A strongly correlated superconductor with reduced dimension attracts much interest since its critical temperature ($T_c$) is often enhanced. Superconducting anisotropy is the key parameter to characterize the dimensional reduction of a superconductor. $\left({\mathrm{Li}}_{0.84}{\mathrm{Fe}}_{0.16}\right){\mathrm{OHFe}}_{0.98}\mathrm{Se}$ (FeSe11111) is a newly found iron-based superconductor by intercalating (Li/Fe)OH layers into FeSe. The large distance between the FeSe layers leads to a highly two-dimensional character of the electronic structure compared to bulk FeSe. By performing out-of-plane angular-dependent torque measurements on FeSe11111 single-crystal samples with $T_c=40$ K, we directly obtain the superconducting anisotropy parameter $\gamma$. It is found that $\gamma$ reaches about 20 at a reduced temperature $t(=T/T_c)=0.8$, suggesting that FeSe11111 is more anisotropic than other families of iron-based superconductors. Further, $\gamma$ shows a temperature-dependent behavior, which points towards a multiband scenario. Our result provides thermodynamic evidence for the two-dimensional nature of the superconductivity in FeSe11111.

An artificial neural network (ANN) with the restricted Boltzmann machine (RBM) architecture was recently proposed as a versatile variational quantum many-body wave function. In this work we provide physical insights into the performance of this ansatz. We uncover the connection between the structure of the RBM and perturbation series, which explains the excellent precision achieved by the RBM ansatz in certain simple models, demonstrated in G. Carleo and M. Troyer [Science 355, 602 (2017)]. Based on this relation, we improve the numerical algorithm to achieve a better performance of the RBM in cases where local minima complicate the convergence to the global one. Furthermore, we study the performance of a sparse RBM for approximating ground states of random, translationally invariant models in one dimension, as well as random matrix-product states. We find that the error in approximating such states exhibits a broad distribution and shows a positive correlation with the entanglement properties of the targeted state.

The trilayer nickelate ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10-\delta }$ was investigated by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. The crystal structure data suggest a higher Ni valence in the inner perovskitelike layer. At ambient pressure the resistivity shows a jump at 162 K, indicating a metal-to-metal transition (MMT). The MMT is also characterized by a magnetic susceptibility drop, a sharp specific-heat peak, and an isotropic lattice contraction. Below $\sim 50\mathrm{K}$, a resistivity upturn with $logT$ dependence shows up, accompanied with negative thermal expansion. External hydrostatic pressure suppresses the resistivity jump progressively, coincident with the weakening of the $logT$ behavior. The low-temperature electronic specific-heat coefficient is extracted to be $\sim 150\mathrm{mJ}{\mathrm{K}}^{-2}\mathrm{mol}\text{−}{\mathrm{f}.\mathrm{u}.}^{-1}$, equivalent to $\sim 50\mathrm{mJ}{\mathrm{K}}^{-2}\mathrm{mol}\text{−}{\mathrm{Ni}}^{-1}$, indicating an unusual heavy-electron correlated state. The novel heavy-electron state as well as the logarithmic temperature dependence of resistivity is explained in terms of the ${\mathrm{Ni}}^{3+}$-centered Kondo effect in the inner layer of the ${\left({\mathrm{NdNiO}}_3\right)}_3$ trilayers.

We investigate the higher-order topological insulators on the breathing kagome lattices with $p$-orbital freedom. The bulk topological phase diagram is obtained by calculating the $Z_3$ Berry phase of the $p$-band model. The existence of a different class of orbital corner states is demonstrated on the (zero-dimensional) finite-size triangular structure. We find that the transverse hopping of the $p$ orbital is essential for the corner states to appear on the finite-size structure. The orbital configurations of the corner states are also explored and rich orbital orders are identified. Moreover, we show that the orbital corner states can be realized in a realistic photonic system, which are in good agreement with the predictions of the tight-binding model and then offer a versatile platform for the orbital physics.

The topological electronic properties of orthorhombic-phase ${\mathrm{Mo}}_2\mathrm{C}$ and ${\mathrm{W}}_2\mathrm{C}$ superconductors have been studied based on first-principles electronic structure calculations. Our studies show that both ${\mathrm{Mo}}_2\mathrm{C}$ and ${\mathrm{W}}_2\mathrm{C}$ are three-dimensional strong topological insulators defined on curved Fermi levels. The topological surface states on the (001) surface of ${\mathrm{Mo}}_2\mathrm{C}$ right cross the Fermi level, while those of ${\mathrm{W}}_2\mathrm{C}$ pass through the Fermi level with slight electron doping. These surface states hold helical spin textures and can be induced to become superconducting via a proximity effect, giving rise to an equivalent $p+ip$ type superconductivity. Our results show that ${\mathrm{Mo}}_2\mathrm{C}$ and ${\mathrm{W}}_2\mathrm{C}$ can provide a promising platform for exploring topological superconductivity and Majorana zero modes.

Tunable exciton condensates exhibit anomalous Hall transport owing to mutual Coulomb coupling and have been widely studied in two-dimensional electron gas systems under a strong magnetic field. Here, we explore another framework using topological flat band models in the absence of Landau levels for realizing the many-body exciton phases of two-component fermions under strong intercomponent interactions. By developing a new diagnosis based on the state-of-the-art density-matrix renormalization group and exact diagonalization, we show the theoretical discovery of the emergence of the Halperin (111) quantum Hall effect at a total filling factor $\nu =1$ in the lowest Chern band under strong Hubbard repulsion, which is classified by the unique ground state with bulk charge insulation and spin superfluidity, The topological nature is further characterized by one edge branch of chiral propagating Luttinger modes with level counting 1, 1, 2, 3, 5, 7, consistent with the conformal field theory description. Moreover, with nearest-neighbor repulsions, we propose the Halperin (333) fractional quantum Hall effect at a total filling factor $\nu =1/3$ in the lowest Chern band.

Partially embedded Pb chains are synthesized on a vicinal Si(113) surface in a process of self-organization. The deposition of one monolayer-range amount of Pb orders the substrate over the macroscopic scale and results in extremely long monatomic chains reproducing Si lattice geometry, as demonstrated by reflection high-energy electron diffraction and scanning tunneling microscopy. Angle-resolved photoelectron spectroscopy reveals well-developed one-dimensional metallic bands. Ab initio density functional theory calculations fully support the experimental findings and predict partially embedded zigzag Pb chains with substantially spin-orbit-split one-dimensional electron bands. The present work is experimental evidence of a new type of one-dimensional structures in which one of two chains of atoms building the structure is embedded in the substrate.

We study one-dimensional (1D) and quasi-1D periodic structures as possible platforms for the emergence of Majorana bound states with enhanced robustness against disorder and system inhomogeneity. First, using a simple 1D model, we analytically derive the effective parameters characterizing the minibands generated by the periodic potential. We show that, for strong enough periodic potentials, the higher-energy minibands hosting Majorana bound states have significant advantages compared to their counterparts in uniform systems, including increased topological gaps, enhanced robustness against disorder, and enlarged parameter space regions consistent with the presence of topological superconductivity. We identify the problem of engineering a strong enough periodic potential as a key roadblock to realizing efficient periodic 1D structures. To address this challenge, we propose an efficient implementation of the periodic potential based on quasi-1D channels realized in two-dimensional (2D) semiconductor heterostructures proximity coupled to superconductor strips of periodically modulated width. Our numerical study of the modulated channel device shows excellent agreement with the simple 1D model, reveals a topological phase diagram that is quite insensitive to the details of the confining potential associated with screening by the superconductor, and demonstrates that engineering patterned 2D structures represents a powerful and versatile approach to realizing robust Majorana bound states.

The geometric configuration and energy of a hydrogen molecule centered between two point-shaped tips of equal charge are calculated with the variational quantum Monte Carlo (QMC) method without the restriction of the Born-Oppenheimer (BO) approximation. The ground-state nuclear distribution, stability, and low-vibrational excitations are found to deviate significantly from the BO treatment based on a potential energy surface obtained with the same QMC accuracy. The quantum-mechanical distribution of the molecular axis direction and the bond length at a subnanometer level is fundamental for understanding nanomechanical dynamics with embedded hydrogen. The cylindrical symmetry of the tip arrangement yields a uniform azimuthal distribution of the molecular axis vector relative to the tip-tip axis. For fixed tip separation, the QMC sampling shows that the polar angle distribution of the molecular axis is centered around the equatorial plane for positive tip charge (transverse alignment) and around the tip-tip direction for negative tip charge (bridge alignment). These deviations from spherical symmetry are magnified as the tip-tip distance decreases. Our results thus show that the molecular orientation in the junction can be controlled by the tip charge and separation, suggesting an application in the field of molecular machines.

Based on the Bethe-Salpeter equation eigenstates, we present a first-principles many-body formalism for calculating the two-photon absorption (TPA) coefficient of semiconductors. We apply this formalism to calculate the TPA spectra of $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{W}{\mathrm{S}}_2$ monolayers. The all-electron full-potential linearized augmented plane wave based functions are used for solving the Bethe-Salpeter equation. The calculated TPA spectra exhibit significant excitonic effects when compared to those based on the independent particle approximation. The physical origin of TPA excitonic transitions of $\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{W}{\mathrm{S}}_2$ monolayers are revealed by tracing the sum-over-states process. We show that the spin-orbit coupling effect leads to characteristic double peaks with an interval of half spin-orbit splitting energy. These double peaks mainly originate from the transitions at the vicinity of $K$ point. We compare our calculated two-photon absorption spectra to experimental and other theoretical results.

Recent achievements in the field of gate-defined semiconductor quantum dots reinforce the concept of a spin-based quantum computer consisting of nodes of locally connected qubits which communicate with each other via superconducting circuit resonator photons. In this paper, we theoretically demonstrate a versatile set of quantum gates between adjacent spin qubits defined in semiconductor quantum dots situated within the same node of such a spin-based quantum computer. The electric dipole acquired by the spin of an electron that moves across a double quantum dot potential in a magnetic field gradient has enabled strong coupling to resonator photons and low-power spin control. Here we show that this flopping-mode spin qubit also provides the tunability to program multiple two-qubit gates. Since the capacitive coupling between these qubits brings about additional dephasing, we calculate the estimated infidelity of different two-qubit gates in the most immediate possible experimental realizations.

We explore proximity effects on transition metal dichalcogenide ribbons deposited on antiferromagnetic (AFM) insulating substrates. We model these hybrid heterostructures using a tight-binding model that incorporates exchange and Rashba fields induced by proximity to the AFM material. The robust edge states that disperse in the midgap of the dichalcogenide are strongly affected by induced exchange fields that reflect different AFM ordering in the substrate. This results in enhanced spin-orbit coupling effects and complex spin projection content for states on zigzag ribbon edges. Gated systems that shift the Fermi level in the midgap range are also shown to exhibit spin-polarized currents on these edges. Antiparallel exchange fields along the edge results in spin currents that can reverse polarization with the applied field. The added functionality of these hybrid structures can provide spintronic devices and versatile platforms to further exploit proximity effects in diverse material systems.

Integrating the carrier tunability of a functional two-dimensional material electronic device with a direct probe of energy- and momentum-resolved electronic excitations is essential to gain insights on how many-body interactions are influenced during device operation. Here, we use microfocused angle-resolved photoemission in order to analyze many-body interactions in back-gated graphene supported on hexagonal boron nitride. By extracting the doping-dependent quasiparticle dispersion and self-energy, we observe how these interactions renormalize the Dirac cone and impact the electron mobility of our device. Our results are not only limited to a finite energy range around the Fermi level, as in electron transport measurements, but describe interactions on a much wider energy scale, extending beyond the regime of hot carrier excitations.

We consider the phase diagram of the most general SU(4)-symmetric two-site Hamiltonian for a system of two fermions per site (i.e., self-conjugate $\mathbf{6}$ representation) on the square lattice. It is known that this model hosts magnetic phases breaking SU(4) symmetry and quantum disordered dimerlike phases breaking lattice translation symmetry. Motivated by a previous work [O. Gauthé, S. Capponi, and D. Poilblanc, Phys. Rev. B 99, 241112(R) (2019)], we investigate the possibility of the existence of SU(4) quantum spin liquid phases in this model, using SU(4)-symmetric projected entangled pair states (PEPS) of small bond dimensions, which can be classified according to point group and charge (C) symmetries. Among several (disconnected) families of SU(4)-symmetric PEPS, breaking or not C-symmetry, we identify critical or topological spin liquids which may be stable in some regions of the phase diagram. These results are confronted to exact diagonalization (ED) and density matrix renormalization group (DMRG) calculations.

We present a compressive sensing approach for the long-standing problem of Matsubara summation in many-body perturbation theory. By constructing low-dimensional, almost isometric subspaces of the Hilbert space we obtain optimum imaginary time and frequency grids that allow for extreme data compression of fermionic and bosonic functions in a broad temperature regime. The method is applied to the random phase and self-consistent $GW$ approximation of the grand potential. Integration and transformation errors are investigated for Si and ${\mathrm{SrVO}}_3$.

Energy transport can reveal information about interacting many-body systems beyond other transport probes. In particular, in one dimension it has been shown that the energy current is directly proportional to the central charge, thus revealing information about the degrees of freedom of critical systems. In this paper, we explicitly verify this result in two cases for translationally invariant systems based on explicit microscopic calculations. More importantly, we generalize the result to nontranslation invariant setups and use this to study a composite system of two subsystems possessing different central charges. We find a bottleneck effect, meaning the smaller central charge limits the energy transport.

We develop a general theoretical framework based on $Z$ classification to count the number of topological bound states at a junction of chiral-symmetric one-dimensional systems. The formulation applies to general multiway junctions composed of an arbitrary number of channels and an arbitrary joint structure. By using the formula, we calculate the zero-energy bound states in various types of two-way and three-way junctions of semiconducting graphene nanoribbons. We then consider periodic two-dimensional networks of graphene nanoribbons and show that the topological junction states form isolated energy bands inside the bulk energy gap, which can be viewed as a two-dimensional crystal of the effective atoms. Depending on the $Z$ number of a single junction, we have a different set of effective atomic orbitals, resulting in various types of nanoscale metamaterials, which are often accompanied by flat bands. The system would provide an ideal platform for quantum simulator to emulate a strongly-interacting fermion system on various types of lattices.

We study the influence of a thermal environment on a nonadiabatic spin-flip driving protocol of spin-orbit qubits. The driving protocol operates by moving the qubit, trapped in a harmonic potential, along a nanowire in the presence of a time-dependent spin-orbit interaction. We consider the harmonic degrees of freedom to be weakly coupled to a thermal bath. We find an analytical expression for the Floquet states and derive the Lindblad equation for a strongly nonadiabatically driven qubit. The Lindblad equation corrects the dynamics of an isolated qubit with Lamb shift terms and a dissipative behavior. Using the Lindblad equation, the influence of a thermal environment on the spin-flip protocol is analyzed.

The time evolution of the lattice structure in graphene after ultrashort laser excitation results in a complex dynamics of electrons and ions. In particular, a femtosecond laser pulse heats up the electrons, which then couple strongly to particular optical phonon modes, called SCOPs (strongly coupled optical phonons), located around the $\mathrm{\Gamma }$ and K (K') points of the Brillouin zone. In this paper, a fully ab initio description of the ultrafast structural response of graphene to femtosecond laser excitation is presented. Our atomistic simulations show that upon intense ultrafast laser excitation, a biexponential decay of the Bragg peaks takes place, in agreement with experiments. The calculated time-dependent phonon energies show that SCOPs equilibrate considerably faster with phonons having frequencies above 10 THz than with lower frequency phonon modes.

Pressure effects have been widely studied in spin-crossover (SCO) solids due to their immediate influence on the thermal dependence of the high-spin (HS) fraction. In most of the cooperative SCO materials, the applied pressure shifts the transition temperatures upward and decreases the thermal hysteresis widths to such an extent that it vanishes at some critical pressure. However, several other unexpected experimental features were found in the literature, showing that the applied pressure may (i) induce an increase of the thermal hysteresis width or even (ii) lead to a reentrant behavior on the thermal hysteresis whose width first increases for low applied pressures and then decreases at high-pressure values. These nonstandard behaviors have been classified as anomalous, even though the transition temperature always increases under pressure. In this theoretical contribution, we rationalize all these behaviors by describing the spin-crossover system under pressure with an elastic description accounting for the difference of lattice parameters between the low-spin (LS) and HS phases including the pressure effects. The analytical study of this elastic model in the homogeneous mechanical system demonstrated its isomorphism with an Ising-like model with infinitely long-range interactions, in which the pressure acts linearly on the ligand field and nonlinearly on the strength of the interactions. The resolution of this model brings to light the existence of a pressure-induced interplay between these two contributions allowing one to recover a wide range of normal and abnormal observed experimental thermal dependences of the spin transition under pressure.