We present a comprehensive study of vortex structures in $d$-wave superconductors from large-scale renormalized mean-field theory of the square-lattice $t\text{−}{t}^{\prime }\text{−}J$ model, which has been shown to provide a quantitative modeling for high-$T_c$ cuprate superconductors. With an efficient implementation of the kernel polynomial method for solving electronic structures, self-consistent calculations involving up to ${10}^5$ variational parameters are performed to investigate the vortex solutions on lattices of up to ${10}^4$ sites. By taking into account the strong correlation of the model, our calculations shed new light on two puzzling results that have emerged from recent scanning tunneling microscopy experiments. The first concerns the issue of the zero-biased-conductance peak (ZBCP) at the vortex core for a uniform $d$-wave superconducting state. Despite its theoretical prediction, the ZBCP was not observed in most doping range of cuprates except in heavily over-doped samples at low magnetic field. The second issue is the nature of the checkerboard charge-density waves (CDWs) with a period of about eight unit cells in the vortex halo at optimal doping. Although it has been suggested that such bipartite structure arises from low-energy quasiparticle interference, another intriguing scenario posits that the checkerboard CDWs originate from an underlying bidirectional pair-density wave (PDW) ordering with the same period. We present a coherent interpretation of these experimental results based on systematic studies of the doping and magnetic-field effects on vortex solutions with and without a checkerboard structure. Due to the small size of Cooper pairs, the vortex core has a radius of about three unit cells, which results in a strong spatial dependence on pairing fields. This may be an important mechanism for the formation of PDW states inside the vortex core.

Understanding the fundamental mechanisms of optoelectronic excitation and relaxation pathways on the single-molecule level has only recently been started by combining scanning tunneling microscopy (STM) and spectroscopy (STS) with STM-induced luminescence (STML). In this paper, we investigate cationic and anionic fluorescence of individual zinc phthalocyanine (ZnPc) molecules adsorbed on ultrathin NaCl films on Ag(111) by using STML. They depend on the tip-sample bias polarity and appear at threshold voltages that are correlated with the onset energies of particular molecular orbitals, as identified by STS. We also find that the fluorescence is caused by a single-electron tunneling process. Comparing with results from density functional theory calculations, we propose an alternative many-body picture to describe the charging and electroluminescence mechanism. In this paper, we provide aspects toward well-defined voltage selectivity of bipolar electrofluorochromism as well as fundamental insights regarding the role of transiently charged states of emitter molecules within organic light-emitting diode devices.

Helical edge states in topological insulators generate a counterpropagating spin current on the two parallel edges. We here propose antihelical edge states of magnons in patterned antiferromagnetic thin films, which host a copropagating spin current on the two parallel edges, where the two magnon modes with opposite chirality act like the spin. The embedded heavy metal dot array in the thin film induces interfacial Dzyaloshinskii-Moriya interactions (iDMIs), drives the magnon bands into nontrivial topological phases, characterized by spin Chern number. The resulting helical edge modes lead to spin current with the direction dependent on the sign of the iDMI parameter. In a strip geometry, we combine two subsystems with two embedded metal dot arrays, which give opposite iDMI parameters. Antihelical edge states emerge, compensated by the counterpropagating bulk-confined states. Helical and antihelical edge states are verified by the micromagnetic simulations. We consider these results promising and inspiring for further developments of magnon spintronic devices based on antiferromagnets.

Single and double quantum dot (QD) transistors have been used to investigate entropy transitions in the single particle limit. Precisely controlled QD electron states allow a few-particle thermodynamic system to be defined. Charge stability diagrams are calculated to find the Gibbs entropy $S$ vs bias voltage, providing a framework to define single particle entropy diagrams. The calculation method is applied to experimental dopant atom QD transistor characteristics. As multiple states become occupied, $S$ increases in a stepwise manner towards $S=kln\mathrm{\Omega }$, where $\mathrm{\Omega }$ is the total number of microstates, retaining the Boltzmann interpretation of entropy. The $T\text{−}S$ diagram vs gate voltage, where $T$ is temperature, reflects underlying single particle state transitions and enables the definition of heat cycles. These diagrams approximate the behavior of macroscopic phase changes in magnetic, liquid-vapor, and superconducting systems.

We experimentally investigate the stability of a quantum gas with repulsive interactions in an optical one-dimensional lattice subjected to periodic driving. Excitations of the gas in the lowest lattice band are analyzed across the complete stability diagram, from slow to fast driving frequencies and from weak to strong driving strengths. To interpret our results, we expand the established analysis based on parametric instabilities to include modulational instabilities. Extending the concept of modulational instabilities from static to periodically driven systems provides a convenient mapping of the stability in a static system to the cases of slow and fast driving. At intermediate driving frequencies, we observe an interesting competition between modulational and parametric instabilities. We experimentally confirm the existence of both types of instabilities in driven systems and probe their properties. Our results allow us to predict stable and unstable parameter regions for the minimization of heating in future applications of Floquet driving.

We explore the nonequilibrium dynamics of a one-dimensional Fermi-Hubbard system as a sensitive testbed for the capabilities of the time-dependent two-particle reduced density matrix (TD2RDM) theory to accurately describe time-dependent correlated systems. We follow the time evolution of the out-of-equilibrium finite-size Fermi-Hubbard model initialized by a quench over extended periods of time. By comparison with exact calculations for small systems and with matrix product state calculations for larger systems but limited to short times, we demonstrate that the TD2RDM theory can accurately account for the nonequilibrium dynamics in the regime from weak to moderately strong interparticle correlations. We find that the quality of the approximate reconstruction of the three-particle cumulant (or correlation) required for the closure of the equations of motion for the reduced density matrix is key to the accuracy of the numerical TD2RDM results. We identify the size of the dynamically induced three-particle correlations and the amplitude of cross correlations between the two- and three-particle cumulants as critical parameters that control the accuracy of the TD2RDM theory when current state-of-the-art reconstruction functionals are employed.

We contribute to fulfill the long-lasting gap in the understanding of the spatial search with multiple marked vertices. The theoretical framework is that of discrete-time quantum walks (QW), i.e., local unitary matrices that drive the evolution of a single particle on the lattice. QW based search algorithms are well understood when they have to tackle the fundamental problem of finding only one marked element in a $d\text{−}\text{dimensional}$ grid and it has been proven they provide a quadratic advantage over classical searching protocols. However, once we consider searching more than one element, the behavior of the algorithm may be affected by the spatial configuration of the marked elements and even the quantum advantage is no longer guaranteed. Here our main contribution is threefold: (i) we provide sufficient conditions for optimality for a multi-items QWSearch algorithm; (ii) we provide analytical evidence that almost, but not all spatial configurations with multiple marked elements are optimal; and (iii) we numerically show that the computational advantage with respect to the classical counterpart is not always certain and it does depend on the proportion of searched elements over the total number of grid points.

Normative models are often used to describe how humans and animals make decisions. These models treat deliberation as the accumulation of uncertain evidence that terminates with a commitment to a choice. When extended to social groups, such models often assume that individuals make independent observations. However, individuals typically gather evidence from common sources, and their observations are rarely independent. Here we ask: For a group of ideal observers who do not exchange information, what is the impact of correlated evidence on decision accuracy? We show that even when agents are identical, correlated evidence causes decision accuracy to depend on temporal decision order. The first decider is less accurate than a lone observer, and early deciders are less accurate than late deciders. These phenomena occur despite the fact that the rational observers use the same decision criterion, so they are equally confident in their decisions. We analyze discrete and macroscopic evidence-gathering models to explain why the first decider is less accurate than a lone observer when evidence is correlated. Pooling the decisions of early deciders using a majority rule does not rescue accuracy results in only a modest accuracy gain. Although we analyze an idealized model, we believe that our analysis offers insights that do not depend on exactly how groups integrate evidence and form decisions.

Fault-tolerant, error-corrected quantum computation is commonly acknowledged to be crucial to the realization of large-scale quantum algorithms that could lead to extremely impactful scientific or commercial results. Achieving a universal set of quantum gate operations in a fault-tolerant, error-corrected framework suffers from a conservation of unpleasantness. In general, no matter what error-correction technique is employed, there is always one element of a universal gate set that carries a significant resource overhead—either in physical qubits, computational time, or both. Specifically, this is due to the application of non-Clifford gates. A common method for realizing these gates for stabilizer codes such as the surface code is a combination of three protocols: state injection, distillation, and gate teleportation. These protocols contribute to the resource overhead compared with logical operations such as a CNOT gate and contribute to the qubit resources for any error-corrected quantum algorithm. In this paper, we introduce a very simple protocol to potentially reduce this overhead for non-Clifford gates: transversal injection. Transversal injection modifies the initial physical states of all data qubits in a stabilizer code before standard encoding and results in the direct preparation of a large class of single qubit states, including resource states for non-Clifford logic gates. Preliminary results hint at high-quality fidelities at larger distances and motivate further research on this technique.

Quantum systems of indistinguishable particles are commonly described using the formalism of second quantization, which relies on the assumption that any admissible quantum state must be either symmetric or antisymmetric under particle permutations. Coherence-induced many-body effects such as superradiance, however, can arise even in systems whose constituents are not fundamentally indistinguishable as long as all relevant dynamical observables are permutation-invariant. Such systems are not confined to symmetric or antisymmetric states and therefore require a different theoretical approach. Focusing on noninteracting systems, here we combine tools from representation theory and thermodynamically consistent master equations to develop such a framework. We characterize the structure and properties of the steady states emerging in permutation-invariant ensembles of arbitrary multilevel systems that are collectively weakly coupled to a thermal environment. As an application of our general theory, we further explore how these states can in principle be used to enhance the performance of quantum thermal machines. Our group-theoretical framework thereby makes it possible to analyze various limiting cases that would not be accessible otherwise. In addition, it allows us to show that the properties of multilevel ensembles differ qualitatively from those of spin ensembles, which have been investigated earlier using the standard Clebsch-Gordan theory. Our results have a large scope for future generalizations and pave the way for systematic investigations of collective effects arising from permutation invariance in quantum thermodynamics.

Coupled neuronal oscillators generate a wide range of dynamic activity patterns that drive various behaviors in animals. A single neural circuit can generate multiple activity patterns and switch between them by tuning the coordination between its component oscillators. However, the mechanisms underlying how neural circuits dynamically tune the phase relationship between oscillators are not fully understood. We studied the phase relationships between two unidirectionally coupled oscillators and found that the duration of synaptic input to the driven oscillator controls the phase difference between them. The phase difference can smoothly change over a wide range, from large phase advances to phase delays, with variations in the duration of synaptic input. The control of the phase difference derives from the entrainment properties shared by commonly used neural oscillators, particularly when they are close to their relaxation limit. We applied our findings on the control of phase difference to a chain of unidirectionally synaptically coupled identical oscillators. We demonstrated that the direction and speed of activity propagation in the chain can be controlled by the duration of synaptic input.

Two spatially separated magnetic impurities coupled to itinerant electrons give rise to a dynamically generated exchange (RKKY) inter-impurity interaction that competes with the individual Kondo screening of the impurities. It has been recently shown by Yevtushenko and Yudson [Phys. Rev. Lett. 120, 147201 (2018)] that the RKKY interaction and the RKKY vs Kondo competition become nontrivial on helical edges of two-dimensional topological insulators where there is lock-in relation between the electron spin and its direction of motion. Kondo screening always takes over and dominates at large inter-impurity distances and it can also dominate all the way to short distances if the Kondo coupling is sufficiently large and anisotropic. In the present paper, we study the Kondo-RKKY competition in detail on a qualitative and quantitative level. For this we employ the numerically exact numerical renormalization group (NRG) for a broad parameter scan of two Kondo coupled impurities vs magnetic anisotropy, impurity distance, and temperature, and comment on the role of finite bandwidth. We give a pedagogical introduction on the the setup of the two-impurity setting within the NRG in the helical context. Overall we establish a plain crossover from RKKY to Kondo with increasing impurity distance, which permits an intuitive physical picture by simply comparing length scales set by the Kondo screening cloud vs the thermal length scale vs the impurity distance.

We investigate a ballistic InSb nanoflag-based Josephson junction with Nb superconducting contacts. The high transparency of the superconductor-semiconductor interfaces enables the exploration of quantum transport with parallel short and long conducting channels. Under microwave irradiation, we observe half-integer Shapiro steps that are robust to temperature, suggesting their possible nonequilibrium origin. Our results demonstrate the potential of ballistic InSb nanoflags Josephson junctions as a valuable platform for understanding the physics of hybrid devices and investigating their nonequilibrium dynamics.

We theoretically study the entropy production and the work extracted from a system connected to two reservoirs by periodic modulations of the electrochemical potentials of the reservoirs and the parameter of a system Hamiltonian under isothermal conditions. We find that the modulation of the parameters can drive a geometric state, which is away from a nonequilibrium steady state (NESS). Using this property, we construct a demon in which the entropy production during the first one-cycle is negative such that we can extract the work if we start from the NESS without parameter modulations. We use the Anderson model to implement the demon in a realistic situation.