We present a theoretical approach for understanding the stability of simple metal nanowires, in particular, monovalent metals such as the alkalis and noble metals. Their cross sections are of order 1 nm, so that small perturbations from external (usually thermal) noise can cause large geometrical deformations. The nanowire lifetime is defined as the time required for making a transition into a state with a different cross-sectional geometry. This can be a simple overall change in radius, or a change in the cross-section shape, or both. We develop a stochastic field theoretical model to describe this noise-induced transition process in which the initial and final states correspond to locally stable states on a potential surface derived by solving the Schrödinger equation for the electronic structure of the nanowire numerically. The numerical string method is implemented to determine the optimal transition path governing the lifetime. Using these results, we tabulate the lifetimes of sodium and gold nanowires for several different initial geometries.

We present evidence for the existence and control of bulk-surface coupling in ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$ nanoribbons. Our magnetoresistance measurements reveal that the number of coherent channels contributing to quantum interference in the nanoribbons changes abruptly when the film thickness exceeds the bulk phase relaxation length. We interpret this observation as an evidence for bulk-mediated coupling between metallic states located on opposite surfaces. This hypothesis is supported by additional magnetoresistance measurements conducted under a set of gate voltages and in a parallel magnetic field, the latter of which alters the intersurface coupling in a controllable way.

We introduce a spin-orbital entangled (SOE) resonating valence bond (RVB) state on a square lattice of spins $\frac{1}{2}$ and orbitals represented by pseudospins $\frac{1}{2}$. Like the standard RVB state, it is a superposition of nearest-neighbor hard-core coverings of the lattice by spin singlets, but adjacent singlets are favored to have perpendicular orientations and, more importantly, an orientation of each singlet is entangled with orbitals state on its two lattice sites. The SOE-RVB state can be represented by a projected entangled pair state (PEPS) with a bond dimension $D=4$. This representation helps to reveal that the state is a superposition of striped coverings conserving a topological quantum number. The stripes are a critical quantum spin liquid. We propose a spin-orbital Hamiltonian supporting a SOE-RVB ground state.

We present ab initio quasiparticle calculations for electronic excitations and the fundamental band gap of the strongly correlated transition-metal oxide CuO using the $GW$ approximation of many-body perturbation theory. Problems related to the suitability of the method for strongly correlated materials and issues of self-consistency are addressed. We explain why quasiparticle self-consistent $GW$ strongly overestimates the band gap of CuO. Apart from the band gap, electron addition and removal spectra in the quasiparticle approximation including lifetime and matrix-element effects are found to be in excellent agreement with the quasiparticle excitations in direct and inverse photoemission data.

All-electron path integral Monte Carlo (PIMC) and density functional theory molecular dynamics (DFT-MD) simulations provide a consistent, first-principles investigation of warm dense neon plasmas in the density-temperature range of 1–15 g ${\mathrm{cm}}^{-3}$ and ${10}^4–{10}^9$ K. At high temperatures, DFT-MD becomes intractable because of too many partially occupied bands, while at lower temperatures, PIMC is intractable because of the free-particle approximation of fermion nodes. In combination, PIMC and DFT-MD pressures and internal energies provide a coherent equation of state with a region of overlap in which the two methods cross-validate each other. Pair-correlation functions at various temperatures and densities provide details of the plasma structure and the temperature-driven ionization process. The electronic density of states of neon shows that a gap persists for the highest density-temperature conditions studied here with DFT-MD. Finally, the computed shock Hugoniot curves show an increase in compression as the first and second shells are ionized.

The ${\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x$ alloy system is an established ideal transition-metal system possessing a composition-induced paramagnetic-to-ferromagnetic quantum phase transition (QPT) at the critical concentration $x_c\sim 0.026$ in bulk. A low-temperature non-Fermi liquid (NFL) behavior around $x_c$ usually indicates the presence of quantum criticality (QC) in this system. In this work, we explore the existence of such a QPT in nanoparticles of this alloy system. We synthesized single-phase, polydispersed and 40–50 nm mean diameter crystalline nanoparticles of ${\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x$ alloys, with $x$ near $x_c$ and beyond, by a chemical reflux method. In addition to the determination of the size, composition, phase, and crystallinity of the alloys by microscopic and spectroscopic techniques, the existence of a possible QPT was explored by resistivity and dc magnetization measurements. A dip in the value of the exponent $n$ near $x_c$, and a concomitant peak in the constant $A$ of the $A{T}^n$ dependence of the low-temperature ($T$) resistivity indicate the presence of a quantum-like phase transition in the system. The minimum value of $n$, however, remains within the Fermi liquid regime ($n>2$). The dc magnetization results suggest an anticipatory presence of a superparamagnetic-to-ferromagnetic QPT in the mean-sized nanoparticles. The observation of a possible quantum critical NFL behavior ($n<2$) through resistivity is argued to be inhibited by the electron-magnon scatterings present in the smaller nanoparticles.

We calculate core-level spectra for pristine and doped free-standing graphene sheets. Instructions for how to perform the calculations are given in detail. Although pristine graphene is not metallic the core-level spectrum presents low-energy tailing which is characteristic of metallic systems. The peak shapes vary with doping level in a characteristic way. The spectra are compared to experiments and show good agreement. We compare to two different pristine samples and to one doped sample. The pristine samples are one with quasi-free-standing epitaxial graphene on SiC obtained by hydrogen intercalation and one with a suspended graphene sheet. The doped sample is a gold supported graphene sheet. The gold substrate acts as an acceptor so the graphene sheet gets $p$ doped.

The electron number parity of the ground state of a semiconductor nanowire proximity coupled to a bulk superconductor can alternate between the quantized values $\pm 1$ if parameters such as the wire length $L$, the chemical potential $\mu$, or the magnetic field $B$ are varied inside the topological superconductor phase. The parity jumps, which may be interpreted as changes in the occupancy of the fermion state formed from the pair of Majorana modes at opposite ends of the wire, are accompanied by jumps $\delta N$ in the charge of the nanowire, whose values decrease exponentially with the wire length. We study theoretically the dependence of $\delta N$ on system parameters, and compare the locations in the $\mu \text{−}B$ plane of parity jumps when the nanowire is or is not proximity coupled to a bulk superconductor. We show that, despite the fact that the wave functions of the Majorana modes are localized near the two ends of the wire, the charge-density jumps have spatial distributions that are essentially uniform along the wire length, being proportional to the product of the two Majorana wave functions. We explain how charge measurements, say by an external single-electron transistor, could reveal these effects. Whereas existing experimental methods require direct contact to the wire for tunneling measurements, charge sensing avoids this issue and provides an orthogonal measurement to confirm recent experimental developments. Furthermore, by comparing density of states measurements which show Majorana features at the wire ends with the uniformly distributed charge measurements, one can rule out alternative explanations for earlier results. We shed light on a parameter regime for these wire-superconductor hybrid systems, and propose a related experiment to measure spin density.

The effects of isovalent doping on orbital ordering in the perovskite ${\mathrm{Y}}_{1-x}{\mathrm{La}}_x{\mathrm{VO}}_3$ were investigated using neutron diffraction and the pair distribution function analysis. ${\mathrm{YVO}}_3$ is a prototype for orbital ordering, exhibiting two consecutive transitions to G-type and C-type ordering with decreasing temperature. Evidence for local orbital ordering above the transition temperature of $T_{OO}\sim 200$ K is presented, obtained from the temperature dependence of the oxygen-oxygen octahedral correlations. This suggests that locally, G-type orbital ordering is present above the $T_{OO}$ temperature but it is short range. With doping, it is expected that orbital ordering disappears altogether. By 30% of doping, as in ${\mathrm{Y}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{VO}}_3$, even though no orbital ordering is expected, we find that locally, C-type orbital correlations are most likely present below the magnetic transition temperature $T_N$, allowing for a direct paramagnetic to orbitally ordered/antiferromagnetic transition in this composition.

Atomic-scale elemental maps of materials acquired by core-loss inelastic electron scattering often exhibit an undesirable sensitivity to the unavoidable elastic scattering, making the maps counterintuitive to interpret. Here, we present a systematic study that scrutinizes the energy-loss and sample-thickness dependence of atomic-scale elemental maps acquired using 100-keV incident electrons in a scanning transmission electron microscope. For single-crystal silicon, the balance between elastic and inelastic scattering means that maps generated from the near-threshold $\mathrm{Si}-L$ signal (energy loss of 99 eV) show no discernible contrast for a thickness of $0.5\lambda$ ($\lambda$ is the electron mean-free path, here approximately 110 nm). At greater thicknesses we observe a counterintuitive “negative” contrast. Only at much higher energy losses is an intuitive “positive” contrast gradually restored. Our quantitative analysis shows that the energy loss at which a positive contrast is restored depends linearly on the sample thickness. This behavior is in very good agreement with our double-channeling inelastic scattering calculations. We test a recently proposed experimental method to correct the core-loss inelastic scattering and restore an intuitive “positive” chemical contrast. The method is demonstrated to be reliable over a large range of energy losses and sample thicknesses. The corrected contrast for near-threshold maps is demonstrated to be (desirably) inversely proportional to sample thickness. Implications for the interpretation of atomic-scale elemental maps are discussed.

Thermal phonon transport at length scales comparable to mean free paths is governed by the Boltzmann equation, which is challenging to solve due to its high dimensionality. Here, we present an analytical Green's function for the frequency-dependent, multidimensional Boltzmann equation under the relaxation-time approximation. The new analytical solution is valid from diffusive to ballistic transport regimes and rigorously includes frequency dependence of phonon properties. We demonstrate that our result enables simple closed-form solutions for a number of multidimensional problems for which the only prior solution methods have been computationally expensive numerical simulations.

$\mathrm{Pb}\left(\mathrm{F}{\mathrm{e}}_{0.5}\mathrm{N}{\mathrm{b}}_{0.5}\right){\mathrm{O}}_3$ (PFN), one of the few relaxor multiferroic systems, has a $G$-type antiferromagnetic transition at $T_{\mathrm{N}}=143\mathrm{K}$ and a ferroelectric transition at $T_{\mathrm{C}}=385\mathrm{K}$. By using high-resolution neutron-diffraction experiments and a total scattering technique, we paint a comprehensive picture of the long- and short-range structures of PFN: (i) a clear sign of short-range structural correlation above $T_{\mathrm{C}}$, (ii) no sign of the negative thermal expansion behavior reported in a previous study, and (iii) clearest evidence thus far of magnetoelectric coupling below $T_{\mathrm{N}}$. We conclude that at the heart of the unusual relaxor multiferroic behavior lies the disorder between ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Nb}}^{5+}$ atoms. We argue that this disorder gives rise to short-range structural correlations arising from O disorder in addition to Pb displacement.

We present a pulsed magnetic field study on the magnetic and magnetostriction properties of Ni-Mn-$Z$ $(Z=\mathrm{In}$, Sn, and Sb) based Heusler shape-memory alloys. These materials generally display a field-induced magnetostructural transition that could lead to an irreversible phase transition, when measured near the martensitic transition temperature. Here, we show that independently of the transition temperature, the critical field for the phase transition sensitively depends on the main-group element in the sample. Irrespective of their compositions, all samples display a magnetization of around $2{\mu }_B/\mathrm{f}.\mathrm{u}.$ in the martensite phase and about $6{\mu }_B/\mathrm{f}.\mathrm{u}.$ in the cubic austenite phase. Our magnetic and magnetostriction measurements at low temperatures exhibit a partial or complete arrest of the high-field austenite phase below the reverse martensitic transition. This results in a large irreversibility with a hysteresis width as high as 24 T. We introduce a theoretical model to discuss the experimental results.

There is a multiplicity of charge-ordered, pairing-based, or pair density wave theories of the cuprate pseudogap, albeit arising from different microscopic mechanisms. For mean-field schemes (of which there are many) we demonstrate here that they have precise implications for two-body physics in the same way that they are able to address the one-body physics of photoemission spectroscopy. This follows because the full vertex can be obtained exactly from the Ward-Takahashi identity. As an illustration, we present the spin response functions, finding that a recently proposed pair density wave (Amperean pairing) scheme is readily distinguishable from other related scenarios.

We present harmonic transverse voltage measurements of current-induced thermoelectric and spin-orbit torque (SOT) effects in ferromagnet/normal-metal bilayers, in which thermal gradients produced by Joule heating and SOT coexist and give rise to ac transverse signals with comparable symmetry and magnitude. Based on the symmetry and field dependence of the transverse resistance, we develop a consistent method to separate thermoelectric and SOT measurements. By addressing first ferromagnet/light-metal bilayers with negligible spin-orbit coupling, we show that in-plane current injection induces a vertical thermal gradient whose sign and magnitude are determined by the resistivity difference and stacking order of the magnetic and nonmagnetic layers. We then study ferromagnet/heavy-metal bilayers with strong spin-orbit coupling, showing that second harmonic thermoelectric contributions to the transverse voltage may lead to a significant overestimation of the antidamping SOT. We find that thermoelectric effects are very strong in Ta(6 nm)/Co(2.5 nm) and negligible in Pt(6 nm)/Co(2.5 nm) bilayers. After including these effects in the analysis of the transverse voltage, we find that the antidamping SOTs in these bilayers, after normalization to the magnetization volume, are comparable to those found in thinner Co layers with perpendicular magnetization, whereas the fieldlike SOTs are about an order of magnitude smaller.

We investigate theoretically how ferromagnetic resonance (FMR) in a multiferroic heterostructure provides quantitative information on multiferroic coupling. In particular, we analyze the asymmetry of the angular dependence of FMR on the direction of resonance magnetic fields as experimentally reported for ${\mathrm{Co}/\mathrm{BaTiO}}_3$ [Jedrecy et al., Phys. Rev. B 88, 121409(R) (2013)] and for ${\mathrm{permalloy}/\mathrm{Pb}\left(\mathrm{MgNb}\right)\mathrm{O}}_3-{\mathrm{PbTiO}}_3$ [Nan et al., Sci. Rep. 4, 3688 (2014)]. Based on both analytical expressions for the dependence of FMR on the magnetic-field orientation and full numerical simulations which account for mutually coupled polarization $\mathbf{P}$ and magnetization $\mathbf{M}$ dynamics, we conclude that it is the linear magnetoelectric coupling in $\mathbf{P}$ and $\mathbf{M}$ originating from spin-dependent screening that leads to the experimentally observed asymmetry in FMR angular behavior. This suggests that angular resolved FMR is well suited to quantify the type and the strength of magnetoelectric coupling. In addition, we predict a pronounced thickness dependence of the resonance asymmetries and show how the FMR angular asymmetry can be tuned by an external static electric field. The possibility to observe electric-field-assisted FMR in the absence of an external static magnetic field is also discussed.

The classification of electron systems according to their topology has been at the forefront of condensed matter research in recent years. It has been found that systems of the same symmetry, previously thought of as equivalent, may in fact be distinguished by their topological properties. Moreover, the nontrivial topology found in some insulators and superconductors has profound physical implications that can be observed experimentally and can potentially be used for applications. However, characterizing a system's topology is not always a simple task, even for a theoretical model. When translation and other symmetries are present in a quadratic model, the topological invariants are readily defined and easily calculated in a variety of symmetry classes. However, once interactions or disorder come into play the task becomes difficult, and in many cases prohibitively so. The goal of this paper is to test whether the entanglement entropy and entanglement spectrum bare signatures of the system's topology. Using quadratic models of superconductors we demonstrate that these entanglement properties are sensitive to changes in topology.

Phase transitions in charge and orbital degrees of freedom occur successively in the $A$-site ordered perovskite manganite ${\mathrm{SmBaMn}}_2{\mathrm{O}}_6$ at $T_{\mathrm{CO}1}\approx 370\mathrm{K}$ and $T_{\mathrm{CO}2}\approx 190\mathrm{K}$. To determine the $3d$ electronic state in each phase, we performed crystal structure analyses on a single-crystal sample at 150, 300, and 400 K using synchrotron x-ray diffraction. The results indicate that the charge and orbital ordering on the distorted and alternately tilted ${\mathrm{MnO}}_6$ octahedra should accompany electric polarization in each ${\mathrm{MnO}}_2$ layer. The stacking of the polarized ${\mathrm{MnO}}_2$ layers along the $c$ axis changes from a staggered configuration to a ferroic configuration at $T_{\mathrm{CO}2}$.

We survey polaron formation in the Barisić-Labbé-Friedel and Su-Schrieffer-Heeger (BLF-SSH) model using acoustic phonons in the adiabatic limit. Multiple different numerical optimization routines and strong-coupling analytical calculations are used to find a robust ground-state energy for a wide range of coupling strengths. The electronic configuration and accompanying ionic distortions of the polaron were determined, as well as a nonzero critical coupling strength for polaron formation in two and three dimensions.