Local structures of ${\mathrm{Pd}}_{40}{\mathrm{Ni}}_{40}{\mathrm{P}}_{20}$ bulk metallic glass were investigated by combining anomalous x-ray scattering close to the Pd and Ni $K$ absorption edges, x-ray diffraction, neutron diffraction, and reverse Monte Carlo modeling, from which partial structure factors $S_{ij}\left(Q\right)$ and partial pair distribution functions $g_{ij}\left(r\right)$ as well as three-dimensional atomic configurations were carefully obtained around the constituent elements. A disagreement is found in the local structures with an ab initio molecular dynamics simulation by Guan et al., i.e., the existence of the P-P nearest-neighboring configurations is clarified in the present experimental result. From the Voronoi tessellation analysis, a preference of the pure icosahedral configurations is observed around the Ni atoms, whereas the local configurations around the Pd and P atoms are rather distorted icosahedra. A persistent homology analysis was carried out to identify meaningful shape characteristics of the intermediate-range atomic configuration of large rings.

We present an embedding scheme for the locally self-consistent method to study disordered electron systems. We test this method in a tight-binding basis and apply it to the single band Anderson model. The local interaction zone is used to efficiently compute the local Green's function of a supercell embedded into a local typical medium. We find a quick convergence as the size of the local interaction zone which reduces the computational costs as expected. This method captures the Anderson localization transition and accurately predicts the critical disorder strength. The present work opens the path towards the development of a typical medium embedding scheme for the $O\left(N\right)$ multiple scattering methods.

The unconventional transport and magnetic properties of perovskitelike lanthanum cobalt oxide ${\mathrm{LaCoO}}_3$ have been studied for more than five decades. This highly correlated electron system exhibits a variety of peculiar properties that are desirable for environmentally friendly energy solutions, fuel cell technologies, novel diesel engines, and oxyfuel power plants. However, the true spin state of the ${\mathrm{Co}}^{3+}$ ion is an important but still unresolved issue that underlies these applications. Although many theoretical models have been proposed, finding supporting experimental evidence of spin-state transitions is extremely difficult. Not until recently have new advanced scattering methods emerged allowing unprecedented precision in determining the crystal structure of ${\mathrm{LaCoO}}_3$. In this work, we combine high-resolution extended x-ray absorption fine structure, x-ray powder diffraction, and neutron powder and single-crystal diffraction over a broad range of temperatures, from 2 up to 1000 K, as well as quantum mechanical modeling to study the spin-state transition in ${\mathrm{LaCoO}}_3$ and in a reference sample of ${\mathrm{LaGaO}}_3$. Our results suggest that the Co ions are mainly in a low-spin state at temperatures below 150 K, with a minority of ions in a high-spin state. With an increase in the temperature the gradual transition from low- to intermediate-spin state occurs up until 550 K. At the metal-insulator transition at 550 K, the long-range domains of the intermediate-spin states become a dominant contribution. Above 550 K, a transition from intermediate- to high-spin state is observed. It is established that a slight change in the degree of $pd$ hybridization can lead to the appearance of a spin-state transition which might be induced by both temperature and surface effects in powder crystallites.

In view of recent experiments, the determination of magnetic anisotropy (MA) is revisited, thus leading to unexpected relations between the pertinent tensors. The sudden perturbation of the preferential magnetic direction triggers magnetization dynamics, which can be traced on the femtosecond timescale. Modeling this process provides indirect information on magnetic anisotropy. In magnetic molecular systems, the interplay between the zero-field splitting given by the $D$ tensor and the Zeeman interaction given by the $G$ tensor can be measured spectroscopically, likewise indirectly providing information on MA, but never fully uncovering these tensors. Here, a theory that bridges the gap between the physical experiments, the microscopic mechanisms, and the effective models is established. Expressions for the magnetic anisotropy in terms of the $D$ and $G$ tensors are derived in a covariant form. A minimal experiment where these parameters can be determined is proposed. In particular, by means of the Hellmann-Feynman theorem, we are able to account for the mutual orientation of the principal axes of the $G$ and $D$ tensors, which is an important effect for molecular magnets.

Various ${\mathrm{Co}}_2$-based Heusler compounds are predicted to be half-metallic ferromagnets with Weyl points. These systems with lack of time inversion symmetry possess a momentum space Berry curvature that can introduce exotic transport properties. The present study, on epitaxially grown ${\mathrm{Co}}_2\mathrm{TiSn}$ films, is an approach to understand and explore this possibility. The theoretical investigation shows that the Berry curvature is significant for the total anomalous Hall effect in experimentally grown ${\mathrm{Co}}_2\mathrm{TiSn}$ films. The deviation between the theory and experiment is due to the influence of side jump and skew scattering mechanisms. From a theoretical point of view, the intrinsic contribution to the anomalous Hall effect originates from partially gaped nodal lines due to the symmetry reduction induced by the lack of time reversal symmetry. Furthermore, from hard x-ray photoelectron spectroscopy measurements, we establish an electronic structure in the film that is comparable to the theoretical density of states calculations. These results provide intuitive insight into Heusler spintronics rooted in topological electronic structure.

The magnetocaloric effect (MCE), which is the refrigeration based on the variation of the magnetic entropy, is of great interest in both technological applications and fundamental research. The MCE is quantified by the magnetic Grüneisen parameter ${\mathrm{\Gamma }}_{\mathrm{mag}}$. We report on an analysis of ${\mathrm{\Gamma }}_{\mathrm{mag}}$ for the classical Brillouin-like paramagnet for a modified Brillouin function taking into account a zero-field splitting originated from the spin-orbit (SO) interaction and for the one-dimensional Ising (1DI) model under a longitudinal field. For the Brillouin-like model with SO interaction and the longitudinal 1DI model, a sign-change in the MCE is observed for vanishing $T$ and $B$. SO interaction leads to a narrowing of the enhancement of ${\mathrm{\Gamma }}_{\mathrm{mag}}$ for $T$ and $B\to 0$. Our findings emphasize the relevance of ${\mathrm{\Gamma }}_{\mathrm{mag}}$ for exploring critical points. Also, we show that the Brillouin model with and without SO interaction can be recovered from the 1DI model in the regime of high temperatures and vanishing coupling constant $J$.

For electron-doped cuprates, the strong suppression of antiferromagnetic spin correlation by efficient reduction annealing by the “protect-annealing” method leads to superconductivity not only with lower Ce concentrations but also with higher transition temperatures. To reveal the nature of this superconducting state, we have performed angle-resolved photoemission spectroscopy measurements of protect-annealed electron-doped superconductors ${\mathrm{Pr}}_{1.3-x}{\mathrm{La}}_{0.7}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ and directly investigated the superconducting gap. The gap was found to be consistent with $d$-wave symmetry, suggesting that strong electron correlation persists and hence antiferromagnetic spin fluctuations remain a candidate that mediates Copper pairing in the protect-annealed electron-doped cuprates.

We report measurements of longitudinal temperature gradient in thermal counterflow of superfluid ${}^4\mathrm{He}$ performed directly in the 17.5 cm long channel of a square cross section of 7 mm side using static temperature sensors placed on its wall and one small sensor movable along its axis. For various temperatures and heat inputs, we verify linear temperature gradient in the bulk and detect a “boundary layer”—a region with increased temperature gradient—adjacent to the planar heater. We verify this result using three different heater geometries, all of which display similar thickness of this region but differ in exact details of the temperature profile. We propose that the observed temperature gradient enhancement is at least partly due to an increase of the density of the vortex tangle induced by the inhomogeneity of areal heat input distribution and support this claim by numerical simulations.

We use powder x-ray diffraction to study the effect of pressure on the crystal structure of the honeycomb rhodate ${\mathrm{Li}}_2{\mathrm{RhO}}_3$. We observe low-pressure ($P<P_{c1}=6.5\mathrm{GPa}$) and high-pressure ($P>P_{c2}=14\mathrm{GPa}$) regions corresponding to the monoclinic $C2/m$ symmetry, while a phase mixture is observed at intermediate pressures. At $P>P_{c2}$, the honeycomb structure becomes distorted and features short Rh-Rh bonds forming zigzag chains stretched along the crystallographic $a$ direction. This is in contrast to dimerized patterns observed in triclinic high-pressure polymorphs of $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ and $\alpha \text{−}{\mathrm{RuCl}}_3$. Density-functional theory calculations at various pressure conditions reveal that the observed rhodium zigzag-chain pattern is not expected under hydrostatic pressure but can be reproduced by assuming anisotropic pressure conditions.

We present a study of the different high-pressure polymorphs of $\mathrm{GdV}{\mathrm{O}}_4$ and its stability. Powder x-ray diffraction and Raman experiments show a phase transition from a zircon- to a scheelite-type structure taking place at 6.8(4) GPa. Ab initio density functional theory calculations support this conclusion. The equations of state of these two phases are reported. In addition, we studied the pressure evolution of the Raman modes for the zircon and scheelite phases, showing good agreement between calculations and experiments. For the sake of completeness, we performed optical-absorption measurements up to 16 GPa, showing a band-gap collapse at the transition point. Beyond 20 GPa a second phase transition to a monoclinic fergusonite structure takes place as a consequence of a mechanical instability. A third transition is observed at around 29.3 GPa in Raman experiments. According to our calculations, this fourth polymorph corresponds to an orthorhombic structure described by space group Cmca. This phase involves an increase of the atomic coordination number of vanadium and gadolinium. The results are compared to those reported on isomorphic compounds.

The deconfined quantum critical point was proposed as a second-order quantum phase transition between two broken-symmetry phases beyond the Landau-Ginzburg-Wilson paradigm. However, numerical studies cannot completely rule out a weakly-first-order transition because of strong violations of finite-size scaling. We demonstrate that the fidelity is a simple probe to study the deconfined quantum critical point. We study the ground-state fidelity susceptibility close to the deconfined quantum critical point in a spin chain using the large-scale finite-size density-matrix renormalization-group method. We find that the finite-size scaling of the fidelity susceptibility obeys the conventional scaling behavior for continuous phase transitions, supporting the idea that the deconfined quantum phase transition is continuous. We numerically determine the deconfined quantum critical point and the associated correlation length critical exponent from the finite-size-scaling theory of the fidelity susceptibility. Our results are consistent with recent results obtained directly from the matrix product states for infinite-size lattices using others observables. Our work provides a useful probe to study critical behaviors at the deconfined quantum critical point from the concept of quantum information.

We have studied structural and magnetic properties of ${\left(\mathrm{FePt}\right)}_{1-x}\mathrm{T}{\mathrm{b}}_x$ thin films grown at elevated temperatures on MgO(100) substrates. The incorporation of Tb into the chemically ordered $\mathrm{L}{1}_0$ FePt lattice gives rise to considerable changes in the structural and magnetic properties, depending on the Tb content and the deposition temperature. When grown at $770{}^{\circ }\mathrm{C}$, pronounced $L{1}_0$ ordering of the initial FePt phase is present. It vanishes gradually with Tb addition, thereby forming additional crystalline Tb-Pt phases at higher Tb content. These structural changes are accompanied by a strong reduction of the perpendicular magnetic anisotropy with increasing Tb content, finally resulting in a soft magnetic material with in-plane easy-axis magnetization. Extended x-ray absorption fine-structure measurements were performed to examine the possible incorporation of Tb into the FePt lattice. Based on the number of Pt first neighbors, the transformation from the initial tetragonal $L{1}_0$ structure to the chemically disordered fcc ($A$1) phase with increasing Tb content could be firmly concluded. In a further sample series, FePt was grown at a lower temperature of 530 °C, which leads to reduced initial $L{1}_0$ chemical ordering. Adding Tb results in strong elastic stress in the lattice, eventually causing full amorphization of the film. Interestingly, in this case, a spin-reorientation transition from in-plane to out-of-plane magnetic easy axis is found at low temperatures, which is associated with an anisotropic chemical short-range order present in the amorphous phase. Furthermore, x-ray magnetic circular dichroism studies at the $\mathrm{Fe}{L}_{3,2}$ and Tb $M_{5,4}$ edges for all samples reveal strong antiferromagnetic coupling between Fe and Tb, resulting in a reduction of the net magnetization of the film.

We study the correlation between chemical composition and vortex dynamics in Ni-doped $\mathrm{CaK}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{N}{\mathrm{i}}_x\right)}_4\mathrm{A}{\mathrm{s}}_4$ ($x=0$, 0.015, 0.025, 0.03, and 0.05) single crystals by performing measurements of the critical current densities $J_c$ and the flux creep rates $S$. The magnetic relaxation of all the crystals is well described by the collective creep theory. The samples display a glassy exponent μ within the predictions for vortex bundles in a weak pinning scenario and relatively small characteristic pinning energy ($U_0<100\mathrm{K}$). The undoped crystals display modest $J_c$ values at low temperatures and high magnetic fields applied along the $c$ axis. $J_c\left(T\right)$ dependences at high fields display an unusual peak. The enhancement in $J_c\left(T\right)$ matches with an increase in $U_0$ and the appearance of a second peak in the magnetization. As Ni doping increases, whereas there is a monotonic decrease in $T_c$ there is a nonmonotonic change in $J_c$. Initially $J_c$ increases, reaching a maximum value for $x=0.015$, and then $J_c$ decreases for $x\ge 0.025$. This change in $J_c$($x$) is coincident with the onset of antiferromagnetic order. The magnetic field dependence of $J_c\left(H\right)$ also manifests a change in behavior between these $x$ values. The analysis of the vortex dynamics for small and intermediate magnetic fields shows a gradual evolution in the glassy exponent μ with Ni content, $x$. This implies that there is no appreciable change in the mechanism that determines the vortex relaxation.

We present a worm-type Monte Carlo study of several typical models in the three-dimensional (3D) U(1) universality class, which include the classical 3D XY model in the directed flow representation and its Villain version, as well as the 2D quantum Bose-Hubbard (BH) model with unitary filling in the imaginary-time world-line representation. From the topology of the configurations on a torus, we sample the superfluid stiffness ${\rho }_s$ and the dimensionless wrapping probability $R$. From the finite-size scaling analyses of ${\rho }_s$ and of $R$, we determine the critical points as $T_c\left(\mathrm{XY}\right)=2.2018441\left(5\right)$ and $T_c\left(\mathrm{Villain}\right)=0.33306704\left(7\right)$ and ${(t/U)}_c\left(\mathrm{BH}\right)=0.0597291\left(8\right)$, where $T$ is the temperature for the classical models, and $t$ and $U$ are, respectively, the hopping and on-site interaction strength for the BH model. The precision of our estimates improves significantly over that of the existing results. Moreover, it is observed that at criticality, the derivative of a wrapping probability with respect to $T$ suffers from negligible leading corrections and enables a precise determination of the correlation length critical exponent as $\nu =0.67183\left(18\right)$. In addition, the critical exponent $\eta$ is estimated as $\eta =0.03853\left(48\right)$ by analyzing a susceptibilitylike quantity. We believe that these numerical results would provide a solid reference in the study of classical and quantum phase transitions in the 3D U(1) universality, including the recent development of the conformal bootstrap method.

In a previous paper [Bischoff and Jeckelmann, Phys. Rev. B 96, 195111 (2017)] we introduced a density-matrix renormalization group method for calculating the linear conductance of one-dimensional correlated quantum systems and demonstrated it on homogeneous spinless fermion chains with impurities. Here we present extensions of this method to inhomogeneous systems, models with phonons, and the spin conductance of electronic models. The method is applied to a spinless fermion wire-lead model, the homogeneous spinless Holstein model, and the Hubbard model. Its capabilities are demonstrated by comparison with the predictions of Luttinger liquid theory combined with Bethe ansatz solutions and other numerical methods. We find a complex behavior for quantum wires coupled to interacting leads when the sign of the interaction (repulsive/attractive) differs in the wire and leads. The renormalization of the conductance given by the Luttinger parameter in purely fermionic systems is shown to remain valid in the Luttinger liquid phase of the Holstein model with phononic degrees of freedom.

Supersymmetry, a symmetry between fermions and bosons, provides a promising extension of the standard model but is still lacking experimental evidence. Recently, the interest in supersymmetry has arisen in the condensed matter community owing to its potential emergence at the continuous quantum phase transition. In this paper, we demonstrate that $2+1\mathrm{D}$ supersymmetry, relating massive Majorana and Ising fields, might emerge at the first-order quantum phase transition of the Ising magnetization by tuning a single parameter. Although the emergence of the SUSY is only allowed in a finite range of scales due to the existence of relevant masses, the scale range can be large when the masses before scaling are small. We show that the emergence of supersymmetry is accompanied by a topological phase transition for the Majorana field, where its non-zero mass changes the sign but keeps the magnitude. An experimental realization of this scenario is proposed using the surface state of a $3+1\mathrm{D}$ time-reversal invariant topological superconductor with surface magnetic doping.

An analysis of the stability of topological states induced by Coulomb repulsion on decorated honeycomb lattices is presented. Based on a mean-field treatment of a spinless extended Hubbard model on the decorated honeycomb lattice we show how the quantum anomalous Hall (QAH) phase is a robust topological phase which emerges at various electron fillings and involves either quadratic band crossing points (QBCP) or Dirac points. The topological QAH phase is also found to be most stable against thermal fluctuations up to moderate temperatures when the Coulomb repulsion is maximally frustrated and at half filling. We show how a topological metal can be induced from the QAH for certain electron doping ranges. Electrons on the Fermi surface of such metallic states are characterized by having nonzero Berry phases which can give rise to nonquantized intrinsic Hall conductivities.

Search for parafermions and Fibonacci anyons, which are excitations obeying non-Abelian statistics, is driven both by the quest for deeper understanding of nature and prospects for universal topological quantum computation. However, physical systems that can host these exotic excitations are rare and hard to realize in experiments. Here we study the domain walls and the edge states formed in spin transitions in the fractional quantum Hall effect. Effective theory approach and exact diagonalization in a disk and torus geometries proves the existence of the counterpropagating edge modes with opposite spin polarizations at the boundary between the two neighboring regions of the two-dimensional electron liquid in spin-polarized and spin-unpolarized phases. By analytical and numerical analysis, we argue that these systems can host parafermions when coupled to an $s$-wave superconductor and are experimentally feasible. We investigate settings based on $\nu =2/3,4/3$, and $5/3$ spin transitions and analyze spin-flipping interactions that hybridize counterpropagating modes. Finally, we discuss spin-orbit interactions of composite fermions.