The iron-based superconductor FeSe isovalently substituted with S displays an abundance of remarkable phenomena that have not been fully understood, at the center of which are apparent zero-energy excitations in the superconducting state in the tetragonal phase. The phenomenology has been generally consistent with the proposal of the so-called ultranodal states where Bogoliubov Fermi surfaces (BFSs) are present. Recently, nuclear magnetic resonance measurements have seen unusually large upturns in the relaxation rate as temperature decreases to nearly zero in these systems, calling for theoretical investigations. In this paper, we calculate the spin susceptibility of an ultranodal superconductor including correlation effects within the random phase ap- proximation. Although the noninteracting mean-field calculation rarely gives an upturn in the low-temperature relaxation rate within our model, we found that correlation strongly enhances scattering between coherent parts of the BFS, resulting in robust upturns when the interaction is strong. Our results suggest that, in addition to the presence of BFSs, correlation and multiband physics also play important roles in the low-energy excitations of the system.

We synthesized ${\mathrm{CaRuO}}_3$ (001) thin films on a set of substrates and investigated their electronic and magnetic properties via combining magnetotransport measurements with first-principles density-functional theory calculations. The experimental results indicate that a moderate strain can introduce the Kondo effect in the system, leading to a significant modulation of the non-Fermi liquid behavior. Moreover, when the strain reaches a certain threshold, the system undergoes a metal-semiconductor transition, accompanied by a transition from a nonmagnetic state to a plausible G-type antiferromagnetic state. We attribute the observed phenomena in ${\mathrm{CaRuO}}_3$ to strain-induced disruption of the delicate balance between the itinerant and the local Ru $4d$ electrons. These findings shed light on the intriguing magnetic and non-Fermi liquid behavior of ${\mathrm{CaRuO}}_3$, systematically tailored by heteroepitaxial strain.

Understanding the mechanisms of qubit decoherence is a crucial prerequisite for improving the qubit performance. In this paper, we discuss the effects of residual Bogolyubov quasiparticles in Schrödinger cat qubits, either of the dissipative or Kerr type. The major difference from previous studies of quasiparticles in superconducting qubits is that the Schrödinger cat qubits are operated under nonequilibrium conditions. Indeed, an external microwave drive is needed to stabilize cat states, which are superpositions of coherent degenerate eigenstates of an effective stationary Lindbladian in the rotating frame. We present a microscopic derivation of the master equation for cat qubits and express the effect of the quasiparticles as dissipators acting on the density matrix of the cat qubit. This enables us to determine the conditions under which the quasiparticles give a substantial contribution to the qubit errors.

Among the wide variety of single fluorescent defects investigated in silicon, numerous studies have focused on color centers with a zero-phonon line around $1.28\mu \mathrm{m}$ and identified to a common carbon complex in silicon, namely the $G$ center. However, inconsistent estimates regarding their quantum efficiency cast doubt on the correct identification of these individual emitters. Through a comparative analysis of their single-photon emission properties, we demonstrate that these single color centers are split in two distinct families of point defects. A first family consists of the genuine single $G$ centers with a well-identified microscopic structure and whose photoluminescence has been investigated on ensemble measurements since the 1960s. The remaining defects belong to another color center, which we will refer to as the $G^{★}$ center, whose atomic configuration has yet to be determined. These results provide a safeguard against future defect misidentifications, which is crucial for further development of quantum technologies relying on $G$ or $G^{★}$ center quantum properties.

Spin-ordered states close to metal-insulator transitions are poorly understood theoretically and challenging to probe in experiments. Here, we propose that the quantum twisting microscope, which provides direct access to the energy-momentum resolved spectrum of single-particle and collective excitations, can be used as a novel tool to distinguish between different types of magnetic order. To this end, we calculate the single-particle spectral function and the dynamical spin-structure factor for both a ferromagnetic and antiferromagnetic generalized Wigner crystal formed in fractionally filled moiré superlattices of transition metal dichalcogenide heterostructures. We demonstrate that magnetic order can be clearly identified in these response functions. Furthermore, we explore signatures of quantum phase transitions in the quantum twisting microscope response. We focus on the specific case of triangular moiré lattices at half filling that have been proposed to host a topological phase transition between a chiral spin liquid and a ${120}^{\circ }$ ordered state. Our work demonstrates the potential for quantum twisting microscopes to characterize quantum magnetism in moiré heterostructures.

We investigate the phase diagram of the half-filled triangular-lattice Hubbard model using the density-matrix renormalization group method. By systematically studying the ground-state properties with the ratio of on-site Coulomb repulsion and bandwidth, we first identify two competing intermediate phases in the vicinity of metal-insulator transition on narrow cylinders with width $L_y=3$: a gapped chiral spin liquid phase and a gapless phase with stripy antiferromagnetic quasi-long-range order. Then, as the system width increases to width $L_y=4,6,7$, the spin chirality order persists in a wider range of parameters with significantly enhanced amplitude, giving rise to a robust chiral spin liquid phase toward the two-dimensional limit. The stripy antiferromagnetic quasi-long-range order phase, however, is fragile and eventually disappears. We further extrapolate the amplitude of the spin chirality order parameter as a function of cylinder width and also address the previous discrepancies on the nature of intermediate phases.

In this work we propose a solid-state platform for creating quantum simulators based on implanted spin centers in semiconductors. We show that under the presence of an external magnetic field, an array of $S=1$ spin centers interacting through magnetic dipole-dipole interaction can be mapped into an effective spin-half system equivalent to the XYZ model in an external magnetic field. Interestingly, this system presents a wide range of quantum phases and critical behaviors that can be controlled via magnetic field and orientational arrangement of the spin centers. We demonstrate our interacting spin chain can be tuned to both isotropic Heisenberg model and transverse-field Ising universality class. Notably, our model contains a line where the system is in a critical floating phase that terminates at Berezinskii-Kosterlitz-Thouless and Pokrovsky-Talapov transition points. We propose this system as a solid-state quantum simulator for the floating phase based on spin centers.

Nonreciprocal superconductivity, also known as the superconducting diode effect, has been extensively studied in the presence of a magnetic field or some form of ferromagnetic order breaking time-reversal symmetry. We here show that another class of magnetic order known as altermagnetism, which also breaks time-reversal symmetry but does not exhibit a finite net magnetic moment, can also give rise to a superconducting diode effect. Whether this is the case depends on the combination of the system's point group and altermagnetic order parameter, which we explore systematically for two-dimensional crystalline systems. If the superconducting electrons are in a centrosymmetric crystalline environment, an electric field $E_z$ (or other sources of inversion symmetry breaking) can be used to turn on and tune the nonreciprocity, yielding an electric-field-tunable diode effect; there are also noncentrosymmetric point groups, which are not reached by applying $E_z\ne 0$ in a centrosymmetric crystal, but still allow for an altermagnetic order parameter with nonreciprocal superconductivity. Depending on the residual magnetic point group, the zeros of the critical current asymmetry, $J_c\left(\widehat{\mathit{n}}\right)-J_c(-\widehat{\mathit{n}})$, are pinned along high-symmetry crystalline directions $\widehat{\mathit{n}}={\widehat{\mathit{e}}}_j$ or are free to rotate in the plane of the system. In some cases, the zeros can be rotated by tuning the electric field $E_z$. We discuss all of these phenomena both on the general level using exact symmetry arguments and more explicitly by constructing and solving minimal lattice models. We provide experimental setups to realize the altermagnetic superconducting diode effect.

Van der Waals heterostructures of two-dimensional transition metal dichalcogenides provide a unique platform to engineer optoelectronic devices tuning their optical properties via stacking, twisting, or straining. Using ab initio many-body perturbation theory, we predict the electronic and optical (absorption and photoluminescence spectra) properties of ${\mathrm{MoS}}_2/{\mathrm{WS}}_2$ and ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterobilayers with different stacking and twisting. We analyze the valley splitting and optical transitions, and we explain the enhancement or quenching of the inter- and intralayer exciton states. We fully include transitions within the entire Brillouin Zone, contrary to predictions based on continuum models which only consider energies near the K point. As a result, we predict an interlayer exciton with significant electron density in both layers and a mixed intralayer exciton distributed over both ${\mathrm{MoSe}}_2$ and ${\mathrm{WSe}}_2$ in a twisted Se-based heterostructure. We propose that it should be possible to produce an inverted order of the excitonic states in some ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2$ heterostructures, where the energy of the intralayer ${\mathrm{WSe}}_2$ exciton is lower than that in ${\mathrm{MoSe}}_2$. We predict the variability across different stacking of the exciton peak positions ($\sim 100\mathrm{meV}$) and the exciton radiative lifetimes, from pico- to nanoseconds, and even microseconds in twisted bilayers. The control of exciton energies and lifetimes paves the way toward applications in quantum information technologies and optical sensing.

Recently, a wide family of electronic orders was unveiled in graphene flat bands, such as spin- and valley-polarized phases as well as nematic momentum-polarized phases, stabilized by exchange interactions via a generalized Stoner mechanism. Momentum polarization involves orbital degrees of freedom and is therefore expected to impact resistivity in a way that is uniquely sensitive to the ordering type. Under pocket polarization, carrier distribution shifts in $k$ space and samples the band mass in regions defined by the displaced momentum distribution. This makes transport coefficients sensitive to pocket polarization, resulting in the Ohmic resistivity decreasing with temperature. In addition, it leads to current switching and hysteresis under strong $E$ field. Being robust in the presence of electron-phonon scattering, this behavior can serve as a telltale sign of pocket polarization order. The fast timescale and low dissipation of the switching cycle may be advantageous for highly applicable memory-dependent resistors, i.e., memristors.

We show the appearance of an unconventional Majorana zero mode whose wave function splits into multiple parts located at different ends of different one-dimensional topological superconductors, hereafter referred to as a multilocational Majorana zero mode. Specifically, we discuss the multilocational Majorana zero modes in a three-terminal Josephson junction consisting of topological superconductors, which forms an elemental qubit of fault-tolerant topological quantum computers. We also demonstrate anomalously long-ranged nonlocal resonant transport phenomena caused by the multilocational Majorana zero mode.

The addition of magnetic impurities in topological insulators (TIs) can drive ferromagnetic order that leads to quantum anomalous Hall transport well below the Curie temperature. The fragility of the quantized regime has been ascribed to the random nature of the magnetic moment distribution. Here, we refine this hypothesis by using inelastic neutron scattering and density-functional theory calculations to show that two antagonistic components define the magnetism in Mn-substituted SnTe, thereby limiting the effectiveness of dilute magnetic TIs. One component is strongly bound antiferromagnetic dimers that compete with ferromagnetic order. The other component consists of undimerized moments where ferromagnetic order develops via long-range interactions.

We introduce a message-passing neural-network (NN)-based wave function Ansatz to simulate extended, strongly interacting fermions in continuous space. Symmetry constraints, such as continuous translation symmetries, can be readily embedded in the model. We demonstrate its accuracy by simulating the ground state of the homogeneous electron gas in three spatial dimensions at different densities and system sizes. With orders of magnitude fewer parameters than state-of-the-art NN wave functions, we demonstrate better or comparable ground-state energies. Reducing the parameter complexity allows scaling to $N=128$ electrons, previously inaccessible to NN wave functions in continuous space, allowing future work on finite-size extrapolations to the thermodynamic limit. We also show the capability of the Ansatz to quantitatively represent different phases of matter.

We describe two developments of tensor network influence functionals [in particular, influence functional matrix product states (IF-MPS)] for quantum impurity dynamics within the fermionic setting of the Anderson impurity model. The first provides the correct extension of the IF-MPS to continuous time by introducing a related mathematical object, the boundary influence functional MPS. The second connects the dynamics described by a compressed IF-MPS to that of a quantum embedding method with a time-dependent effective bath undergoing nonunitary dynamics. Using these concepts, we implement higher-order time propagators for the quench dynamics of the Anderson impurity model within the boundary IF-MPS formalism. The calculations illustrate the ability of the current formulation to efficiently remove the time-step error in standard discrete-time IF-MPS implementations as well as to interface with state-vector propagation techniques. They also show the advantages of IF-MPS dynamics, with its associated highly compact effective bath dynamics, over state-vector propagation with a static bath discretization.

Collective plasmon modes, riding on top of drifting electrons, acquire a fascinating nonreciprocal dispersion characterized by ${\omega }_p\left(\mathbf{q}\right)\ne {\omega }_p(-\mathbf{q})$. The classical plasmonic Doppler shift arises from the polarization of the Fermi surface due to the applied DC bias voltage. Here, we predict an additional quantum contribution to the plasmonic Doppler shift originating from the quantum metric of the Bloch wavefunction. We systematically compare the classical and quantum corrections to the Doppler shifts by investigating the drift-induced nonreciprocal plasmon dispersion in graphene and in twisted bilayer graphene. We show that the quantum plasmonic Doppler shift dominates in moiré systems at large wave vectors, yielding plasmonic nonreciprocity up to 20% in twisted bilayer graphene. Our findings highlight the significance of the quantum corrections to plasmonic Doppler shift in moiré systems and motivate the design of innovative nonreciprocal photonic devices with potential technological implications.

Dzyaloshinskii-Moriya interaction (DMI) is found in the epitaxial Ni/Cu(001) system by imaging magnetic domain wall profiles with high resolution and by Brillouin light scattering. The domain walls have right-handed chirality and deviate from both the Néel and Bloch configurations for Ni thicknesses up to 9 nm. From a detailed structural analysis it is found that strain in the Ni film is inhomogeneous with film thickness. We suggest that this strain field is responsible for the symmetry breaking that is required for a sizable DMI to appear, even in absence of a heavy metal.

First-principle calculations of the Raman intensities of a ${\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2^{+}$ centrosymmetric dimer lead to a full reconsideration of the assignment of the three C=C stretching phonons in $\kappa$-phase BEDT-TTF salts. In contrast with previous interpretation, it is found that the presence of the out-of-phase coupling of the antisymmetric external C=C stretching mode has also to be taken into account. This mode, infrared-active for a single BEDT-TTF molecule, implies a strong intramolecular charge oscillation along BEDT-TTF long molecular axis. In the consequent reassignment of the C=C spectral region, a very broad band appearing in the cross polarized Raman spectra is interpreted as due to interdimer electron-molecular vibration (e-mv) coupling, to be contrasted with the well-known intradimer e-mv coupling that induces very strong infrared absorptions. Analysis of the data in terms of a properly developed e-mv scheme yields an evaluation of the Hubbard parameters relevant to the so-called effective dimer model often used to interpret the physical properties of $\kappa$-BEDT-TTF salts. Finally, the Raman spectra of an asymmetric ${\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}^{+0.6}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}^{+0.4}$ dimer have been calculated in order to interpret the Raman spectra of the intriguing charge-ordered (CO), ferroelectric phase of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_2\mathrm{Cl}$. It turns out that in the asymmetric dimer the strong intradimer the e-mv induced infrared C=C stretching absorption should appear with huge intensity also in Raman. The absence of such band in the spectra of $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Hg}{\left(\mathrm{SCN}\right)}_2\mathrm{Cl}$ ferroelectric phase points to an alternative CO pattern with respect to the one generally accepted so far.

By Avila's global theory, we analytically reveal that the non-Hermitian mobility edge will take on a ring structure in the complex plane, which we name as the “mobility ring”. The universality of the mobility ring has been checked and supported by the Hermitian limit, $PT$-symmetry protection, and without $PT$-symmetry cases. Further, we study the evolution of mobility ring versus quasiperiodic strength, and find that in the non-Hermitian system, there will appear multiple mobility ring structures. With cross reference to the multiple mobility edges in the Hermitian case, we give the expression of the maximum number of mobility rings. Finally, by comparing the results of Avila's global theorem and self-duality method, we show that self-duality relation has its own limitations in calculating the critical point in non-Hermitian systems. As we know, the general non-Hermitian system has a complex spectrum, which determines that the non-Hermitian mobility edge can but exhibit a ring structure in the complex plane.

Multiorbital electronic models hosting a nontrivial band topology in the regime of strong electronic interactions are an ideal playground for exploring a host of complex phenomenology. We consider here a sign-problem-free and time-reversal symmetric model with isolated topological (Chern) bands involving both spin and valley degrees of freedom in the presence of a class of repulsive electronic interactions. Using a combination of numerically exact quantum Monte Carlo computations and analytical field-theoretic considerations, we analyze the phase diagram as a function of the flat-band filling, temperature, and relative interaction strength. The low-energy physics is described in terms of a set of intertwined orders—a spin-valley Hall (SVH) insulator and a spin-singlet superconductor (SC). Our low-temperature phase diagram can be understood in terms of an effective SO(4) pseudospin nonlinear sigma model. Our work paves the way for building more refined and minimal models of realistic materials, including moiré systems, to study the universal aspects of competing insulating phases and superconductivity in the presence of nontrivial band topology.

The ground-state phonon pressure is an analog to the famous Casimir pressure of vacuum produced by zero-point photons. The acoustic Casimir forces are, however, many orders of magnitude weaker than the electromagnetic Casimir forces, as the typical speed of sound is 100 000 times smaller than the speed of light. Because of its weakness, zero-point acoustic Casimir pressure was never observed, although the pressure of artificially introduced sound noise on a narrow aperture has been reported. However, the magnitude of Casimir pressure increases as $1/L^3$ with the decrease of the sample size $L$, and reaches piconewtons in the submicron scales. We demonstrate and measure the acoustic Casimir pressure induced by zero-point phonons in solid helium adsorbed on a carbon nanotube. We have also observed Casimir-like “pushing out” thermal phonons with the decreasing temperature or the length. We also show that all thermodynamic quantities are size dependent, and therefore in the mesoscopic range $L\lesssim \hslash c/\left(k_{B}T\right)$ quadruple points are possible on the phase diagram where four different phases coexist. Due to the smallness of solid helium sample, temperature of Bose-Einstein condensation (BEC) of vacancies is relatively high, $10-100\mathrm{mK}$. This allowed us to experimentally discover the BEC in a system of zero-point vacancies, predicted more than 50 years ago.