The extended Hubbard model describes fermions on a lattice coupled through on-site, $U$, and first-neighbor, $V$, interactions. In the context of high-$T_c$ cuprates, antiferromagnetic fluctuations may lead to an attractive channel, hence to superconductivity. Despite interest in the two-dimensional version of the model, the current knowledge about the phase diagram is still far from complete. Here, we report on the results of extensive determinant quantum Monte Carlo simulations for this model at half filling, in which we have used the average sign of the product of fermionic determinants as an additional observable to locate critical points. We arrive at a ground state phase diagram in the $U\text{−}V$ plane in which the boundaries involving antiferromagnetic, charge-ordered, $s$- and $d$-wave superconductivity, and phase-separated phases are quantitatively set with good accuracy. We have also proposed a partial phase diagram, $T_c(U,V)$, featuring critical temperatures for the charge-density wave and $s$-wave superconducting phases.

The formalism of the Rokhsar-Kivelson (RK) model has been frequently used to study topological phase transitions in 2D in terms of the deformed wave functions, which are RK-type wave functions. A key drawback of the deformed wave functions is that the obtained quantum critical points are RK-type, in the sense that the equal-time correlation functions are described by 2D conformal field theories (CFTs). The generic Lorentz invariant quantum critical points described by $(2+1)\mathrm{D}$ CFTs can not be obtained from the deformed wave functions. To address this issue, we generalize the deformed wave-function approach to the deformed thermofield double (TFD) state methodology. Through this extension, we can effectively reconstruct the absent temporal dimension at the RK-type quantum critical point. We construct deformed TFD states for a $(1+1)\mathrm{D}$ quantum phase transition from a symmetry-protected topological phase to a symmetry-breaking phase and for generic $(2+1)\mathrm{D}$ topological phase transitions from a ${\mathbb{Z}}_2$ topologically ordered phase to a trivial paramagnetic phase.

Polymorphs of boron provide various types of atomic structures that lead to the emergence of physical phenomena and become playgrounds to examine electronic states. There are lithium-boron compounds that embed quasi-one-dimensional (quasi-1D) boron incommensurately in the hexagonal lithium sublattices. Based on first-principles calculations, $k$ groups, and topological invariants, we unveiled that the quasi-1D structures show double $\pi$ bonding orbitals, unavoidable band crossings, and nodal lines around the Fermi level. The origin of this phenomenon is attributed to the interaction between B-B chains through sublattice Li atoms and to the nonsymmorphic crystal symmetry.

Density functional theory (DFT) calculations confirm that the structurally ordered ${\mathrm{Fe}}_2\mathrm{VAl}$ Heusler alloy is nonmagnetic narrow-gap semiconductor. This compound easily crystallizes in various disordered phases with high concentration of antisite defects. We study the effect of structural disorder on the electronic structure, magnetic, and electronic transport properties of the full Heusler alloy ${\mathrm{Fe}}_2\mathrm{VAl}$ and its off-stoichiometric equivalent ${\mathrm{Fe}}_2{\mathrm{VAl}}_{1.35}$. Data analysis in relation to ab initio calculations indicates an appearance of antisite disorder mainly due to Fe-V and Fe-Al stoichiometric variations. The data for weakly magnetic ${\mathrm{Fe}}_2{\mathrm{VAl}}_{1.35}$ are discussed with respect to ${\mathrm{Ni}}_2\mathrm{VAl}$. ${\mathrm{Fe}}_2{\mathrm{VAl}}_{1.35}$ can be classified as a nearly ferromagnetic metal with a pronounced spin-glassy contribution, which, however, does not have a predominant effect on its thermoelectric properties. The figure of merit $ZT$ is at 300 K about 0.05 for the Fe sample and 0.02 for Ni one, respectively. However, it is documented that the narrow $d$ band resulting from Fe/V site exchange can be responsible for the unusual temperature dependencies of the physical properties of the ${\mathrm{Fe}}_2{\mathrm{TiAl}}_{1.35}$ alloy, characteristic of strongly correlated electron systems. As an example, the magnetic susceptibility of ${\mathrm{Fe}}_2{\mathrm{VAl}}_{1.35}$ exhibits singularity characteristic of a Griffiths phase, appearing as an inhomogeneous electronic state below $T_G\sim 200$ K. We also performed numerical analysis which supports the Griffiths phase scenario.

The Tamura model is a particular type of first Born approximation of the phonon-isotope scattering problem. The expression for the mode-resolved phonon-isotope scattering rates in this model, derived in 1983, is still widely used in ab initio transport calculations. While the original work emphasized its applicability to low-energy acoustic phonons only, nevertheless, it has also been applied to optical phonons in the field of phonon transport. The model has the salient feature of being a perturbation theory on top of a virtual-crystal background. As such, this approach does not correspond to the proper methodology for solving the phonon-substitution defect problem in the respective limit. Here we explore three avenues to go beyond the Tamura model and carry out calculations on a set of common materials to compare the different approaches. This work allows a systematic improvement of the treatment of phonon-isotope scattering in ab initio phonon transport while unifying it with the general phonon-substitution scattering problem.

In the framework of the microscopic Bogoliubov-de Gennes theory, we investigate the topological defect states under out-of-plane magnetic flux in a mesoscopic superconducting rectangular system with spin-orbit (SO) interaction. The $s+id$ superconducting phase with time-reversal symmetry breaking can be realized by suitable choice of model parameters. For the perfect rectangle with pure Dresselhaus SO interaction, the unclosed domain wall carrying two one-component vortices and double-quanta coreless skyrmions can arise stably with increasing flux. Interestingly, when the admixed Rashba SO coupling is included, the evolution of vortex matter toward the single-quanta coreless skyrmion with unit topological charge $Q=1$ favors to take place in the present rectangular system. The relative strength of coupled condensates can be effectively tuned by the strengths of Rashba SO coupling and next-nearest-neighbor hopping, giving rise to rich multiple and hybrid topological defect configurations containing even number of $Q=1$ skyrmions. Moreover, an appropriate in-plane Zeeman potential can drive the splitting of enclosed vortex chains, accompanied with the emergence of obvious single-quanta skyrmionic characters. In particular, the number and location of topological defects are highly sensitive to the surface indentation defect. The single-skyrmionic and multiskyrmionic states consisting of odd number of $Q=1$ skyrmions can also be produced by introducing a defect in the middle of the short or the long edge of the rectangle. Our investigation may shed new light on the single-quanta skyrmions and provide useful information for observing and tuning topological defect states in multicomponent superconducting systems.

Dissipation is a common occurrence in real-world systems and is generally considered detrimental to transport. In this paper, we examine the transport properties of a narrow quantum anomalous Hall system with dissipation applied on one edge. When the Fermi level resides within the hybridization gap, we find that, while transport is suppressed on one edge, it is significantly enhanced on the other. We reveal that the enhancement of the conductance on the other edge arises from dissipation-induced gap closure, which is deeply rooted in the point-gap topology of the system, resulting in a reduction of the decaying coefficient. When the dissipation is very large, we find that the low-energy physics is nearly indistinguishable from a narrower system, whose dissipation amplitude is inversely proportional to that of the original one. To get more physical intuition, we demonstrate that the low-energy physics can be well captured by a pair of coupled counterpropagating chiral edge states, one of which has a modified group velocity and an effective dissipation. We also briefly discuss the possible experimental realizations of this enhanced unidirectional transport.

Advanced tensor network numerical methods are used to explore the fidelity of repeated SWAP operations on a system comprising 20–100 quantum dot spin qubits in the presence of valley leakage and electrostatic crosstalk. The fidelity of SWAP gates is largely unaffected by Zeeman splitting and valley splitting, except when these parameters come into resonance. The fidelity remains independent of the overall valley phase for valley eigenstates, while for generic valley states, some minor corrections arise. We analyze the fidelity scaling for long qubit chains without valley effects, where crosstalk represents the only error source.

It has proven difficult to distinguish between topological Majorana bound states and nontopological Andreev bound states and to measure the unique properties of the former. In this work, we aim to alleviate this problem by proposing and theoretically analyzing a different type of fusion protocol based on transport measurements in a Majorana box coupled to normal leads. The protocol is based on switching between different nanowire pairs being tunnel coupled to one of the leads. For a Majorana system, this leads to switching between different states associated with parity blockade. The charge being transmitted at each switch provides a measurement of the Majorana fusion rules. Importantly, the result is different for a system with nontopological Andreev bound states. The proposed protocol only requires measuring a DC current combined with fast gate control of the tunnel couplings.

We report an implementation of the recursion method that addresses quantum many-body dynamics in the nonperturbative regime. The method essentially amounts to constructing a Lanczos basis in the space of operators and solving coupled Heisenberg equations in this basis. The reported implementation has two key ingredients: a computer-algebraic routine for the symbolic calculation of nested commutators and a procedure to extrapolate the sequence of Lanczos coefficients according to the universal operator growth hypothesis. We apply the method to calculate infinite-temperature correlation functions for spin-$1/2$ systems on one- and two-dimensional lattices. In two dimensions the accessible timescale is large enough to essentially embrace the relaxation to equilibrium. The method allows one to accurately calculate transport coefficients. As an illustration, we compute the diffusion constant for the transverse-field Ising model on a square lattice.

We found a plateau in the magnitude of the isolated magnetic moments as a function of the anisotropy of the transfer integral ($t^{\prime }/t$) in the gapless quantum spin liquid (QSL) phase of an $S=1/2$ triangular lattice molecular solid $X{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$, accomplished by a fine tuning of $t^{\prime }/t$ through the mixing of cations, $X$. In contrast, the magnetic susceptibility at the lowest temperature in the QSL phase parametrized by $t^{\prime }/t$ evinces an unconventional suppression approaching the quantum phase transition, implying significant critical fluctuations.

We investigate the impact of transport current on the superconducting order parameter in superconducting islands in full-shell epitaxial Al-InAs nanowires. Depending on the device layout, the suppression of superconductivity occurs in three fundamentally different ways-by a critical current in the case of superconducting reservoirs and by a critical voltage or by a critical Joule power in the case of normal reservoirs. In the latter case, the collapse of the superconducting state depends on the ratio of the dwell time and the electron-phonon relaxation time of quasiparticles in the island. For low resistive and high resistive coupling to the reservoirs, respectively, a relaxation-free regime and a strong electron-phonon relaxation regime are realized. Our results shed light on the potential shortcomings of finite-bias transport spectroscopy in floating islands.

It has been recently established that a nonlinear valley current can be generated through propagating a surface acoustic wave (SAW) in two-dimensional Dirac materials. So far, the SAW-driven valley currents have been attributed to the warped Fermi surface or the Berry phase effect. Here, we demonstrate that the tilt mechanism can also lead to a nonlinear valley Hall current (VHC) when propagating SAWs in materials with the tilted Dirac cone placed on a piezoelectric substrate. It has been found that the nonlinear VHC exhibits a $sin\theta$ dependence on the orientation of the tilt with respect to the SAW. In addition, this tilt-induced nonlinear acoustic VHC shows independence of the relaxation time, distinguished from the contributions from the Berry phase or trigonal warping. Remarkably, the magnitude of the nonlinear acoustic VHC from the tilt mechanism in the uniaxially strained graphene is 2 orders larger than those reported in ${\mathrm{MoS}}_2$ stemming from the Berry phase effect and the warping effect.

Despite decades of intensive studies, some aspects of charge transport through carbon nanotubes (CNTs) remain poorly understood. Here, we explore the transport properties of field-effect devices based on metallic CNTs, where the density of states is supposed to be constant and a field effect is not anticipated. The observed current-voltage characteristics are qualitatively different from these expectations. The conductance of our devices shows significant modulation as a function of gate voltage. However, the minimal conductance does not show a temperature dependence, with the on/off ratio being constant in a wide temperature range. Still, when an axial magnetic field is applied, an Arrhenius temperature dependence of the minimal conductance is observed. The observed effects are naturally explained by the CNT's band-structure modifications under the metallic electrodes at the CNT edges. Our results provide experimental proof of the predicted structural deformations of CNTs in side-contacted devices.

A spin-wave formalism has been developed for Heisenberg antiferromagnets with combined exchange and dipolar interactions. Magnon-magnon interactions, represented by higher-order contributions to the spin Hamiltonian, are included to have the magnon energies renormalized with temperature. Distinct from conventional perturbation theory, these contributions are random-phase decoupled prior to diagonalization. This allows incorporation in the coefficients of the lowest-order Hamiltonian, which is diagonalized by a standard Bogoliubov transformation. In this way, all higher-order terms taken into account fully contribute to the corrections of both the magnon modes and their energies, whereas in conventional perturbation theory only the energies are corrected by a selection of higher-order terms written in the diagonal representation of the unperturbed zero-order Hamiltonian. Excellent agreement with experiment is found for the temperature dependence of the sublattice magnetization in representative one-, two-, and three-dimensional antiferromagnets up to at least 80% of the Néel temperature. This is a marked improvement over the classical perturbation approach to incorporate magnon-magnon interactions. It is thereby essential to allow for phase differences between spin waves in all spatial directions, irrespective of the dimensionality of the exchange interactions. The formalism includes faithful renormalization of the magnon modes and their energies by the model itself, rather than letting the energy gap fall with temperature in a semiempirical way. This leaves only the zero-temperature spin-wave parameters as inputs.

A long excitation coil piercing a superconducting ring is used to generate an ever increasing persistent current in the ring, until the current destroys the order parameter. Given that the penetration depth $\lambda$ is known, this experiment measures, hypothetically, the coherence length $\xi$. We examine various aspects of this theoretically driven hypothesis by testing niobium rings with different dimensions, and by comparing the results to the known values of $\xi$. We then apply the method to two ${\mathrm{La}}_{1.875}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$ rings at $T\to 0$. In one, the current flows in the ${\mathrm{CuO}}_2$ planes, hence it is set by ${\xi }_{ab}$. In the other, the current must cross planes and is determined by ${\xi }_c$. We find that ${\xi }_c=1.3\pm 0.1$ nm, and ${\xi }_{ab}<2.3\mathrm{nm}$, indicating that at low temperatures the Cooper pairs are three dimensional.

We unravel the effect of La substitution and hence antisite disorders on the exchange bias (EB) mechanism in ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ $(x=0, 0.2)$ double-perovskite samples using the detailed analysis of the field cooled magnetization isotherms $\left(M\text{−}H\right)$ and training effect. The field dependence of the freezing temperature deviates from both Gabay-Toulouse and de Almeida-Thouless lines and the analysis suggests that the $x=0$ sample follows a different universality class with moderate anisotropy in the frozen spins. Interestingly, we find that the EB effect is significantly suppressed in the $x=0.2$ sample due to increase [decrease] in the size of ferromagnetic (FM) [cluster glass (CG)] domain, which reduces the effective disordered interface responsible for the EB. The changes in fraction of FM, antiferromagnetic, and CG-like interactions with the La substitution and applied magnetic field are found to be crucial in governing the EB effect in these samples. Furthermore, the training effect measurements show the unequal shift in the left and right branches of the $M\text{−}H$ loops and their different evolution with the field cycles $\left(n\right)$. The analysis reveals that the rotatable spins relax approximately one order of magnitude faster than the frozen spins at the disordered interface. We find a possible correlation between the observed EB effect and the antisite disorders in these samples.

In this paper, we study the electronic structure of a helically magnetized magnetic Weyl semimetal and propose a multistate memory device utilizing the transport in such a magnetic superlattice. For both Bloch- and Néel-type spiral magnetic textures, we report magnetization-tunable flat bands in the superlattice direction via the onset of bulk axial Landau levels (LLs), find the analytic zeroth axial LL wave functions and energy spectrum of both systems, and determine a condition for band flattening. We show that both Bloch- and Néel-type devices using this electronic structure have application in neuromorphic memristive devices and do not rely on the topology or spin polarization of the contacts, with Bloch-type devices allowing for an additional chirality filtering for next-generation computing and memory.