We calculate the elongation or contraction force $F$ on a point contact (length $L$) connecting two superconductors with a phase difference $\varphi$. When $L$ is small compared with the superconducting coherence length ${\xi }_0$, this force is given by $F=-({\mathrm{\Delta }}_0/\pi {\xi }_0)ln({\xi }_0/L)cos\varphi$ per spin-degenerate transverse mode. Quantum fluctuations in states from the continuous spectrum outside the superconducting gap ${\mathrm{\Delta }}_0$ give the dominant contribution to this force, which may be understood as the superconducting counterpart of the electromagnetic Casimir force. We compare with earlier work that only included contributions from the discrete spectrum of Andreev levels.

We investigate the emergence of a myriad of phases in the strong coupling regime of the dipolar Hubbard model in two dimensions. By using a combination of numerically unbiased methods in finite systems with analytical perturbative arguments, we show the versatility that trapped dipolar atoms possess in displaying a wide variety of many-body phases, which can be tuned by changing the collective orientation of the atomic dipoles. We further investigate the stability of these phases to thermal fluctuations in the strong coupling regime, highlighting that they can be accessed with current techniques employed in cold atoms experiments on optical lattices. Interestingly, both quantum and thermal phase transitions are signaled by the behavior of density-density and local moment-local moment correlations, which are available through quantum gas microscopy for cold atoms to be used as probes for the onset of different phases.

We propose a scheme that generates period-doubled responses via periodically driving certain Hamiltonians hosting quantum many-body scars, akin to recent experimental observations in driven Rydberg atom arrays. Our construction takes advantage of an su(2) spectrum generating algebra associated with the static quantum-scarred Hamiltonian, which enacts a $\pi$ rotation within the scar subspace after one period of time evolution with appropriately chosen driving parameters. This yields period-doubled (subharmonic) responses in local observables for any choice of initial state residing in the scar subspace. The quasienergy spectrum features atypical $\pi$-paired eigenstates embedded in an otherwise fully thermal spectrum. The protocol requires neither a large driving frequency nor a large driving amplitude and is thus distinct from the prethermalization physics in previous investigations of the driven PXP model. We demonstrate our scheme using several spin-1/2 and spin-1 quantum scarred models possessing an exact su(2) spectrum generating algebra, as well as a symmetry-deformed PXP model, where the su(2) algebra is only approximate. Our results extend the class of models hosting quantum many-body scars that can be leveraged to yield time-crystalline behaviors under periodic driving.

We investigate the existence of linear-in-momentum spin orbit interactions in the valence band of Ge/GeSi heterostructures using an atomistic tight-binding method. We show that symmetry breaking at the Ge/GeSi interfaces gives rise to a linear Dresselhaus-type interaction for heavy holes. This interaction results from the heavy-hole/light-hole mixings induced by the interfaces and can be captured by a suitable correction to the minimal Luttinger-Kohn, four bands $\mathbf{k}·\mathbf{p}$ Hamiltonian. It is dependent on the steepness of the Ge/GeSi interfaces, and is suppressed if interdiffusion is strong enough. Besides the Dresselhaus interaction, the Ge/GeSi interfaces also make a contribution to the in-plane gyromagnetic $g$ factors of the holes. The tight-binding calculations also highlight the existence of a small linear Rashba interaction resulting from the couplings between the heavy-hole/light-hole manifold and the conduction band enabled by the low structural symmetry of Ge/GeSi heterostructures. These interactions can be leveraged to drive the hole spin. The linear Dresselhaus interaction may, in particular, dominate the physics of the devices for out-of-plane magnetic fields. When the magnetic field lies in-plane, it is, however, usually far less efficient than the $g$-tensor modulation mechanisms arising from the motion of the dot in nonseparable, inhomogeneous electric fields and strains.

Using large-scale density-matrix renormalzation group calculations and minimally augmented spin-wave theory, we demonstrate that the phase diagram of the quantum $S=\frac{1}{2}{J}_1$–$J_3$ ferro-antiferromagnetic model on the honeycomb lattice differs dramatically from the classical one. It hosts the double-zigzag and Ising-$z$ phases as unexpected intermediaries between ferromagnetic and zigzag states that are also extended beyond their classical regions of stability. In broad agreement with quantum order-by-disorder arguments, these collinear phases replace the classical spiral state.

Recently, unusual $\pi$ phase shifts in Little-Parks experiments performed on two systems derived from the layered superconductor $2H\text{−}{\mathrm{TaS}}_2$ were reported. These systems share the common feature that additional layers have been inserted between the $1H\text{−}{\mathrm{TaS}}_2$ layers. In both cases, the $\pi$ phase shift has been interpreted as evidence for the emergence of exotic superconductivity in the $1H$ layers. Here, we propose an alternative explanation assuming that superconductivity in the individual $1H$ layers is of conventional $s$-wave nature derived from the parent $2H\text{−}{\mathrm{TaS}}_2$. We show that a negative Josephson coupling between otherwise decoupled neighboring $1H$ layers can explain the observations. Furthermore, we find that the negative coupling can arise naturally assuming a tunneling barrier containing paramagnetic impurities. An important ingredient is the suppression of non-spin-flip tunneling due to spin-momentum locking of Ising type in a single $1H$ layer together with the inversion symmetry of the double layer. In the exotic superconductivity scenario, it is challenging to explain why the critical temperature is almost the same as in the parent material and, in the $4\mathit{Hb}$ case, the superconductivity's robustness to disorder. Both are nonissues in our picture, which also exposes the common features that are special in these two systems.

We explore from first-principles calculations the ferroelectric material ${\mathrm{Pb}}_5{\mathrm{Ge}}_3{\mathrm{O}}_{11}$ as a model for controlling the spin-orbit interaction (SOC) in crystalline solids. The SOC has a surprisingly strong effect on the structural energy landscape by deepening the ferroelectric double well. We observe that this effect comes from a specific Pb Wyckoff site that lies on the verge of a natural cavity channel of the crystal. We also find that a unique cavity state is formed by the empty $6p$ states of another Pb site at the edge of the cavity channel. This cavity state exhibits a sizable spin splitting with a mixed Rashba-Weyl character and a topologically protected crossing of the related bands. We also show that the ferroelectric properties and the significant SOC effects are exceptionally robust in the presence of n-type doping at levels of up to several electrons per unit cell. We trace the provenance of these original effects to the unique combination of the structural cavity channel and the chemistry of the Pb atoms with $6p$ orbitals localizing inside the channel.

We study a multiorbital Hubbard–Su-Schrieffer-Heeger model for the one-dimensional (1D) corner-shared cuprates in the adiabatic and nonadiabatic limits using exact diagonalization and determinant quantum Monte Carlo. At half filling and in the adiabatic limit, lattice dimerization can be achieved only over a narrow range of couplings slightly below a critical coupling $g_c$. Beyond this value, the sign of the effective hopping changes, and the lattice becomes unstable. Strong lattice fluctuations replace the dimerization state in the nonadiabatic case. We also examine the model's temperature-dependent uniform and dynamical magnetic susceptibilities and compare them to the results of an effective spin-$1/2$ Heisenberg model. In doing so, we demonstrate that lattice fluctuations induced by the $e$-ph interaction suppress the effective superexchange interaction when $g<g_c$. Our results elucidate the effect of bond-stretching phonons in the parent cuprate compounds in general and are particularly relevant to 1D cuprates, where strong $e$-ph interactions have recently been inferred.

We introduce a classification of the radial spin textures in momentum space that emerge at the high-symmetry points in crystals characterized by nonpolar chiral point groups $(D_2,$ $D_3,$ $D_4,$ $D_6,$ $T,$ $O)$. Based on the symmetry constraints imposed by these point groups in a vector field, we study the general expression for the radial spin textures up to third order in momentum. Furthermore, we determine the high-symmetry points of the 45 nonpolar chiral space groups supporting a radial spin texture. These two principles are used to screen materials databases for archetypes that go beyond the basic hedgehog radial spin texture. Among the selected materials we highlight the axion insulator candidate ${\mathrm{Ta}}_2{\mathrm{Se}}_8\mathrm{I}$, the material proposed for dark matter detection ${\mathrm{Ag}}_3\mathrm{Au}{\mathrm{Te}}_2$, and heazlewoodite ${\mathrm{Ni}}_3{\mathrm{S}}_2$, a conventional metal predicted to exhibit current-induced spin polarization. We point out that the symmetry analysis proposed in this Letter is more general and extends to studying other vector properties in momentum space.

We explore a full dynamical phase diagram by means of a double quench protocol that depends on a relaxation time as the only control parameter. The protocol comprises two fixed quenches and an intermediate relaxation time that determines the phase in which the quantum state is placed after the final quench. We apply it to an anharmonic Lipkin-Meshkov-Glick model. We show that its two excited-state quantum phase transitions split the spectrum into three different phases: two symmetry-breaking ones, characterized by different constants of motion, and a disordered phase. As a consequence, our protocol allows us to explore all the dynamical phase transitions arising from two kinds of quenches: the typical one, leading the system from a symmetry-breaking to a disordered phase, and another one in which the system transitions between two different symmetry-breaking phases. We characterize all of them in terms of the constants of motion appearing in all three phases of the model.

Renormalization group theory allows continuous variation of critical exponents along a marginal direction (when there is one), keeping the scaling relations invariant. We propose a superuniversality hypothesis (SUH) suggesting that, up to constant scale factors, the scaling functions along the critical line must be identical to that of the base universality class even when all the critical exponents vary continuously. We demonstrate this in the Ashkin-Teller (AT) model on a two-dimensional square lattice where two different phase transitions occur across the self-dual critical line: while magnetic transition obeys the weak-universality hypothesis where exponent ratios remain fixed, the polarization exhibits a continuous variation of all critical exponents. The SUH not only explains both kinds of variations observed in the AT model, it also provides a unified picture of continuous variation of critical exponents observed in several other contexts.

We propose a practical implementation of a two-qubit entanglement engine which denotes a scheme to generate quantum correlations through purely dissipative processes. On a diamond platform, the electron spin transitions of two nitrogen-vacancy (NV) centers play the role of artificial atoms (qubits), interacting through a dipole-dipole Hamiltonian. The surrounding carbon-13 nuclear spins act as spin baths playing the role of thermal reservoirs at well-defined temperatures and exchanging heat through the NV center qubits. In our scheme, a key challenge is therefore to create a temperature gradient between two spin baths, for which we propose to exploit the recent progress in dynamical nuclear polarization, combined with microscopy superresolution methods. We discuss how these techniques should allow us to initialize such a long lasting out-of-equilibrium polarization situation between them, effectively leading to suitable conditions to run the entanglement engine successfully. Within a quantum master equation approach, we make theoretical predictions using state-of-the-art values for experimental parameters. We obtain promising values for the concurrence, reaching theoretical maxima.

We formulate a generalized pseudospin formalism for multiband superconductors in the presence of an external perturbing electromagnetic field. Our theory naturally captures the effects of quantum band geometric quantities and is valid even for flat-band superconductors. As an interesting consequence of our theory, we show that there is an interband pairing fluctuations induced by the external field and mediated by the quantum band geometry. Surprisingly, this interband fluctuation is independent of the band gap, which can be understood from the geometric nature of such novel fluctuations. We derive the generalized equation of motion for the multiband pseudospin and the self-consistency equation. We present a formal solution to the pseudospin equation of motion in powers of the perturbing electromagnetic field. As a simple illustration of our theory, we calculate the Leggett modes for the two-band case.

We combine nanosecond laser shock compression with in situ picosecond x-ray diffraction to provide structural data on iron up to 275 GPa. We constrain the extent of hcp-liquid coexistence, the onset of total melt, and the structure within the liquid phase. Our results indicate that iron, under shock compression, melts completely by 258(8) GPa. A coordination number analysis indicates that iron is a simple liquid at these pressure-temperature conditions. We also perform texture analysis between the ambient body-centered-cubic (bcc) $\alpha$, and the hexagonal-closed-packed (hcp) high-pressure $\epsilon -\mathrm{phase}$. We rule out the Rong-Dunlop orientation relationship (OR) between the $\alpha$ and $\epsilon -\mathrm{phase}\mathrm{s}$. However, we cannot distinguish between three other closely related ORs: Burger's, Mao-Bassett-Takahashi, and Potter's OR. The solid-liquid coexistence region is constrained from a melt onset pressure of 225(3) GPa from previously published sound speed measurements and full melt [246.5(1.8)–258(8) GPa] from x-ray diffraction measurements, with an associated maximum latent heat of melting of 623 J/g. This value is lower than recently reported theoretical estimates and suggests that the contribution to the earth's geodynamo energy budget from heat release due to freezing of the inner core is smaller than previously thought. Melt pressures for these nanosecond shock experiments are consistent with gas gun shock experiments that last for microseconds, indicating that the melt transition occurs rapidly.

Single crystals of $R{\mathrm{Ni}}_4\mathrm{Cd}$ $(R$ = Ce, Nd, Sm, and $\mathrm{Gd}-\mathrm{Tm})$ are grown by Cd flux and their physical properties are investigated by means of x-ray diffraction, magnetization, electrical resistivity, and specific heat measurements. Except for $R=\mathrm{Ce}$, the unit cell volume of $R{\mathrm{Ni}}_4\mathrm{Cd}$ follows a lanthanide contraction, implying a $3+$ valence state of rare-earth ions in this series. At high temperatures, magnetic susceptibility curves for $R=\mathrm{Nd}$ and Gd–Tm follow the Curie-Weiss behavior. The obtained Curie-Weiss temperature $\left({\theta }_p\right)$ for $R=\mathrm{Gd}\text{–}\mathrm{Tm}$ is small and negative. The temperature dependence of the electrical resistivity shows a metallic behavior for all $R{\mathrm{Ni}}_4\mathrm{Cd}$ compounds. Of the thermodynamic and transport property measurements, only ${\mathrm{GdNi}}_4\mathrm{Cd}$ indicates an antiferromagnetic ordering below $T_N=4.5\mathrm{K}$, while the remaining compounds show no signature of magnetic transition down to 1.8 K. The Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange mechanism is examined for its relevance to $T_N$ and ${\theta }_p$ of isostructural metallic $R{T}_{4}X$ $(R=\mathrm{Gd}\text{–}\mathrm{Tm}$, $T=\mathrm{Cu} \mathrm{and} \mathrm{Ni}, \mathrm{and} X=\mathrm{Cd}, \mathrm{In}, \mathrm{Mg}, \mathrm{Pd}, \mathrm{Ag}, \mathrm{and} \mathrm{Au})$ compounds. Although the variation in ${\theta }_p$ can be qualitatively explained by the RKKY sum, the values of ${\theta }_p$ in this family are highly dependent on their lattice parameter. Unlike the $R{\mathrm{Cu}}_{4}X$, the strength of the exchange interaction between molecular fields produced by rare-earth planes is strongly suppressed by the lattice contraction in $R{\mathrm{Ni}}_{4}X$ compounds.

Fe–Ga alloys show an unusually large increase in magnetostriction compared to pure Fe and are one of the most interesting Fe-based alloys for this reason. However, the origin of the large magnetostriction and its relation to the chemical ordering on the underlying bcc phase is still under debate. To gain further understanding of the extraordinary magnetoelastic characteristics of this system, we investigate the effect of Ga-concentration and ordering on the spin-wave spectra and stiffness. The magnetic interactions in the Fe–Ga alloys are obtained by ab initio electronic structure calculations and the magnon spectra are modeled using atomistic spin dynamics modeling. Our results agree with available experimental data and show softening of the magnon modes with increasing Ga-concentration and a strong reduction of the spin-wave stiffness due to atomic ordering.

Recently, gate-mediated supercurrent suppression in superconducting nanobridges has been reported in many experiments. This could be either a direct or an indirect gate effect. The microscopic understanding of this observation has not been clear until now. Using the quasiclassical Green's function method, we show that a small concentration of magnetic impurities at the surface of the bridges can significantly help to suppress superconductivity and hence the supercurrent inside the systems while a gate field is applied. This is because the gate field can enhance the depairing through the exchange interaction between the magnetic impurities at the surface and the superconductor. We also obtain a symmetric suppression of the supercurrent with respect to the gate field, a signature of a direct gate effect. We discuss the parameter range of applicability of our model and how it is able to qualitatively capture the main aspects of the experimental observations. Future experiments can verify our predictions by modifying the surface with magnetic impurities.

One of the most famous proximity effects at ferromagnet/superconductor (FM/SC) interfaces is partial conversion of singlet superconductivity to triplet pairing correlations. Due to the presence of macroscopic exchange field in the ferromagnet the Cooper pairs penetrating into the ferromagnet from the superconductor acquire a finite momentum there. The finite-momentum pairing manifests itself, in particular, as a nonmonotonic dependence of the critical temperature of the bilayer on the thickness of the FM layer. Here we predict that despite the absence of the macroscopic exchange field the critical temperature of the antiferromagnet/superconductor (AFM/SC) bilayers also exhibit nonmonotonic (oscillating) dependence on the AFM layer thickness. It is a manifestation of the proximity-induced Néel-type triplet correlations, which acquire finite total pair momentum and oscillate in the AFM layer due to the Umklapp electron-scattering processes at the AFM/SC interface. Our prediction can provide a possible explanation for a number of recently published experimental observations of the critical temperature of AFM/SC bilayers.

Due to the observed superconductivity, the alternating twisted trilayer graphene (ATTLG) has drawn great research interest very recently, in which three single-layered graphene (SLG) are stacked in an alternating twist way. If one or several of the SLG in ATTLG are replaced by multilayer graphene, we get a double-twisted multilayer graphene (DTMLG). In this work, we theoretically illustrate that, if the DTMLG has a mirror symmetry along the $z$ direction like the ATTLG, there exists a mirror symmetry decomposition (MSD), by which the DTMLG can be exactly decoupled into two subsystems with opposite parity. The two subsystems are either a twisted multilayer graphene (single twist) or a multilayer graphene, depending on the stacking configuration. Such MSD can give a clear interpretation of all the novel features of the moiré band structures of DTMLG, e.g., the fourfold degenerate flat bands, and the enlarged magic angle. Meanwhile, in such DTMLG, the parity becomes a new degree of freedom of the electrons, so that we can define a parity-resolved Chern number for the moiré flat bands. More importantly, the MSD implies that all the novel correlated phases in the twisted multilayer graphene should also exist in the corresponding DTMLGs since they have the same Hamiltonian in form. Specifically, according to the MSD, we predict that the superconductivity should exist in the $(1+3+1)\text{−}\mathrm{DTMLG}$.