We use first-principles calculations, at the density-functional-theory (DFT) and GW levels, to study both the electron-phonon interaction for acoustic phonons and the “synthetic” vector potential induced by a strain deformation (responsible for an effective magnetic field in case of a nonuniform strain). In particular, the interactions between electrons and acoustic phonon modes, the so-called gauge-field and deformation potential, are calculated at the DFT level in the framework of linear response. The zero-momentum limit of acoustic phonons is interpreted as a strain of the crystal unit cell, allowing the calculation of the acoustic gauge-field parameter (synthetic vector potential) within the GW approximation as well. We find that using an accurate model for the polarizations of the acoustic phonon modes is crucial to obtain correct numerical results. Similarly, in the presence of a strain deformation, the relaxation of atomic internal coordinates cannot be neglected. The role of electronic screening on the electron-phonon matrix elements is carefully investigated. We then solve the Boltzmann equation semianalytically in graphene, including both acoustic and optical phonon scattering. We show that, in the Bloch-Grüneisen and equipartition regimes, the electronic transport is mainly ruled by the unscreened acoustic gauge field, while the contribution due to the deformation potential is negligible and strongly screened. We show that the contribution of acoustic phonons to resistivity is doping and substrate independent, in agreement with experimental observations. The first-principles calculations, even at the GW level, underestimate this contribution to resistivity by $\approx 30\%$. At high temperature ($T>270$ K), the calculated resistivity underestimates the experimental one more severely, the underestimation being larger at lower doping. We show that, besides remote phonon scattering, a possible explanation for this disagreement is the electron-electron interaction that strongly renormalizes the coupling to intrinsic optical-phonon modes. Finally, after discussing the validity of the Matthiessen rule in graphene, we derive simplified forms of the Boltzmann equation in the presence of impurities and in a restricted range of temperatures. These simplified analytical solutions allow us the extract the coupling to acoustic phonons, related to the strain-induced synthetic vector potential, directly from experimental data.

We present a theoretical model for electrolytically top-gated graphene, in which we analyze the effects of dielectric saturation of water due to possibly strong electric fields near the surface of a highly charged graphene, as well as the steric effects due to the finite size of salt ions in an aqueous electrolyte. By combining two well-established analytical models for those two effects, we show that the total capacitance of the solution-gated graphene is dominated by its quantum capacitance for gating potentials $\lesssim 1\mathrm{V}$, which is the range of primary interest for most sensor applications of graphene. On the other hand, at the potentials $\gtrsim 1\mathrm{V}$ the total capacitance is dominated by a universal capacitance of the electric double layer in the electrolyte, which exhibits a dramatic decrease of capacitance with increasing gating potential due to the interplay of a fully saturated dielectric constant of water and ion crowding near graphene.

Transition-metal-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystals, $X_{0.05}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ ($X=\mathrm{Cr}$, Co, Ni, and Cu), are prepared and studied by powder x-ray diffraction and extended x-ray absorption fine structure (EXAFS) measurements to get the local structures around the dopants. Cr atoms substitute Bi atoms and Co atoms may substitute Bi atoms or form ${\mathrm{Co}}_3{\mathrm{Se}}_4$ ($C2/m$) clusters. Cu is also found taking the Bi substitutional site, which differs from the reported superconductor phase of Cu-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, suggesting the dependence of site occupancy of Cu in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ on the process of sample growing. For ${\mathrm{Ni}}_{0.05}{\mathrm{Bi}}_2{\mathrm{Se}}_3$, Ni atoms form ${\mathrm{Ni}}_3{\mathrm{Se}}_4$ ($C2/m$) clusters. The nearest neighbors of $X$ ($X=\mathrm{Cr}$, Co, and Cu) are Se atoms, and the $X$-Se bond lengths are extracted from EXAFS as 2.50 Å for Cr-Se, 2.40 Å for Co-Se, and 2.38 Å for Cu-Se, which show the direct evidences of dramatic structural relaxations around $3d$ dopants. The bond information of local structures around dopants is valuable for subsequent theoretical studies, and can hardly be obtained from other techniques.

Since conduction electrons of a metal effectively screen the local electric dipole moments, it was widely believed that the ferroelectric- (FE-) like distortion cannot occur in metals. Recently, metallic ${\mathrm{LiOsO}}_3$ was discovered to be the first clear-cut example of an Anderson-Blount “ferroelectric” metal, which at 140 K undergoes a ferroelectriclike structural transition similar to insulating ${\mathrm{LiNbO}}_3$. This is very surprising because the mechanisms for structural phase transitions are usually quite distinct in metals and insulators. Through performing first-principles calculations, here we reveal that the local polar distortion in ${\mathrm{LiOsO}}_3$ is solely due to the instability of the $A$-site Li atom, in contrast to the ${\mathrm{LiNbO}}_3$ case where the second-order Jahn-Teller effect of the $B$-site Nb ion also plays an additional role. More importantly, the ferroelectriclike long-range order of the local polar distortion is found to be due to the predominantly ferroelectric short-range pair interactions between the local polar modes, which are not screened by conduction electrons. Furthermore, we predict that ${\mathrm{LiNbO}}_3$-type ${\mathrm{MgReO}}_3$ is also a ferroelectric metal but with a much higher structural transition temperature by 391 K than ${\mathrm{LiOsO}}_3$. Our paper not only unravels the origin of FE-like distortion in ${\mathrm{LiNbO}}_3$-type ferroelectric metals, but also provides a clue for designing other multifunctional ferroelectric metals.

We use a sample-dependent analysis, based on medians and quantiles, to analyze the behavior of the overlap probability distribution of the Sherrington-Kirkpatrick and $3D$ Edwards-Anderson models of Ising spin glasses. We find that this approach is an effective tool to distinguish between replica symmetry breaking–like and droplet-like behavior of the spin-glass phase. Our results are in agreement with a replica symmetry breaking–like behavior for the $3D$ Edwards-Anderson model.

Materials with heavier atomic masses usually possess lower lattice thermal conductivity ($\kappa$). The reported $\kappa$ of IrSb3 skutterudite is about 35% higher than that of CoSb3, despite Ir being much heavier than Co. We study the lattice dynamics and $\kappa$ of $\mathrm{CoSb}{}_3$ and $\mathrm{IrSb}{}_3$ from first principles. We unveil the physical reasons for the difference in $\kappa$ by comparing all the influential factors: phonon velocities, anharmonicity characterized by the third-order interatomic force constants, the weighted phase space $W$, and the atomic mass. We find the increased mass from Co to Ir is ultimately the dominant factor resulting in the increase of $\kappa$ in $\mathrm{IrSb}{}_3$, and the other factors tend to reduce $\kappa$. Larger mass leads to smaller thermal displacements causing weaker anharmonic scattering. Our work provides deeper insight to understand the correlation of $\kappa$ of systems sharing the same crystal structure. We also find that the decreases in acoustic phonon frequencies and Debye temperature in $\mathrm{IrSb}{}_3$ are almost entirely due to the mass increase from Co to Ir.

We numerically investigate magnetization, pinning, ordering, and dynamics of vortices interacting with pinning arrangements which have a density gradient. We focus on conformal crystal structures obtained by conformally transforming a spatially uniform periodic array, as well as nonconformal gradient structures and structures with quasiperiodic order. The conformal structures feature a density gradient and local ordering. Using magnetization simulations we find that conformal pinning arrays exhibit enhanced pinning compared to nonconformal gradient arrays as well as compared to random, periodic, and quasiperiodic arrays, for a broad range of fields. The effectiveness of conformal arrays arises from the continuum of length scales introduced into the arrays by the conformal transformation, allowing for a broad range of local commensuration effects. At higher vortex fillings above the range of conformal effectiveness, we show that a nonconformal rectangular gradient array exhibits strong pinning due to a novel commensuration effect and vortex ordering. Using transport simulations where vortices are driven along the gradient and at an angle to the gradient, we confirm the effectiveness of conformal pinning at increasing the critical current. For a rotated drive, the gradient arrays produce a strong vortex guidance effect in the direction perpendicular to the gradient.

In this work, we examine the lattice behavior of the economically interesting superhard material, tungsten tetraboride (${\mathrm{WB}}_4$), in a diamond anvil cell under nonhydrostatic compression up to 48.5 GPa. From the measurements of lattice-supported differential stress, significant strength anisotropy is observed in ${\mathrm{WB}}_4$. The (002) planes are found to support the highest differential stress of 19.7 GPa within the applied pressure range. This result is in contrast to ${\mathrm{ReB}}_2$, one of the hardest transition metal borides known to date, where the same planes support the least differential stress. A discontinuous change in the slope of c/a ratio is seen at 15 GPa, suggesting a structural phase transition that has also been observed under hydrostatic compression. Speculations on the possible relationship between the observed structural changes, the strength anisotropy, and the orientation of boron-boron bonds along the $c$ direction within the ${\mathrm{WB}}_4$ structure are included.

The structural phase transition of ${\mathrm{EuTiO}}_3$ near $T_{\mathrm{S}}$ = 282 K is investigated by temperature- and magnetic-field-dependent measurements of the linear thermal expansion (TE) on a polycrystalline sample. At $T_{\mathrm{S}}$ a distinct anomaly is seen in the linear coefficient which is similar to the anomaly seen at the cubic-to-tetragonal phase transition in ${\mathrm{SrTiO}}_3$. The anomaly is well described by the thermal displacement correlation function which exhibits an abrupt change at the structural phase transition caused by a double-well potential in the oxygen-oxygen interaction. In accordance with previous data a pronounced magnetic-field effect on the TE is observed which might yield an alternative potential for applications.

By studying a nonlinear susceptibility on supercooled glycerol, we show that applying a static field $E_{st}$ increases the glass transition temperature $T_g$ by an amount quadratic in $E_{st}$. This has important consequences: (i) it reinforces the relation between the two paths put forward in the last years to unveil the dynamical correlation volume close to $T_g$; (ii) it clarifies the interpretation of nonlinear measurements; (iii) it yields a new control parameter of the glass transition, which paves the way for experiments deepening our understanding of glasses.

We have investigated by inelastic neutron scattering (INS), neutron diffraction, and magnetometry the magnetic properties of the mononuclear complex $\left[{\mathrm{Mn}}^{3+}{\left(\mathrm{pyrol}\right)}_3\left(\mathrm{tren}\right)\right]$ in both high-spin (${}^{5}E$, HS, $S=2$) and low-spin (${}^3{T}_1$, LS, $S=1$) states. The system presents a spin transition (ST) around 47 K with a small hysteresis width ($T_{\text{ST},\uparrow }=47.5$ K and $T_{\text{ST},\downarrow }=46$ K) characteristic of an efficient collective transition process. In the HS state, the INS spectrum at 56 K and zero magnetic field is accounted for by a zero-field splitting with $D=-5.73\left(3\right)$ ${\mathrm{cm}}^{-1}$ and $\left|E\right|=+0.47\left(2\right)$ ${\mathrm{cm}}^{-1}$ which may be the result of a dynamic Jahn-Teller effect reported in the literature. In the LS state, a single magnetic peak at 4.87 meV is observed, still at zero field. Despite the existence of an unquenched orbital moment ($L=1$) in the ground ${}^3{T}_1$ state, we argue that it may be described by a genuine $S=1$ spin Hamiltonian owing to the existence of a strong trigonal distortion of the ${\mathrm{Mn}}^{3+}$ coordination octahedron. The observed peak corresponds to a transition $\Delta M=+1$ within the $S=1$ ground state split by a large single-ion anisotropy term $D=+39.3$ ${\mathrm{cm}}^{-1}$. A full spin-Hamiltonian model is proposed based on these first INS results obtained in a thermal ST molecular magnetic system.

The compound ${\mathrm{Sm}}_2{\mathrm{Co}}_{17}$ displays magnetic properties amenable to permanent magnet applications owing to both the $3d$ electrons of Co and the $4f$ electrons of Sm. The long-standing description of the magnetic interactions between the Sm and Co ions implies a truly ferromagnetic configuration, but some recent calculations challenge this axiom, suggesting at least a propensity for ferrimagnetic behavior. We have used high-pressure synchrotron x-ray techniques to characterize the magnetic and structural properties of ${\mathrm{Sm}}_2{\mathrm{Co}}_{17}$ to reveal a robust ferromagnetic state. The local Sm moment is at most weakly affected by compression, and the ordered moments show a surprising resilience to volumetric compressions of nearly 20%. Density functional theory calculations echo the magnetic robustness of ${\mathrm{Sm}}_2{\mathrm{Co}}_{17}$.

The recent experimental demonstration of spin-polarized supercurrents offers a venue for establishment of a superconducting analog to conventional spintronics. Whereas domain-wall motion in purely magnetic structures is a well-studied topic, it is not clear how domain-wall dynamics may influence superconductivity and whether some functional property can be harnessed from such a scenario. Here, we demonstrate that domain walls in superconducting systems offer a unique way of controlling the quantum state of the superconductor. Considering both the diffusive and ballistic limits, we show that moving the domain wall to different locations in a Josephson junction will change the quantum ground state from being in a 0 state to a $\pi$ state. Remarkably, we also show that domain-wall motion can be used to turn on and off superconductivity: the position of the domain wall determines the critical temperature $T_c$ and thus whether the system is in a resistive state or not, causing even a quantum phase transition between the dissipationless and normal state at $T=0$. In this way, one achieves dynamical control over the superconducting state within a single sample by utilizing magnetic domain wall motion which has interesting consequences in terms of a domain-wall-controlled superconducting magnetoresistance effect.

We study the dynamics of doublons in the half-filled Hubbard model on the triangular lattice by using cellular dynamical mean-field theory. Investigating the nearest-neighbor dynamical correlations, we demonstrate that a nearest-neighbor doublon-holon pair shows a strong attraction, in particular in the insulating phase. The nearest-neighbor doublon and holon dynamics obtained indicate that the characteristic time scales inversely with the Coulomb interaction in the short-time region in both the metallic and insulating phases. We find that the metallic phase has larger fluctuations in their dynamics. In the long-time region, the metallic phase has persistent fluctuations in various nearest-neighbor configurations, while the fluctuations are vanishingly small in the insulating phase. We also calculate the on-site dynamical correlation of doublons and find that the lifetime of the doublon is longer in the metallic phase than in the insulating phase.

The archetypical strongly correlated Mott-phenomena compound ${\mathrm{V}}_2{\mathrm{O}}_3$ is known to show a paramagnetic metal-insulator transition driven by doping with chromium atoms and/or (negative) pressure. Via charge self-consistent density-functional theory+dynamical mean-field theory calculations we demonstrate that these two routes cannot be understood as equivalent. An explicit description of Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3$ by means of supercell calculations and the virtual crystal approximation is performed. Introducing chromium's additional electron to the system is shown to modify the overall many-body electronic structure substantially. Chromium doping increases electronic correlations which in addition induce charge transfers between Cr and the remaining V ions. Thereby the transition-metal orbital polarization is increased by the electron doping, in close agreement with experimental findings.

The spin-$1/2$ anisotropic triangular model, which is believed to describe the materials ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ and ${\mathrm{Cs}}_2{\mathrm{CuBr}}_4$, among others, is dominated by incommensurate spiral physics and is thus extremely resistant to numerical analysis on small system sizes. In this paper, we use twisted boundary conditions and exact diagonalization techniques to study the ground state of this model as a function of exchange parameter strength. With these boundary conditions we are able to extract the inter- and intrachain ordering $q$ vectors for the $\frac{J^{\prime }}{J}<1$ region finding very close agreement with recent DMRG results on much larger systems. Our results suggest a phase transition between a long-range incommensurate spiral ordered phase, and a more subtle phase with short-range spiral correlations with the $q$ vector describing the incommensurate correlations varying smoothly through the transition. In the latter phase correlations between next-nearest chains exhibit an extremely close competition between predominantly antiferromagnetic and ferromagnetic correlations. Further analysis suggests that the antiferromagnetic next-nearest chain correlations may be slightly stronger than the ferromagnetic ones. This difference is found to be slight but in line with previous renormalization group predictions of a collinear antiferromagnetic ordering in this region.

We examine non-Abelian topological defects in an Abelian lattice model in two dimensions. We first construct an exact solvable lattice model that exhibits coexisting and intertwined topological and classical orders. The anyon types of quasiparticle excitations are permuted by lattice symmetry operations like translations, rotations, and reflections. The global anyon permutation symmetry has a group structure of $S_3$, the permutation group of three elements. Topological crystalline defects—dislocations and disclinations—change the anyon type of an orbiting quasiparticle. They exhibit multichannel order-dependent fusion rules and projective braiding operations. Their braiding and exchange statistics breaks modular invariance and violates the conventional spin-statistics theorem. We develop a framework to characterize these unconventional properties that originate from the semiclassical nature of defects.

Just as phosphorus in silicon produces a hydrogenic defect, the double donor selenium in silicon is an analog of helium. We have measured the impurity absorption spectrum at high magnetic field, and we show that the odd-parity excited states of Si:Se behave identically to those of Si:P. This fact allows us to isolate the electron-electron interactions (exchange and correlation) in the ground state from the quadratic Zeeman effect. The field tuning allows us to put upper limits on the strength of some of these interactions (e.g., at 30 T the electron-electron correlation interaction in the ground state of Se is less than about 40 μeV; at 30 T the quadratic Zeeman energy in the ground state of P is less than about 200 μeV).

A combination of theoretical and computational methods is used to study the electronic properties of three-terminal graphitic nanowiggles (3TGNWs). 3TGNWs consisting of three GNWs connected at ${120}^{\circ }$ to each other have been recently synthesized. Theory predicts that individual GNWs possess a broad set of electronic and magnetic properties. It follows that the presence of different magnetic states in single GNWs yields a rich set of possibilities in assembled GNWs. Here, we analyze twelve distinct magnetic states in one 3TGNW structure and determine that each state corresponds to unique electronic transport properties. We also show that these nanostructures possess attractive characteristics amenable to being exploited as basic building blocks for operative electronic devices.