By means of ab initio calculations in conjunction with the random-phase approximation (RPA) within the full-potential linearized augmented plane wave method, we study the screening of the Coulomb interaction in ${\mathrm{Nb}}_x\mathrm{Co}$ ($1\le x\le 9$) clusters. In addition, these results are compared with pure bcc Nb bulk. We find that for all clusters the on-site Coulomb interaction in RPA is strongly screened, whereas the intersite nonlocal Coulomb interaction is weakly screened and for some clusters it is unscreened or even antiscreened. This is in strong contrast with pure Nb bulk, where the intersite Coulomb interaction is almost completely screened. Furthermore, constrained RPA calculations reveal that the contribution of the Co $3d \to 3d$ channel to the total screening of the Co $3d$ electrons is small. Moreover, we find that both the on-site and intersite Coulomb interaction parameters decrease in a reasonable approximation linearly with the cluster size and for clusters having more than 20 Nb atoms a transition from 0D to 3D screening is expected to take place.

In strongly correlated organic materials it has been pointed out that charge ordering could also achieve electronic ferroelectricity at the same critical temperature $T_{co}$. A prototype of such phenomenon are the quasi-one-dimensional $({\mathrm{TMTTF})}_{2}X$ Fabre salts. However, the stabilization of a long-range ferroelectric ground state below $T_{co}$ requires the break of inversion symmetry, which should be accompanied by a lattice deformation. In this paper we investigate the role of the monovalent counteranion $X$ in such mechanism. For this purpose, we measured the quasistatic dielectric constant along the $c^{*}$-axis direction, where layers formed by donors and anions alternate. Our findings show that the ionic charge contribution is three orders of magnitude lower than the intrastack electronic response. The $c^{*}$ dielectric constant $\left({\epsilon }_{c^{*}}^{\prime }\right)$ probes directly the charge response of the monovalent anion $X$, since the anion mobility in the structure should help to stabilize the ferroelectric ground state. Furthermore, our ${\epsilon }_{c^{*}}^{\prime }$ measurements show that the dielectric response is thermally broaden below $T_{co}$ if the ferroelectric transition occurs in the temperature range where the anion movement begin to freeze in their methyl groups cavity. In the extreme case of the ${\mathrm{PF}}_6\text{−}{\mathrm{H}}_{12}$ salt, where $T_{co}$ occurs at the freezing point, a relaxor-type ferroelectricity is observed. Also, because of the slow kinetics of the anion sublattice, global hysteresis effects and reduction of the charge response upon successive cycling are observed. In this context, we propose that anions control the order-disorder or relaxation character of the ferroelectric transition of the Fabre salts. Yet, our results show that x-ray irradiation damages change the well-defined ferroelectric response of the ${\mathrm{AsF}}_6$ pristine salt into a relaxor.

The charge-density waves (CDW) in the framework Landau theory are visualized to manifest in complex $R\mathrm{Ni}{\mathrm{O}}_3$ ($R$ = rare-earth) nickelates in which the structure controls the incipient charge order in the weak localization limit. Any consequent effect demonstrating these nickelates in the rare category of CDW conductors with controlled charge-lattice interactions has been elusive so far. Employing terahertz time-domain spectroscopy along selective crystal axes, we present evidence of the CDW in epitaxial strain-modulated crystal structures of prototypical $\mathrm{NdNi}{\mathrm{O}}_3$. A finite peak structure at 5 meV in the terahertz conductivity displays all the characteristics of a CDW condensate in (110)- and (111)-oriented $\mathrm{NdNi}{\mathrm{O}}_3$ thin films. Contrasting charge dynamics of collective CDW mode and Drude conductivity emerging across dissimilar orientations helps disentangle charge ordering from the metal-insulator transition and establish a structure-property cause-effect relationship which may propose novel attributes in emerging oxide electronics.

We introduce an entanglement branching operator to split a composite entanglement flow in a tensor network which is a promising theoretical tool for many-body systems. We can optimize an entanglement branching operator by solving a minimization problem based on squeezing operators. The entanglement branching is a new useful operation to manipulate a tensor network. For example, finding a particular entanglement structure by an entanglement branching operator, we can improve a higher-order tensor renormalization group method to catch a proper renormalization flow in a tensor network space. This new method yields a new type of tensor network states. The second example is a many-body decomposition of a tensor by using an entanglement branching operator. We can use it for a perfect disentangling among tensors. Applying a many-body decomposition recursively, we conceptually derive projected entangled pair states from quantum states that satisfy the area law of entanglement entropy.

We introduce an error measure for matrix-product states without requiring the relatively costly two-site density-matrix renormalization group (2DMRG). This error measure is based on an approximation of the full variance $\langle \psi \left|{(\widehat{H}-E)}^2\right|\psi \rangle$. When applied to a series of matrix-product states at different bond dimensions obtained from a single-site density-matrix renormalization group (1DMRG) calculation, it allows for the extrapolation of observables towards the zero-error case representing the exact ground state of the system. The calculation of the error measure is split into a sequential part of cost equivalent to two calculations of $\langle \psi \left|\widehat{H}\right|\psi \rangle$ and a trivially parallelized part scaling like a single operator application in 2DMRG. The reliability of this error measure is demonstrated by four examples: the $L=30,S=1/2$ Heisenberg chain, the $L=50$ Hubbard chain, an electronic model with long-range Coulomb-like interactions, and the Hubbard model on a cylinder with a size of $10\times 4$. Extrapolation in this error measure is shown to be on par with extrapolation in the 2DMRG truncation error or the full variance $\langle \psi \left|{(\widehat{H}-E)}^2\right|\psi \rangle$ at a fraction of the computational effort.

Topological quantum states have been proposed and investigated on two-dimensional flat surfaces or lattices with different geometries like the plane, cylinder, and torus. Here, we study quantum anomalous Hall (QAH) or Chern insulator (CI) states on two-dimensional singular surfaces (such as conical and helicoidlike surfaces). Such singular geometries can be constructed based on the disk geometry and a defined unit sector with $n$-fold rotational symmetry. The singular geometry induces exotic and intriguing features of CI/QAH states, such as in-gap and in-band core states, charge fractionalization, and multiple branches of edge excitations.

We study spin entanglement of the quasiparticles of the local Fermi liquid excited in nonlinear current through a quantum dot described by the Anderson impurity model with two degenerate orbitals coupled to each other via an exchange interaction. Applying the renormalized perturbation theory, we obtain the precise form of the cumulant generating function and cross-correlations for the currents with spin angled to arbitrary directions, up to third order in the applied bias voltage. It is found that the exchange interaction gives rise to spin-angle dependency in the cross-correlation between the currents through the two different orbitals and also brings an intrinsic cross-correlation of currents with three different angular momenta.

The electronic structure of the Kondo lattice $\mathrm{CeI}{\mathrm{n}}_3$ has been studied by on-resonant angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. A weakly dispersive quasiparticle band has been observed directly with an energy dispersion of 4 meV by photoemission, implying the existence of weak hybridization between the $f$ electrons and conduction electrons. The hybridization is further confirmed by the formation of the hybridization gap revealed by temperature-dependent scanning tunneling spectroscopy. Moreover, we find the hybridization strength in $\mathrm{CeI}{\mathrm{n}}_3$ is much weaker than that in the more two-dimensional compounds $\mathrm{CeCoI}{\mathrm{n}}_5$ and $\mathrm{CeIrI}{\mathrm{n}}_5$. Our results may be essential for the complete microscopic understanding of this important compound and the related heavy-fermion systems.

Nonsymmorphic space group symmetries can generate exotic band crossings in topological metals and semimetals. Here, based on symmetry analysis and first-principles calculations, we reveal rich band-crossing features in the existing layered compounds ${\mathrm{Ta}}_3{\mathrm{SiTe}}_6$ and ${\mathrm{Nb}}_3{\mathrm{SiTe}}_6$, enabled by nonsymmorphic symmetries. We show that in the absence of spin-orbit coupling (SOC), these three-dimensional (3D) bulk materials possess accidental Dirac loops and essential fourfold nodal lines. In the presence of SOC, there emerges an hourglass Dirac loop—a fourfold degenerate nodal loop, on which each point is a neck point of an hourglass-type dispersion. We show that this interesting type of band crossing is protected and dictated by the nonsymmorphic space group symmetries and it gives rise to drumheadlike surface states. Furthermore, we also investigate these materials in the monolayer form. We show that these two-dimensional (2D) monolayers host nodal lines in the absence of SOC and the nodal lines transform to essential spin-orbit Dirac points when SOC is included. Our work suggests a realistic material platform for exploring the fascinating physics associated with nonsymmorphic band crossings in both 3D and 2D systems.

Graphene supported on a transition metal dichalcogenide substrate offers a novel platform to study the spin transport in graphene in the presence of a substrate-induced spin-orbit coupling while preserving its intrinsic charge transport properties. We report the first nonlocal spin transport measurements in graphene completely supported on a 3.5-nm-thick tungsten disulfide (${\mathrm{WS}}_{\text{2}}$) substrate, and encapsulated from the top with an 8-nm-thick hexagonal boron nitride layer. For graphene, having mobility up to 16 000 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, we measure almost constant spin signals both in electron and hole-doped regimes, independent of the conducting state of the underlying ${\mathrm{WS}}_2$ substrate, which rules out the role of spin-absorption by ${\mathrm{WS}}_2$. The spin-relaxation time ${\tau }_{\text{s}}$ for the electrons in graphene-on-${\mathrm{WS}}_2$ is drastically reduced down to $\sim 10$ ps from ${\tau }_{\text{s}}\sim 800$ ps in graphene-on-${\mathrm{SiO}}_2$ on the same chip. The strong suppression of ${\tau }_{\text{s}}$ along with a detectable weak antilocalization signature in the quantum magnetoresistance measurements is a clear effect of the ${\mathrm{WS}}_2$-induced spin-orbit coupling (SOC) in graphene. Via the top-gate voltage application in the encapsulated region, we modulate the electric field by 1 V/nm, changing ${\tau }_{\text{s}}$ almost by a factor of four, which suggests electric-field control of the in-plane Rashba SOC. Further, via the carrier-density dependence of ${\tau }_{\text{s}}$, we also identify the fingerprints of the D'yakonov-Perel' type mechanism in the hole-doped regime at the graphene-${\mathrm{WS}}_2$ interface.

We analyze the effects of electron-electron interactions and disorder on a Rashba double-nanowire setup coupled to an $s$-wave superconductor, which has been recently proposed as a versatile platform to generate Kramers pairs of Majorana bound states in the absence of magnetic fields. We identify the regime of parameters for which these Kramers pairs are stable against interaction and disorder effects. We use bosonization, perturbative renormalization group, and replica techniques to derive the flow equations for various parameters of the model and evaluate the corresponding phase diagram with topological and disorder-dominated phases. We confirm aforementioned results by considering a more microscopic approach, which starts from the tunneling Hamiltonian between the three-dimensional $s$-wave superconductor and the nanowires. We find again that the interaction drives the system into the topological phase and, as the strength of the source term coming from the tunneling Hamiltonian increases, strong electron-electron interactions are required to reach the topological phase.

It is generally claimed that physical processes which display scale-invariant power-law distributions are subjected to a dynamic criticality that prohibits a well-defined kinetic law. In this paper, we demonstrate the coexistence of these two apparently contradicting behaviors during the same physical process—the motion of type-II twin boundaries in martensite Ni-Mn-Ga. The process is investigated by combined measurements of the temporal twin-boundary velocity and the acoustic emitted energy. Velocity values are extracted from high-resolution force measurements taken during displacement-driven mechanical tests, as well as from force-driven magnetic tests, and cover an overall range of six orders of magnitude. Acoustic emission (AE) is measured during mechanical tests. Velocity values follow a normal distribution whose characteristic value is determined by the material's kinetic relation, and its width scales with the average velocity. In addition, it is observed that velocity distributions are characterized by a heavy tail at the right (i.e., faster) end that exhibits a power law over more than one and a half orders of magnitude. At the same time, the AE signals follow a scale-invariant power-law distribution, which is not sensitive to the average twin-boundary velocity. The coexistence of these two different statistical behaviors reflects the complex nature of twin-boundary motion and suggests the possibility that the transformation proceeds through physical subprocesses that are close to criticality alongside other processes that are not.

Ferrimagnetic metals show an extraordinary phenomenon of all-optical switching of magnetization. Ongoing research has revealed the relevant physics of crossing a compensation temperature, occurrence of a transient ferromagneticlike state, and transferring angular momentum between sublattices during the all-optical switching. However, most studies were focused on the spin dynamics of local bands, and the role of the conduction band is unknown. Here, we investigate spin generation in conduction bands of ferrimagnetic metals by measuring spin transport from a ferrimagnetic metal to a nonmagnetic metal or to a ferromagnetic metal. We find that two sublattices of ferrimagnets generate spins in the conduction band with different signs and times.

The evolution of magnetic anisotropy in bcc Fe as a function of interstitial boron atoms was investigated in thin films grown by molecular beam epitaxy. The thermodynamic nonequilibrium conditions during film growth allowed one to stabilize an interstitial boron content of about $14\mathrm{at}.\%$ accompanied by lattice tetragonalization. The $c/a$ ratio scaled linearly with the boron content up to a maximum value of 1.05 at $300{}^{\circ }\mathrm{C}$ substrate growth temperature, with a room-temperature magnetization of. In contrast to nitrogen interstitials, the magnetic easy axis remained in-plane with an anisotropy of approximately $-5.1\times {10}^6\mathrm{erg}/{\mathrm{cm}}^3$. Density functional theory calculations using the measured lattice parameters confirm this value and show that boron local ordering indeed favors in-plane magnetization. Given the increased temperature stability of boron interstitials as compared to nitrogen interstitials, this study will help to find possible ways to manipulate boron interstitials into a more favorable local order.

Spatially resolved neutron diffractive imaging with a microchannel plate collimator is used to directly map the nucleation of the skyrmion lattice (SkL) of the B20 compound MnSi at the conical to SkL transition as a function of magnetic field. Our study shows a macroscopic phase separation of the SkL and the conical phase at the border of the SkL phase pocket, reveals that the nucleation of the SkL starts at the edges of the sample, and quantifies the bending of the SkL due to demagnetization. Our study highlights the importance of geometric and demagnetizing effects for the SkL formation regarding the unambiguous interpretation of measurements of bulk properties such as possible phase coexistence and crossover regimes.