We consider a fermionic system for which there exist a single-reference configuration-interaction (CI) expansion of the ground-state wave function that converges, albeit not necessarily rapidly, with the increasing number of particle-hole excitations. For such systems, we show that, whenever the coefficients of Slater determinants (SD) with $l\le k$ excitations can be defined with a number of free parameters $N_{\le k}$ bounded polynomially in $k$, the ground-state energy $E$ only depends on a small fraction of all the wave function parameters and is the solution of equations of the coupled-cluster (CC) form. This generalizes the standard CC method, for which $N_{\le k}$ is bounded by a constant. Based on that result and low-rank tensor decompositions (LRTD), we discuss two possible extensions of the CC approach for wave functions with general polynomial bound for $N_{\le k}$. The most straightforward of those extensions uses the LRTD to represent the amplitudes of the CC cluster operator $T$ which, unlike in the CC case, is not truncated with respect to number of excitations, and the energy and tensor parameters are given by a LRTD-adapted version of standard CC equations. The LRTD can also be used to directly parametrize the wave function coefficients, which involves different equations of the CC form. We derive those equations for the coefficients of SD's with $l\le 4$ excitations, using the CC exponential wave function ansatz with a different type of excitation operator, and a representation of the Hamiltonian in terms of excited particle and hole operators. To complete the proposed computational methods, we construct compact tensor representations of coefficients, or $T$ amplitudes, using superpositions of tree tensor networks which take into account different possible types of entanglement between excited particles and holes. Finally, we discuss why the proposed CC extensions are theoretically applicable at larger coupling strengths than those treatable by the standard CC method.

Polycrystalline $\mathrm{CaF}{\mathrm{e}}_2{\mathrm{O}}_4$ and $\mathrm{CaC}{\mathrm{r}}_{0.5}\mathrm{F}{\mathrm{e}}_{1.5}{\mathrm{O}}_4$ have been investigated by elastic and inelastic neutron scattering. In agreement with previous reports, $\mathrm{CaF}{\mathrm{e}}_2{\mathrm{O}}_4$ undergoes two magnetic transitions, first to a $B$ phase, below $T_{\mathrm{NB}}=200\mathrm{K}$, then to an $A$ phase, below $T_{\mathrm{NA}}=175\mathrm{K}$, while substituted $\mathrm{CaC}{\mathrm{r}}_{0.5}\mathrm{F}{\mathrm{e}}_{1.5}{\mathrm{O}}_4$ undergoes a magnetic transition to the $B$ phase only, at $T_{\mathrm{N}}=125\mathrm{K}$. Each phase corresponds to staggered antiferromagnetic chains coupled either ferromagnetically $(A$ phase) or antiferromagnetically $(B$ phase). In the $A$ phase of $\mathrm{CaF}{\mathrm{e}}_2{\mathrm{O}}_4$, inelastic scattering measurements show clearly defined gapped spin waves, which can be modeled with classical calculations, based on a simple exchange Hamiltonian following the topology of the crystal structure. In contrast, in the $B$ phase of both compounds, the interpretation of the excitation spectrum evades completely the classical approach, even at low temperature. These results are interpreted based on an interchain exchange close to the threshold between ferromagnetic and antiferromagnetic bonding geometry. This induces random interchain coupling, thus creating magnetic exchange disorder whose dominating effect is to blur out the magnetic excitation spectrum of the $B$ phase. A magnetoelastic effect, through which the interchain coupling becomes sizably ferromagnetic, and which is not observed in $\mathrm{CaC}{\mathrm{r}}_{0.5}\mathrm{F}{\mathrm{e}}_{1.5}{\mathrm{O}}_4$, stabilizes the $A$ phase at low temperature in $\mathrm{CaF}{\mathrm{e}}_2{\mathrm{O}}_4$.

Using first-principles calculations, based on disordered local moment (DLM) theory combined with the self-interaction corrected local spin density approximation (SIC-LSDA), we study magnetic correlations in the paramagnetic state of $\mathrm{Gd}X(X$=Cu, Zn, Ga, Ag, Cd, In, Au, Hg, and Tl) intermetallics and their alloys. The predicted magnetic orders and ordering temperatures that these correlations lead to are in overall good agreement with available experiments. The interactions between the Gd $f$-electron local moments are mediated by the valence electrons of the intermetallics which comprise both Gd and $Xd$ bands as well as $sp$ bands. There are RKKY-like features such as dependence on the number of $sp$-valence electrons but other variations manifest themselves in the phase diagrams as regions of incommensurate magnetic ordering, the origin and range of which are related to the binding energies of the alloying anion $d$ states, and their propensity to hybridize with the Gd states at the Fermi level.

Superconductivity in the doped topological crystalline insulator ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$ is studied by first-principles calculation based on superconducting density functional theory (SCDFT) and tunneling spectroscopy. By considering the spin-orbit coupling and frequency dependence of the screened Coulomb interaction in SCDFT, we succeed in reproducing the critical temperature of ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$ quantitatively, in which the spin-orbit coupling is found to play an essential role. The leading gap function is a conventional $s$ wave with moderate anisotropy in $\mathit{k}$ space, and we find that the subdominant odd-parity instability is significantly weaker than the $s$-wave instability. We perform tunneling spectroscopy measurement and confirm that the spectrum is consistent with the calculated gap function.

We show that infrared radiation impinging onto a one-dimensional array of grooves drilled in the superconductor electrode of a long overlap junction can improve matching between fluxon oscillations at THz frequencies and a spoof plasmon of comparable wavelength. This example proves that metamaterials can be very helpful in integrating superconductive and subwavelength optical circuits with optimized matching, bridging the gap between infrared and microwave radiation.

Nickel aluminide $\left(\mathrm{N}{\mathrm{i}}_3\mathrm{Al}\right)$ is an important material for a number of applications, especially when used as a strengthening constituent in high-temperature Ni-based superalloys. Despite this, there is minimal information on its mechanical properties such as strength, plasticity, creep, fatigue, and fracture. In the present work, a first-principles based pure alias shear deformation has been applied to shed light on dislocation characteristics in $\mathrm{N}{\mathrm{i}}_3\mathrm{Al}$ using the predicted stacking fault energy (i.e., the γ surface) and ideal shear strength $\left({\tau }_{\mathrm{IS}}\right)$. Results include direct evidence for the splitting of a $1/2\left[\overline{1}10\right]$ dislocation into two Shockley partials on the $\left\{111\right\}$ plane, which is further supported by the equivalence of the complex stacking fault (CSF) energy ${\gamma }_{\mathrm{CSF}}$ and the antiphase boundary (APB) energy ${\gamma }_{\mathrm{APB}111}$. Estimates of the Peierls stresses using ${\tau }_{\mathrm{IS}}$ and elastic properties suggest the prevalence of edge dislocations in Ni and screw dislocations in $\mathrm{N}{\mathrm{i}}_3\mathrm{Al}$, agreeing with experimental observations regarding the dominance of edge dislocations in the first stage of crystal deformation in fcc metals and the yield-strength anomaly related to screw dislocations in $\mathrm{N}{\mathrm{i}}_3\mathrm{Al}$. The present calculations further point out that the CSF and APB111 are easily formed by shear due to the low-energy barriers, although the lowest planar energies are for the superlattice intrinsic stacking fault and the APB001. Through the case of $\mathrm{N}{\mathrm{i}}_3\mathrm{Al}$, the present work demonstrates that the pure alias shear methodology is not only computationally efficient but also provides valuable insight into the nature of shear-related properties.

Using in situ polarized neutron reflectometry, the depth-resolved evolution of the magnetism and structure in a Pd/Fe/Pd trilayer thin film is measured during growth. The initial film structure of Pd/Fe shows a small proximity-induced magnetism in the underlayer and a magnetization in the Fe layer of $\approx 1.6{\mu }_{\text{B}}$ per Fe atom, less than the expected bulk value of $2.2{\mu }_{\text{B}}$. Deposition of the Pd capping layer initially follows an islandlike growth mode with subsequent coalescence. With increasing Pd deposition the Fe moment and the proximity-induced magnetism in the Pd capping layer decrease. After final deposition of the Pd capping layer, the magnetic profile is structurally and magnetically symmetric across the Fe layer, with magnetism induced in Pd up to 0.92 nm from the interface. Throughout the Pd deposition the Pd/Fe/Pd trilayer structure is becoming increasingly symmetric, a fact which points to a Dzyaloshinskii-Moriya interaction as a likely cause of the observed magnetic behavior.

Superconductors involving electrons with internal degrees of freedom beyond spin can have internally anisotropic pairing states that are impossible in single-band superconductors. As a case in point, in even-parity multiband superconductors that break time-reversal symmetry, nodes of the superconducting gap are generically inflated into two-dimensional Bogoliubov Fermi surfaces. The detection and characterization of these quasiparticle Fermi surfaces requires the understanding of their experimental consequences. In this paper, we derive the low-energy density of states for a broad range of possible nodal structures. Based on this, we calculate the low-temperature form of observables that are commonly employed for the characterization of nodal superconductors, i.e., the single-particle tunneling rate, the electronic specific heat and Sommerfeld coefficient, the thermal conductivity, the magnetic penetration depth, and the NMR spin-lattice relaxation rate, in the clean limit. We also address the question whether the topological invariant of the Bogoliubov Fermi surfaces is associated with topologically protected surface states, with negative results. This work is meant to serve as a guide for experimental searches for Bogoliubov Fermi surfaces in time-reversal-symmetry-breaking superconductors.

We show theoretically that quantized zero-bias conductance peaks should be ubiquitous in superconductor-semiconductor hybrids by employing a zero-dimensional random matrix model with continuous tuning parameters. We demonstrate that a normal metal-superconductor (NS) junction conductance spectra can be generically obtained in this model replicating all features seen in recent experimental results. The theoretical quantized conductance peaks, which explicitly do not arise from spatially isolated Majorana zero modes, are easily found by preparing a contour plot of conductance over several independent tuning parameters, mimicking the effect of Zeeman splitting and voltages on gates near the junction. This suggests that, even stable apparently quantized conductance peaks need not correspond to isolated Majorana modes; rather, the a priori expectation should be that such quantized peaks generically occupy a significant fraction of the high-dimensional tuning parameter space that characterizes the NS tunneling experiments.

We report an experimental work on simulating the Kitaev chain of one-dimensional $p$-wave superconductivity with long-range coupling using superconducting quantum circuits. The effective Hamiltonian of this model in the momentum space is mapped onto that of a qubit driven by a tunable microwave control field. By monitoring the dynamics of the qubit, we generate intuitive yet precise visualization of topological characteristics of different quantum phases of the system. Topological invariants can be directly deduced from the experimental results, without relying on indirect extraction from measurements such as microwave spectroscopy and temporal averaging. Therefore a much enhanced efficiency of measurement can be realized compared to other methods that are often used to simulate topological phase transitions. As a consequence, a comprehensive phase diagram covering a wide range of the parameter space is obtained. Topological phase transitions and quantum multicritical points are clearly demonstrated. In particular, new topological phases emerge as a consequence of long-range coupling. The method used here can be readily generalized to simulate more complex systems on different experimental platforms.

We reconsider the procedure of calculating fermion-boson vertices and the numerical solution of Bethe-Salpeter equations, used in nonlocal extensions of dynamical mean field theory. Because of the frequency dependence of the vertices, a finite-frequency box for matrix inversions is typically used, which often requires some treatment of the asymptotic behavior of vertices. Recently [J. Kuneš, Phys. Rev. B 83, 085102 (2011); A. Tagliavini et al., Phys. Rev. B 97, 235140 (2018)] it was proposed to split the considered frequency box into smaller and larger ones; in the smaller-frequency box the numerically exact vertices are used, while beyond this box asymptotics of vertices are applied. Yet, this method requires numerical treatment of vertex asymptotics (including corresponding matrix manipulations) in the larger-frequency box and/or knowledge of fermion-boson vertices, which may not be convenient for numerical calculations. In the present paper we derive the formulas which treat analytically the contribution of vertices beyond the chosen frequency box, such that only numerical operations with vertices in the chosen small-frequency box are required. The method is tested on the Hubbard model and can be used in a broad range of applications of nonlocal extensions of dynamical mean field theory.

Charge doping is an essential means to tailor a materials' properties. However, besides moving the Fermi level, charge doping is generally not expected to induce topological phase transition (TPT). Here, using first-principles calculations, we demonstrate an electron doping-induced TPT in bulk Bi from a higher-order topological insulator (HOTI) to a TI. The underlying mechanism is revealed to be driven by an electron doping-induced quantum electronic stress (QES), which in turn induces a highly anisotropic lattice expansion to close/reopen the small energy gap in Bi band structure. Our finding significantly resolves an outstanding controversy concerning the topological characterization of bulk Bi among existing experiments and theories, and explains the physical origin of the topological order in Bi (111) thin films. It sheds new light on the fundamental understanding of topological properties of small band gap materials in relation to doping and QES.

Herein, evidence for the long-sought finite hyperfine interaction in the high-pressure hexagonal close-packed $\epsilon$-iron is gained through synchrotron radiation perturbed angular correlation spectroscopy. This method yields an energy splitting of $3.5\left(5\right)\text{neV}$ between the $m_{I_{\text{e}}}=\pm 1/2$ and $m_{I_{\text{e}}}=\pm 3/2$ nuclear sublevels of the iron-57 $14.412\text{-keV}$ nuclear excited state at $30\left(1\right)\text{GPa}$ and room temperature. This energy splitting is related to a nuclear quadrupole hyperfine interaction with an electric field gradient of $eq=1.2\left(2\right)\times {10}^{16}{\mathrm{V}/\mathrm{cm}}^2$. However, there is still a possibility that the splitting of the iron-57 nuclear levels is related to a modest magnetic hyperfine interaction of ca. $0.40\left(5\right)\text{T}$.

Among transition-metal dichalcogenides, mono and few-layers thick ${\mathrm{VSe}}_2$ has gained much recent attention following claims of intrinsic room-temperature ferromagnetism in this system, which have nonetheless proved controversial. Here, we address the magnetic and chemical properties of $\mathrm{Fe}/{\mathrm{VSe}}_2$ heterostructure by combining element sensitive x-ray absorption spectroscopy and photoemission spectroscopy. Our x-ray magnetic circular dichroism results confirm recent findings that both native mono/few-layer and bulk ${\mathrm{VSe}}_2$ do not show intrinsic ferromagnetic ordering. Nonetheless, we find that ferromagnetism can be induced, even at room temperature, after coupling with a Fe thin film layer, with antiparallel alignment of the moment on the V with respect to Fe. We further consider the chemical reactivity at the $\mathrm{Fe}/{\mathrm{VSe}}_2$ interface and its relation with interfacial magnetic coupling.

Thermodynamic properties of graphene bilayers are studied by path-integral molecular dynamics (PIMD) simulations, considering quantization of vibrational modes and anharmonic effects. Bilayer graphene has been studied at temperatures between 12 and 1500 K for zero external stress, using the LCBOPII effective potential. We concentrate on the thermal expansion, in-plane and out-of-plane compressibility, and specific heat. Additional insight into the meaning of our results for bilayer graphene is obtained from a comparison with data obtained from PIMD simulations for monolayer graphene and graphite. They are also analyzed in view of experimental data for graphite. Zero-point and thermal effects on the in-plane and “real” area of bilayer graphene are studied. The thermal expansion coefficient ${\alpha }_{xy}$ of the in-plane area is negative at low temperatures and positive for $T\gtrsim 800\mathrm{K}$. The minimum ${\alpha }_{xy}$ is $-6.6\times {10}^{-6}{\mathrm{K}}^{-1}$ at $T\approx 220\mathrm{K}$. Both in-plane (${\chi }_{xy}$) and out-of-plane (${\chi }_z$) compressibilities of graphene bilayers are found to increase for rising temperature, and turn out to be lower than that corresponding to monolayer graphene and higher than those found for graphite. At 300 K, we find for the bilayer ${\chi }_{xy}=9.5\times {10}^{-2}{\AA }^2/\mathrm{eV}$ and ${\chi }_z=2.97\times {10}^{-2}{\mathrm{GPa}}^{-1}$. Results for the specific heat obtained from the simulations are compared with those given by a harmonic approximation for the vibrational modes. This approach is noticeably accurate at temperatures lower than 200 K.

In this paper we develop a semi-analytical perturbation-theory approach to the calculation of the energy levels (binding energies) and wave functions of excitons in phosphorene. Our method gives the exciton wave function in both real and reciprocal spaces with the same ease. This latter aspect is important for the calculation of the nonlinear optical properties of phosphorene. We find that our results are in agreement with calculations based both on the Bethe-Salpeter equation and on Monte Carlo simulations, which are computationally much more demanding. Our approach thus introduces a simple, viable, and accurate method to address the problem of excitons in anisotropic two-dimensional materials.

Recently the charge density wave (CDW) in vanadium dichalcogenides have attracted increasing research interest, but a real-space investigation on the symmetry breaking of the CDW state in ${\mathrm{VTe}}_2$ monolayer is still lacking. We have investigated the CDW of ${\mathrm{VTe}}_2$ monolayer by low energy electron diffraction (LEED) and scanning tunneling microscope (STM). While the LEED experiments revealed a $(4\times 4)$ CDW transition near 192 K, our low-temperature STM experiments resolved the $(4\times 4)$ lattice distortions and charge-density modulation in real space, and further unveiled a one-dimensional structural modulation that breaks the threefold rotational and mirror symmetries in the CDW state. In accordance with the CDW state at low temperature, a CDW gap of 12 meV was detected by scanning tunneling spectroscopy at 4.9 K. The observed symmetry breaking implies that besides the conventional Fermi surface nesting or the $q$-dependent electron-phonon coupling, some other mechanism may also contribute to breaking the threefold rotational symmetry in the CDW state of ${\mathrm{VTe}}_2$ monolayer.

We study two-terminal transport through two-dimensional periodically driven systems in which all bulk Floquet eigenstates are localized by disorder. We focus on the anomalous Floquet-Anderson insulator (AFAI) phase, a topologically nontrivial phase within this class, which hosts topologically protected chiral edge modes coexisting with its fully localized bulk. We show that the unique properties of the AFAI yield remarkable far-from-equilibrium transport signatures: for a large bias between leads, a quantized amount of charge is transported through the system each driving period. Upon increasing the bias, the chiral Floquet edge mode connecting source to drain becomes fully occupied and the current rapidly approaches its quantized value.