It is now routine to make Bose-Einstein condensates of polaritons with long enough lifetime and low enough disorder to travel ballistically for hundreds of microns in quasi-one-dimensional (1D) wires. We present observations of a nonequilibrium polariton condensate injected at one point in a quasi-1D ring, with a well-defined initial velocity and direction. A clear precession of the circular polarization is seen, which arises from an effective spin-orbit-coupling term in the Hamiltonian. Our theoretical model accurately predicts the experimentally observed behavior and shows that “zitterbewegung” behavior plays a role in the motion of the polaritons.

The recent discovery of spin-triplet superconductivity emerging from a nonmagnetic parent state in ${\mathrm{UTe}}_2$ has stimulated great interest in the underlying mechanism of Cooper pairing. Experimental characterization of short-range electronic and magnetic correlations is vital to understanding these phenomena. Here we use resonant inelastic x-ray scattering (RIXS), x-ray absorption spectroscopy (XAS), and atomic-multiplet-based modeling to shed light on the active debate between $5f^{2}6d^1$-based models with singlet crystal field states versus $5f^3$-based models that predict atomic Kramers doublets and much greater $5f$ itinerancy. The XAS and RIXS data are found to agree strongly with predictions for a $5f^{2}6d^1$-like valence electron configuration with weak intradimer magnetic correlations, and provide a context for interpreting recent investigations of the electronic structure and superconducting pairing mechanism.

We study localization and topological properties, as well as the fate of the critical phase, of non-Hermitian generalizations of the Aubry-André-Harper model with both on-site and off-diagonal incommensurate modulations. Non-Hermiticity arises from nonreciprocal hopping and a complex phase in the potential. In the absence of nonreciprocal hopping, we compute analytically the localization length of single-particle states by applying Avila's global theory. The system has the same phase diagram as Hermitian cases, except that the complex phase renormalizes the strength of the potential. In the presence of nonreciprocal hopping, the phase diagram is analytically determined by a similarity transformation. Due to the presence of the skin effect, induced by nonreciprocity, the skin phase turns into extended and critical phases when the boundary condition changes from open to periodic, while states are localized asymmetrically in the boundary-independent localized phase. The nonreciprocal hopping is in favor of the critical phase under a periodic boundary condition. The spectra are complex, and loops always exist. A winding number is not a proper indicator of the presence of loops, but the topological transition is in agreement with the localization transition.

We use time-resolved hard x-ray diffraction to investigate the structural dynamics of the multiferroic insulator ${\mathrm{TbMnO}}_3$ in the low-temperature antiferromagnetic and ferroelectrically ordered phase. The lattice response following photoexcitation at 1.55 eV is detected by measuring the (0 2 4) and (1 3 $-5$) Bragg reflections. A $0.022\%$ tensile strain, normal to the surface, is seen to arise within 20–40 ps. The magnitude of this transient strain is over an order of magnitude lower than that predicted from laser-induced heating, which we attribute to a bottleneck in the energy transfer between the electronic and lattice subsystems. The timescale for the transient expansion is consistent with that of previously reported demagnetization dynamics. We discuss a possible relationship between structural and demagnetization dynamics in ${\mathrm{TbMnO}}_3$, in which photoinduced atomic motion modulates the exchange interaction, leading to a destruction of the magnetic order in the system.

The recent discovery of quantum many-body scar states has revealed the possibility of having states with low entanglement that violate the eigenstate thermalization hypothesis in nonintegrable systems. Eigenstates with low entanglement entropy are rare but naturally exist in the integrable system of free fermions. Here, we demonstrate analytically that these atypical states would always be eliminated when an arbitrary weak interaction is introduced between the fermions. In particular, we show that the probability of having a many-body scar state with entanglement entropy satisfying a subvolume scaling law decreases double exponentially as the system size. Thus, our results provide a quantitative argument for the disappearance of scar states in interacting fermion systems.

Magnetic heterostructures with ferromagnetic/nonmagmetic interfaces are very interesting materials due to the possibility to switch the magnetization, to control the induced magnetic moment, and also due to interfacial effects. The tuning of the spin-orbit coupling in magnetic materials allows engineering new spintronic devices. Using density functional theory, the induced local magnetic moments in copper and gold surface atoms of ${\mathrm{Cu}}_3\mathrm{Au}\left(001\right)$ due to the deposited alternate magnetic Fe and Co monolayers were studied. To observe the tuning mechanism, the spin density of the $\mathrm{FeCo}/{\mathrm{Cu}}_3\mathrm{Au}\left(001\right)$ as a function of the number of deposited magnetic monolayers was extensively investigated. The Fe/Co deposition induces a hybridization between electronic states of the magnetic and nonmagnetic interface atoms, generating a broadening in $d$ bands of the deposited material, engineering the magnetic moments of the interface atoms. This observed hybridization elucidates the charge variation in the interface atoms for stacking sequences starting with Fe or with Co as the first magnetic layer in direct contact with the ${\mathrm{Cu}}_3\mathrm{Au}\left(001\right)$ substrate. By performing the spin-charge density calculations, we demonstrated that this magnetic induction occurs only for the nonmagnetic Cu-Au atoms at the ${\mathrm{Cu}}_3\mathrm{Au}\left(001\right)$ surface.

Layered rare-earth delafossites $AR{X}_2$ with $R=\text{Yb}$(III) or Ce(III) received a lot of interest as potential hosts for a quantum spin-liquid ground state. Some systems of this family that show no long-range order down to the lowest-measured temperatures, such as ${\mathrm{NaYbO}}_2$, ${\mathrm{NaYbS}}_2$, and ${\mathrm{NaYbSe}}_2$, are presumed to be in a quantum spin-liquid state. However, other isostructural compounds are known to order antiferromagnetically at subkelvin temperatures. Among them, ${\mathrm{KCeS}}_2$ exhibits stripe-yz magnetic order in the triangular-lattice planes that sets in below 400 mK. Here we investigate the spin-wave spectrum of this ordered phase with powder inelastic neutron scattering and describe it using a model Hamiltonian obtained from first-principles calculations based on the complete $|J,m_J\rangle$ multiplet description of the Ce sites with an anisotropic nearest-neighbor exchange interaction. Combining the current understanding of the exchange interaction in the systems with the effects of texturing in the powder, we are able to model the inelastic neutron scattering spectrum with high fidelity.

Nonreciprocal responses of noncentrosymmetric quantum materials attracted recent intensive interests, which is essential for the rectification function in diodes. A recent breakthrough is the discovery of superconducting diode effect. The principle to enlarge the rectification effect is highly desired to guide the design of the superconducting diode. Here, we study theoretically the Josephson junction S/FI/S (S: $d$-wave superconductor, FI: ferromagnetic insulator) on the surface of a topological insulator (TI). The simultaneous existence of $sin\phi$, $cos\phi$, and $sin2\phi$ terms with almost the same order in the Josephson current $I\left(\phi \right)$ is essential to get larger values of the $Q$ factor given by $Q=(I_c^{+}-|I_c^{-}\left|\right)/(I_c^{+}+|I_c^{-}\left|\right)$ with $I_c^{+}=\max \left[I\left(\phi \right)\right]$ and the negative one $I_c^{-}$ for the macroscopic phase difference $\phi$ of two superconductors on TI. We find that it can show a very large diode effect by tuning the crystal axes of $d$-wave superconductors and the magnetization of FI. The difference of the maximum Josephson currents $I_c$'s between the positive and negative directions can be about a factor of 2, where the current-phase relation is modified largely from the conventional one. The relevance of the zero-energy Andreev bound states as Majorana bound states at the interface is also revealed. This result can pave the way to realize an efficient superconducting diode with low-energy cost.

The recently discovered heavy-fermion superconductor ${\mathrm{UTe}}_2$, as a promising candidate for an odd-parity superconductivity, exhibits anomalous magnetic-field reinforced superconductivity with a huge upper critical field of 35 T. However, the thermodynamic properties of this unconventional superconductivity still remain unclear. Herein we present the results of dc magnetization and heat capacity measurements in ${\mathrm{UTe}}_2$ to probe the magnetic response at low temperatures. The obtained isothermal magnetization in the superconducting state of ${\mathrm{UTe}}_2$ clearly demonstrates the anomalous magnetic flux pinning effect, which is most likely related to the peculiar electromagnetic response of the spin-triplet pairing state in the hard-magnetization axis (orthorhombic $b$ axis). The ac susceptibility measurements support the anomalous vortex states in ${\mathrm{UTe}}_2$. We also found evidence for the Lifshitz transition just above $H_{\mathrm{c}2}$ and the anomalous flux pinning anomalies for the easy-magnetization axis ($a$ axis).

Barium titanate (BT) solid solutions are used in a wide range of applications such as piezoelectric actuators and high-performance energy storage devices. The key to achieve and tune desired macroscopic properties is the chemical modification, which is done by substituting Ba or Ti with other homovalent or heterovalent cations. This work uses large-scale molecular dynamics simulations based on an effective Hamiltonian approach to calculate the macroscopic properties of BT solid solutions from first principles, thereby offering a framework for the prediction of properties prior to materials synthesis. To this end, we elaborate on the theoretical description of substitution in effective Hamiltonians as well as their parametrization by density functional theory calculations for two model systems: homovalent substituted ${\mathrm{BaZr}}_x{\mathrm{Ti}}_{1\text{−}x}{\mathrm{O}}_3$ (BZT) and heterovalent substituted ${\mathrm{BaNb}}_x{\mathrm{Ti}}_{1\text{−}x}{\mathrm{O}}_3$ (BNT). The effective Hamiltonian for BZT obtained in this work is first used for benchmarking against other models and experimental data on the phase diagrams and dielectric properties. Subsequently, the effective Hamiltonian is further extended and used to parametrize BNT and compare the model's predictions to the available experimental data. The parameter sets obtained in this work can be used for future studies and provide deep insight into the subject of relaxor ferroelectrics.

There has been a recent upsurge of interest in the quantum properties of magnons for quantum information processing. An important issue is to examine the stability of quantum states of magnons against various relaxation and dephasing channels. Since the interaction of magnons in magnetic systems may fall in the ultrastrong and even deep-strong coupling regimes, the relaxation process of magnon states is quite different from the more common quantum optical systems. Here we study the relaxation and dephasing of magnons based on the Lindblad formalism and derive a generalized master equation that describes the quantum dynamics of magnons. Employing this master equation, we identify two distinct dissipation channels for squeezed magnons, i.e., the local dissipation and collective dissipation, which play a role for both ferromagnets and antiferromagnets. The local dissipation is caused by the independent exchange of angular momentum between the magnonic system and the environment, while the collective dissipation is dressed by the parametric interactions of magnons and it enhances the quantumness and thermal stability of squeezed magnons. Further, we show how this formalism can be applied to study the pure dephasing of magnons caused by four-magnon scattering and magnon-phonon interactions. Our results provide the theoretical tools to study the decoherence of magnons within a full quantum-mechanical framework and further benefit the use of quantum states of magnons for information processing.

Based on first-principles calculations, an antiferromagnetic Ising model on a triangular lattice has been proposed to interpret the order of $\mathrm{Fe}\left(1\right)$-Ge pairs and the formation of $\sqrt{3}\times \sqrt{3}$ superstructures in the ${\mathrm{Fe}}_5{\mathrm{GeTe}}_2$ (F5GT) as well as to predict the existence of similar superstructures in Ni doped F5GT (Ni-F5GT). Our paper suggests that F5GT systems may be considered as a rare structural realization of the well-known antiferromagnetic Ising model on a triangular lattice. Based on the superstructures, a Heisenberg-Landau Hamiltonian, taking into account longitudinal spin fluctuations, is implemented to describe magnetism in both F5GT and Ni-F5GT. We unveil that frustrated magnetic interactions associated with Fe(1), tuned by a tiny Ni doping, is responsible for the experimentally observed enhancement of the $T_c$ to 478 K in Ni-F5GT. Itinerant magnetism, reflected by longitudinal spin fluctuations, are found to only affect the $T_c$'s mildly with a modification of $\sim 5\%$ with respect to that obtained with standard Heisenberg interactions. Our calculations show that at low doping levels, monolayer Ni-F5GT has almost the same magnetic phase diagram as that of the bulk, which indicates a pervasive beyond room-temperature ferromagnetism in this Ni-doped two-dimensional system.

Chromium-doped ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ is a magnetic topological insulator (MTI), which belongs to the $(\mathrm{Sb},{\mathrm{Bi})}_2(\mathrm{Se},{\mathrm{Te})}_3$ family. When doped with the transition metals V, Cr, and Mn this family displays long-range ferromagnetic order above liquid nitrogen temperature and is currently intensely explored for quantum device applications. Despite the large magnetic ordering temperature, the experimental observation of dissipationless electrical transport channels, i.e., the quantum anomalous Hall effect, is limited in these materials to temperatures below $\approx 2$ K. Inhomogeneities in the MTI have been identified as a major concern, affecting the coupling between the Dirac states and the magnetic dopants. Nevertheless, details on the local magnetic order in these materials are not well understood. Here, we report the study of the magnetic correlations in thin films using a combination of muon spin relaxation $\left(\mu \mathrm{SR}\right)$, and magnetic soft x-ray spectroscopy and imaging. $\mu \mathrm{SR}$ provides two key quantities for understanding the microscopic magnetic behavior: The magnetic volume fraction, i.e., the percentage of the material that is ferromagnetically ordered, and the relaxation rate, which is sensitive to the magnetic static $(\approx \mu \mathrm{s})$ and dynamic disorder. By choosing different implantation depths for the muons, one can further discriminate between near-surface and bulk properties. No evidence for a surface enhancement of the magnetic ordering is observed, but, instead, we find evidence of small magnetically ordered clusters in a paramagnetic background, which are coupled. The significant magnetic field shift that is present in all samples indicates a percolation transition that proceeds through the formation and growth of magnetically ordered spin clusters. We further find that fluctuations are present even at low temperatures, and that there appears to be a transition between superparamagnetism and superferromagnetism.

We study the quantum information spreading in one-dimensional free-fermion systems in the presence of localized thermal baths. We employ a nonlocal Lindblad master equation to describe the system-bath interaction, in the sense that the Lindblad operators are written in terms of the Bogoliubov operators of the closed system, and hence are nonlocal in space. The statistical ensemble describing the steady state is written in terms of a convex combination of the Fermi-Dirac distributions of the baths. Due to the singularity of the free-fermion dispersion, the steady-state mutual information exhibits singularities as a function of the system parameters. While the mutual information generically satisfies an area law, at the singular points it exhibits logarithmic scaling as a function of subsystem size. By employing the Fisher-Hartwig theorem, we derive the prefactor of the logarithmic scaling, which depends on the parameters of the baths and plays the role of an effective “central charge.” This is upper bounded by the central charge governing ground-state entanglement scaling. We provide numerical checks of our results in the paradigmatic tight-binding chain and the Kitaev chain.

We derive ab initio low-energy effective Hamiltonians (LEH) for high-temperature superconducting (SC) copper oxides ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CuO}}_6$ (Bi2201, $N_{\ell }=1$, $T_c^{\exp }\sim 10\mathrm{K}$), ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ (Bi2212, $N_{\ell }=2$, $T_c^{\exp }\sim 84\mathrm{K}$), ${\mathrm{HgBa}}_2{\mathrm{CuO}}_4$ (Hg1201, $N_{\ell }=1$, $T_c^{\exp }\sim 90\mathrm{K}$), and ${\mathrm{CaCuO}}_2$ (Ca11, $N_{\ell }=\infty$, $T_c^{\exp }\sim 110\mathrm{K}$), with substantially different values of experimental optimal SC transition temperature $T_c^{\exp }$ and number $N_{\ell }$ of laminated ${\mathrm{CuO}}_2$ planes between the two neighboring block layers. We apply the latest methodology of the multiscale ab initio scheme for correlated electron systems (MACE), and focus on the LEH consisting of one antibonding (AB) $\mathrm{Cu}3d_{x^2-y^2}$/$\mathrm{O}2p_{\sigma }$ orbital centered on each Cu atom. We discuss prominent features of this LEH: (1) The ratio $U/|t_1|$ between the onsite effective Coulomb repulsion (ECR) $U$ and amplitude of nearest-neighbor hopping $t_1$ increases with $T_c^{\exp }$ and $N_{\ell }$, consistently with the expected increase in $d$-wave SC correlation function $P_{dd}$ with $U/|t_1|$. One possible cause of the increase of $U/|t_1|$ with $N_{\ell }$ is the replacement of apical O atoms by Cu atoms from neighboring ${\mathrm{CuO}}_2$ planes when $N_{\ell }$ increases. Furthermore, we show that the increase in distance between Cu and apical O atoms decreases the effective screening (ES) defined as the screening by electrons outside of the LEH and increases $U/|t_1|$. (2) For Hg1201 and Ca11, we examine the variation in $U/|t_1|$ with hole doping per AB orbital $\delta$, and show that $U/|t_1|$ decreases when $\delta$ increases, which may partly account for the disappearance of SC when $\delta$ exceeds the optimal value in experiment. (3) For $N_{\ell }\ge 2$, offsite inter-${\mathrm{CuO}}_2$ plane ECR is comparable to off-site intra-${\mathrm{CuO}}_2$ plane ECR. We discuss contributions of inter-${\mathrm{CuO}}_2$ plane ECR to both $P_{dd}$ and the stability of the SC state.

The usual models for electrical spin manipulation in semiconductor quantum dots assume that the confinement potential is separable in the three spatial dimensions and that the ac drive field is homogeneous. However, the electric field induced by the gates in quantum dot devices is not fully separable and displays significant inhomogeneities. Here we address the electrical manipulation of hole spins in semiconductor heterostructures subject to inhomogeneous vertical electric fields and/or in-plane ac electric fields. We consider Ge quantum dots electrically confined in a Ge/GeSi quantum well as an illustration. We show that the lack of separability between the vertical and in-plane motions gives rise to an additional spin-orbit coupling mechanism (beyond the usual linear and cubic in momentum Rashba terms) that modulates the principal axes of the hole gyromagnetic $g$ matrix. This nonseparability mechanism can be of the same order of magnitude as Rashba-type interactions, and enables spin manipulation when the magnetic field is applied in the plane of the heterostructure even if the dot is symmetric (disk shaped). More generally, we show that Rabi oscillations in strongly patterned electric fields harness a variety of $g$-factor modulations. We discuss the implications for the design, modeling, and understanding of hole spin qubit devices.

Carbon nanotubes are quasi-one-dimensional systems that possess large electronic conductance (for the metallic variants), high mechanical strength, selective emission and detection of light, and can be made chemically functionalized. In this work, we generalize the notion of disorder hyperuniformity, a recently discovered exotic state of matter with hidden long-range order, to quasi-one-dimensional materials. As a proof of concept, we then apply the generalized framework to quantify the density fluctuations in amorphous carbon nanotubes containing randomly distributed Stone-Wales defects. We demonstrate that all of these amorphous nanotubes are hyperuniform; i.e., the infinite-wavelength (normalized) density fluctuations of these systems are completely suppressed, regardless of the diameter, rolling axis, number of rolling sheets, and defect fraction of the nanotubes. We find that these amorphous nanotubes are energetically more stable than nanotubes with periodically distributed Stone-Wales defects. Moreover, certain semiconducting defect-free carbon nanotubes become metallic as sufficiently large amounts of defects are randomly introduced. This structural study of amorphous nanotubes strengthens our fundamental understanding of these systems, and suggests possible exotic physical properties, as endowed by their disordered hyperuniformity. Our findings also shed light on the effect of dimensionality reduction on the hyperuniformity property of materials.

The realization of the ultrastrong coupling between Josephson plasma waves (JPWs) and terahertz (THz) photons in the subwavelength microcavity array is of interest for manipulating the THz cavity quantum electrodynamics (cQED), ultrahigh-resolution sensing and imaging, and quantum information processing. Here, we describe the engineering of ultrastrong light-matter interactions in a deeply subwavelength microcavity array based on the hybrid silicon and high-temperature superconductor (HTS) ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ (BSCCO) van der Waals (vdW) heterostructure. We perform numerical modeling and analytical calculation to describe Josephson THz cQED and the ultrastrong coupling process between THz radiation and the JPWs in Josephson medium which is naturally present in BSCCO vdW. The resonance frequency of microcavities is swept through the Josephson plasma frequency by altering their width. THz reflection demonstrates the anticrossing behavior of ultrastrong coupling with a normalized Rabi frequency (coupling strength) $2{\mathrm{\Omega }}_{\mathrm{R}}/f_{\mathrm{c}}=0.29$ for the BSCCO thickness $t=200$ nm, which increases to the value of 0.87 for $t=800$ nm. Furthermore, the thermal behavior of coupling strength shows modulation of Rabi splitting $2{\mathrm{\Omega }}_{\mathrm{R}}$ with temperature. We show that the normalized Rabi splitting $2{\mathrm{\Omega }}_{\mathrm{R}}/f_{\mathrm{c}}$ is independent of the temperature in the BSCCO superconducting regime, while a weak coupling can be observed above the superconducting transition temperature. The proposed chip-scale THz photonic integrated circuit with subwavelength microcavity metamaterial array shall guide the effort in the development of power-efficient coherent THz sources, quantum sensors, ultrasensitive detectors, parametric amplifiers and tunable bolometers based on BSCCO HTS quantum material.

We predict angle-dependent superfluidity for a new class of two-dimensional materials: transition metal trichalcogenides (TMTC). Within a mean-field approach superfluidity of indirect excitons in TMTC van der Waals heterostructures (vdWHs) is studied. We use different potentials for charged carrier interaction to analyze the influence of the screening on the studied phenomena. Our study demonstrates the angle-dependent superfluidity temperature in TMTC vdWHs: for a given density a maximum and minimum $T_c$ of superfluidity occurs along the chain and $a$ directions, respectively. This work can guide experimental research toward the realization of anisotropic superfluidity in TMTC vdWH. We suggest an experiment for the observation of anisotropic superfluidity in TMTC vdWH.