the accessible regimes render ${\mathrm{GdInO}}_3$ a well-suited example to study the phase diagram of a semiclassical frustrated hexagonal lattice in the presence of weak easy-axis anisotropy of mainly dipolar origin.

The ${\mathrm{ScAu}}_2\mathrm{Al}$ superconducting Heusler-type compound was recently characterized to have the highest critical temperature of $T_c=5.12$ K and the strongest electron-phonon coupling among the Heusler family. In this work, the electronic structure, phonons, electron-phonon coupling, and superconductivity of ${\mathrm{ScAu}}_2\mathrm{Al}$ are studied using ab initio calculations. The spin-orbit coupling significantly changes the electronic structure removing the van Hove singularity from the vicinity of the Fermi level. In the phonon spectrum, low-frequency acoustic modes, additionally softened by the spin-orbit interaction, strongly couple with electrons, leading to the electron-phonon coupling constant $\lambda =1.25$, a record high among Heuslers. The density functional theory for superconductors is then used to analyze superconducting state in this two-band superconductor. The effect of spin fluctuations (SF) on superconductivity is also analyzed. The calculated critical temperatures of $T_c=5.16$ K (4.79 K with SF) agree very well with the experiment, confirming the electron-phonon mechanism of superconductivity and showing a weak spin-fluctuation effect. The superconducting gaps formed on two Fermi surface sheets exhibit moderate anisotropy. Their magnitudes confirm the strong coupling regime, as the reduced average values are $2{\mathrm{\Delta }}_{b_1}/k_B{T}_c\simeq 4.1$ and $2{\mathrm{\Delta }}_{b_2}/k_B{T}_c\simeq 4.3$. Anisotropy of the gaps and large spread in their values significantly affect the calculated quasiparticle density of states.

The impact of spin-orbit interactions in ${\mathrm{NdNiO}}_2/{\mathrm{SrTiO}}_3$(001) and ${\mathrm{NdNiO}}_2/{\mathrm{KTaO}}_3$(001) is explored by performing density functional theory simulations including a Coulomb repulsion term. Polarity mismatch drives the emergence of an interfacial two-dimensional electron gas in ${\mathrm{NdNiO}}_2/{\mathrm{KTaO}}_3$(001) involving the occupation of Ta $5d$ conduction-band states, which is twice as pronounced as in ${\mathrm{NdNiO}}_2/{\mathrm{SrTiO}}_3$(001). We identify a significant anisotropic $k^3$ Rashba spin splitting of the respective $d_{xy}$ states in both systems that results from the broken inversion symmetry at the nickelate-substrate interface and exceeds the width of the superconducting gap. In ${\mathrm{NdNiO}}_2/{\mathrm{KTaO}}_3$(001), the splitting reaches 210 meV, which is comparable to Bi(111) surface states. At the surface, the Ni $3d_{x^2-y^2}$-derived states exhibit an unexpectedly strong linear Rashba effect with ${\alpha }_{\text{R}}\sim 125$ meV $\text{Å}$, exemplifying its orbital selectivity. The corresponding Fermi sheets present a reconstructed circular shape due to the electrostatic doping, but undergo a Lifshitz transition towards a cupratelike topology deeper in the film that coincides with a realignment of their spin texture. These results promote surface and interface polarity as interesting design parameters to control spin-orbit physics as well as the superconducting pairing in infinite-layer nickelate heterostructures.

Heavy fermion materials have been a rich playground for strongly correlated physics for decades. However, engineering tunable and synthesizable heavy fermion materials remains a challenge. We strive to integrate heavy fermion properties into carbon-boron clathrates as a universal structure which can host a diverse array of interesting physical phenomena. Using a combination of density functional theory and dynamical mean-field theory, we study two rare-earth carbon-boron clathrates, ${\mathrm{SmB}}_3{\mathrm{C}}_3$ and ${\mathrm{CeB}}_3{\mathrm{C}}_3$, and explore properties arising from the strong electronic correlations. We find a significant increase in the density of states at the Fermi level in ${\mathrm{CeB}}_3{\mathrm{C}}_3$ as the temperature is lowered, indicating the development of a heavy electron state. In ${\mathrm{SmB}}_3{\mathrm{C}}_3$, a potential Kondo insulating state is identified. Both findings point to rare-earth carbon-boron clathrates as novel strongly correlated materials within a universally tunable structure, offering a fresh platform to innovate upon conventional heavy fermion materials design.

Topological phases realized in time-reversal invariant (TRI) systems are foundational to the experimental study of the broader canon of topological condensed matter as they do not require exotic magnetic orders for realization. We therefore introduce topological skyrmion phases of matter realized in TRI systems as a foundational step towards experimental realization of topological skyrmion phases. A different bulk-boundary correspondence hidden from the tenfold way classification scheme is revealed by the presence of a nontrivial value of a ${\mathbb{Z}}_2$ spin skyrmion invariant. This quantized topological invariant gives a finer description of the topology in two-dimensional (2D) TRI systems as it indicates the presence or absence of robust helical edge states for open boundary conditions, in cases where the ${\mathbb{Z}}_2$ invariant computed with projectors onto occupied states takes a trivial value. Physically, we show that this hidden bulk-boundary correspondence derives from additional spin momentum locking of the helical edge states associated with the topological skyrmion phase.. ARPES techniques and transport measurements can detect these signatures of topological spin-momentum-locking and helical gapless modes. Our work therefore lays the foundation for experimental study of these phases of matter.

We investigated the time-resolved photoluminescence (PL) of InAs, Cu, and monolayer graphene excited by linearly polarized femtosecond pulses. PL depolarization occurred in the femtosecond regime. The establishment of the isotropic momentum distribution of electrons is discussed based on the electron-electron and electron-phonon scattering. This study demonstrates the impact of a steep dispersion relation in the electron band structure on the electron-optical phonon scattering causing the ultrafast depolarization of PL.

In the last decade, motivated by the concept of Planckian relaxation and the possible existence of a quantum critical point in cuprate materials, holographic techniques have been extensively used to tackle the problem of strange metals and high-$T_c$ superconductors. Among the various setups, the linear axion Gubser-Rocha model has often been considered as a promising holographic model for strange metals since it is endowed with the famous linear in $T$ resistivity property. As fiercely advocated by Anderson, beyond $T$-linear resistivity, there are several additional anomalies unique to the strange metal phase, for example, a Fermi-liquid-like Hall angle—the famous problem of two relaxation scales. In this paper, we show that the linear axion holographic Gubser-Rocha model, which presents a single momentum relaxation time, fails in this respect and therefore is not able to capture the transport phenomenology of strange metals. We prove our statement by means of a direct numerical computation, a previously demonstrated scaling analysis, and also a hydrodynamic argument. Finally, we conclude with an optimistic discussion of the possible improvements and generalizations which could lead to a holographic model for strange metals in all their glory.

It is widely believed that the $U\left(1\right)$ spin liquid with a large spinon Fermi surface (SFS state) can be realized in the spin-$\frac{1}{2}{J}_1\text{−}{J}_4$ model on the triangular lattice, when the ring exchange coupling $J_4$ is sufficiently strong to suppress the ${120}^{\circ }$ magnetic ordered state. This belief is supported by many variational studies on this model and seems to be consistent with the observations on the organic spin liquid materials such as $\kappa$-(BEDT-${\mathrm{TTF})}_2{\mathrm{Cu}}_2{\left(\mathrm{CN}\right)}_3$ and ${\mathrm{EtMe}}_3\mathrm{Sb}{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$, which are systems close to their Mott transition and thus have large $J_4$. Here we show through systematic variational search that such a state is never favored in the $J_1\text{−}{J}_4$ model on the triangular lattice. Instead, a state with broken spatial symmetry is favored in the most probable parameter regime for the SFS state and has an energy much lower than that of the SFS state and other proposed variational states. More specifically, we find that for $J_4\ge 0.09J_1$, the model favors a valence bond solid state with a $4\times 6$ period in its local spin correlation pattern and has a variational energy that is about $5\%$ lower than that of the SFS state. This state is separated from the $\pi$-flux state for $J_4\le 0.045J_1$ by an intermediate symmetry-breaking phase with a zigzag pattern in its local spin correlation. We find that the variational phase diagram we got is in qualitative agreement with that obtained from exact diagonalization on a $6\times 6$ cluster.