We improve a recently developed expansion technique for calculating real-frequency spectral functions of any one-dimensional model with short-range interactions, by postprocessing computed Chebyshev moments with linear prediction. This can be achieved at virtually no cost, and in sharp contrast to existing methods based on the dampening of the moments, improves the spectral resolution rather than lowering it. We validate the method for the exactly solvable resonating level model and the single impurity Anderson model. It is capable of resolving sharp Kondo resonances, as well as peaks within the Hubbard bands when employed as an impurity solver for dynamical mean-field theory. Our method works at zero temperature and allows for arbitrary discretization of the bath spectrum. It achieves similar precision as the dynamical density matrix renormalization group, at lower cost. We also propose an alternative expansion, of $\mathbb{1}-exp(-\tau H)$ instead of the usual $H$, which opens the possibility of using established methods for the time evolution of matrix product states to calculate the spectral functions directly.

In noncentrosymmetric crystals such as MnSi, magnetic order can take the form of a skyrmion crystal (SkX). In this phase, conduction electrons coupled to the local magnetic moments acquire a Berry phase, leading to an emergent electromagnetism. Motivated by experimental reports of a non-Fermi-liquid phase in MnSi, in which resistivity is observed to scale as $\Delta \rho \sim T^{3/2}$, here we examine the effect of coupling phonons of an incommensurate SkX to electrons. Despite the formal similarity to a system consisting of a Fermi surface coupled to an electromagnetic field, the Berry phase fluctuations do not lead to non-Fermi-liquid behavior. Instead, we propose a different mechanism in which electrons scatter off columnar fluctuation in a three-dimensional SkX. When the effects of lattice-induced anisotropy are neglected, these fluctuations are ultrasoft and induce an “anomalous Fermi liquid” in which Landau quasiparticles survive but with an anomalous $\Delta \rho \left(T\right)\sim T^{7/4}$ resistivity perpendicular to the columns, and a Fermi-liquid resistivity along them.

Mott insulator to metal transitions under an electric field are currently the subject of numerous fundamental and applied studies. This puzzling effect, which involves nontrivial out-of-equilibrium effects in correlated systems, is indeed at play in the operation of a new class of electronic memories, the “Mott memories.” However, the combined electronic and thermal effects are difficult to disentangle in Mott insulators undergoing such transitions. We report here a comparison between the properties under an electric field of a canonical Mott insulator and a model built on a realistic two-dimensional resistor network able to capture both thermal effects and electronic transitions. This comparison made specifically on the family of narrow gap Mott insulators $A{M}_4{Q}_8$, ($A=\mathrm{Ga}$ or Ge; $M$ = V, Nb or Ta; and $Q=\mathrm{S}$ or Se) unambiguously establishes that the resistive transition experimentally observed under an electric field arises from a purely electronic mechanism.

Ultrasonic measurements performed on the spin-liquid organic compound $\mathrm{Et}{\mathrm{Me}}_3\mathrm{Sb}\left[\mathrm{Pd}\right(\text{dmit}{)}_2{]}_2$ reveal an inhomogeneous elastic behavior above 40 K due to the rotational motion in the $\mathrm{Et}{\mathrm{Me}}_3\mathrm{Sb}$ cations. Two velocity softening anomalies and concomitant attenuation peaks at 40 K and 160 K are respectively attributed to the rotation of methyl and ethyl groups. At lower temperatures, the elastic character is homogeneous and an anomaly is observed below 2 K. A magnetic field study suggests that two kinds of excitations contribute to this anomaly. First, a softening of the velocity toward the lowest temperatures which is not dependent on magnetic field implies other degrees of freedom than spins. Second, spin excitations coupled to longitudinal acoustic phonons yield a velocity softening anomaly that completely disappears for field values higher than 7 T. Consistent attenuation anomalies are also observed for both contributions. These data suggest that only one kind of spin excitation exists in the spin-liquid ground state below 1.5 K. A spinon-phonon interaction likely explains the magnetic field dependent elastic anomalies. However, no spin-gap-like excitations related to a pairing instability of the spinons could be detected in this experiment.

We have investigated fine structure splitting of indirect excitons in ultrapure diamond by high-resolution spectroscopy. The role of the mass-anisotropy splitting has been revealed by comparison with group-theoretical calculations of the reduced matrix elements for phonon scattering of electrons and holes followed by the radiative recombination. The observed energy levels and coupling strengths of the fine structure states are successfully reproduced by inclusion of the mass-anisotropy splitting in addition to the exchange and spin-orbit interactions.

Nanoscale light-matter interaction in the weak-coupling regime has been achieved with unique hyperbolic metamaterial modes possessing a high density of states. Here, we show strong coupling between intersubband transitions (ISBTs) of a multiple quantum well (MQW) slab and the bulk polariton modes of a hyperbolic metamaterial (HMM). These HMM modes have large wave vectors (high-$k$ modes) and are normally evanescent in conventional materials. We analyze a metal-dielectric practical multilayer HMM structure consisting of a highly doped semiconductor acting as a metallic layer and an active multiple quantum well dielectric slab. We observe delocalized metamaterial mode interaction with the active materials distributed throughout the structure. Strong coupling and characteristic anticrossing with a maximum Rabi splitting (RS) energy of up to 52 meV is predicted between the high-$k$ mode of the HMM and the ISBT, a value approximately 10.5 times greater than the ISBT linewidth and 4.5 times greater than the material loss of the structure. The scalability and tunability of the RS energy in an active semiconductor metamaterial device have potential applications in quantum well infrared photodetectors and intersubband light-emitting devices.

Distributed Bragg reflector based semiconductor resonators constitute paradigmatic systems where cavity optomechanical and optoelectronic phenomena can be simultaneously active in the same device. High GHz range mechanical frequencies and ultrastrong optomechanical couplings are additional attractive features for applications. We report here a detailed spectroscopic study of the fundamental optomechanical resonances of such a device. The existent challenge to study vibrational frequencies that are above the bandwidth of current electronics is solved using a purposely made tandem Fabry-Perot-triple spectrometer. A full theoretical description of the Raman process including electronic, vibrational, and optical confinement is presented to describe the experiments. These results open the path for the demonstration of polariton optomechanical phenomena in these devices.

We compute electromagnetic, gravitational, and mixed linear response functions of two-dimensional free fermions in an external quantizing magnetic field at an integer filling factor. The results are presented in the form of the effective action and as an expansion of currents and stresses in wave vectors and frequencies of the probing electromagnetic and metric fields. In addition to the well-studied $U\left(1\right)$ Chern-Simons and Wen-Zee terms we find a gravitational Chern-Simons term that controls the correction to the Hall viscosity due to the background curvature. We relate the coefficient in front of the term with the chiral central charge.

Using combined theoretical and experimental approaches, we studied the structural and electronic origin of the magnetic structure in hexagonal ${\mathrm{LuFeO}}_3$. Besides showing the strong exchange coupling that is consistent with the high magnetic ordering temperature, the previously observed spin reorientation transition is explained by the theoretically calculated magnetic phase diagram. The structural origin of this spin reorientation that is responsible for the appearance of spontaneous magnetization, is identified by theory and verified by x-ray diffraction and absorption experiments.

The high Curie temperature multiferroic compound CuO has a quasidegenerate magnetic ground state that makes it prone to manipulation by the so-called “order-by-disorder” mechanism. First principle computations supplemented with Monte Carlo simulations and experiments show that isovalent doping allows us to stabilize the multiferroic phase in nonferroelectric regions of the pristine material phase diagram with experiments reaching a 250% widening of the ferroelectric temperature window with 5% of Zn doping. Our results allow us to validate the importance of a quasidegenerate ground state on promoting multiferroicity on CuO at high temperatures and open a path to the material engineering of multiferroic materials. In addition we present a complete explanation of the CuO phase diagram and a computation on the incommensurability in excellent agreement with experiment without free parameters.

Synthetic antiferromagnets with strong perpendicular anisotropy can be modeled by layered Ising antiferromagnets. Accounting for the fact that in the experimental systems the ferromagnetic layers, coupled antiferromagnetically via spacers, are multilayers, we propose a description through Ising films where ferromagnetic stacks composed of multiple layers are coupled antiferromagnetically. We study the equilibrium and nonequilibrium properties of these systems where we vary the number of layers in each stack. Using numerical simulations, we construct equilibrium-temperature-magnetic-field phase diagrams for a variety of cases. We find the same dominant features (three stable phases, where one phase boundary ends in a critical end point, whereas the other phase boundary shows a tricritical point at which the transition changes from first to second order) for all studied cases. Using time-dependent quantities, we also study the ordering processes that take place after a temperature quench. The nature of long-lived metastable states are discussed for thin films, whereas for thick films we compute the surface autocorrelation exponent.

Graphene is the only member of the carbon family from zero- to three-dimensional materials for which superconductivity has not been observed yet. At this time, it is not clear whether the quest for superconducting graphene is hindered by technical challenges, or else by the fluctuation of the order parameter in two dimensions. In this area, ab initio calculations are useful to guide experimental efforts by narrowing down the search space. In this spirit, we investigate from first principles the possibility of inducing superconductivity in doped graphene using the fully anisotropic Migdal-Eliashberg theory powered by Wannier-Fourier interpolation. To address a best-case scenario, we consider both electron and hole doping at high carrier densities so as to align the Fermi level to a van Hove singularity. In these conditions, we find superconducting gaps of $s$-wave symmetry, with a slight anisotropy induced by the trigonal warping, and, in the case of $n$-doped graphene, an unexpected two-gap structure reminiscent of ${\mathrm{MgB}}_2$. Our Migdal-Eliashberg calculations suggest that the observation of superconductivity at low temperature should be possible for $n$-doped graphene at carrier densities exceeding ${10}^{15}{\text{cm}}^{-2}$.

The Lagrangian dynamics of micrometer-sized solid particles of hydrogen and deuterium is investigated in thermal counterflow of superfluid ${}^4\mathrm{He}$ at length scales ${\ell }_{\exp }$ straddling about two orders of magnitude across the average distance $\ell$ between quantized vortices by using the particle tracking velocimetry technique. The normalized probability distribution functions of the particle velocities and accelerations change from the shapes typical of quantum turbulence, characterized by power-law tails, at length scales ${\ell }_{\exp }\lesssim \ell$, to forms similar to those obtained in classical turbulent flows, at ${\ell }_{\exp }\gtrsim \ell$, although the power-law behavior of the acceleration distribution tails is less clear than that observed for the particle velocities. Moreover, the acceleration distribution follows a nearly log-normal, classical-like shape, at $\ell \lesssim {\ell }_{\exp }\lesssim L_{\mathrm{int}}$, where $L_{\mathrm{int}}$ denotes the integral length scale, providing thus, within the just defined inertial range, experimental evidence of the existence of classical-like, macroscopic vortical structures in thermal counterflow of superfluid ${}^4\mathrm{He}$, which is traditionally regarded as a quantum flow with no obvious classical analog. Additionally, we report our observations of the added mass effect in quantum turbulence and discuss them in the framework of a developed model of particle dynamics.

We use NMR techniques to investigate the magnetic properties of ${\mathrm{BaCo}}_2{\mathrm{As}}_2$ single crystals, the nonsuperconducting end member of the Co-substituted iron-pnictide high-$T_c$ superconductor $\mathrm{Ba}\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right){}_2{\mathrm{As}}_2$ with $x=1$. We present ${}^{75}\mathrm{As}$ NMR evidence for enhancement of low frequency spin fluctuations below $\sim 100$ K. This enhancement is accompanied by that of static uniform spin susceptibility at the wave vector $\mathbf{q}$ = $\mathbf{0}$, suggesting that the primary channel of the spin correlations is ferromagnetic rather than antiferromagnetic. Comparison between the NMR Knight shift ${}^{75}K$ and bulk susceptibility ${\chi }_{\mathrm{bulk}}$ data uncovers the presence of two separate components of spin susceptibility with distinct temperature dependencies, presumably because multiple electronic bands crossing the Fermi energy play different roles in the electronic properties of ${\mathrm{BaCo}}_2{\mathrm{As}}_2$.

The recent proposal of Romero-Isart et al. [Phys. Rev. Lett. 111, 145304 (2013)] to utilize the vortex lattice phases of superconducting materials to prepare a lattice for ultracold-atom-based quantum emulators raises the need to create and control vortex lattices of different symmetries. Here we propose a mechanism by which honeycomb, hexagonal, square, and kagome vortex lattices could be created in superconducting systems with multiscale intervortex interactions. Multiple scales of the intervortex interaction can be created and controlled in layered systems made of different superconducting materials or with differing interlayer spacings.

We discuss the dependence of self- and interdiffusion coefficients on temperature and composition for two prototypical binary metallic melts, Al-Ni and Zr-Ni, in molecular-dynamics computer simulations and the mode-coupling theory of the glass transition (MCT). Dynamical processes that are mainly entropic in origin slow down mass transport (as expressed through self-diffusion) in the mixture as compared to the ideal-mixing contribution. Interdiffusion of chemical species is a competition of slow kinetic modes with a strong thermodynamic driving force that is caused by nonentropic interactions. The combination of both dynamic and thermodynamic effects causes qualitative differences in the concentration dependence of self-diffusion and interdiffusion coefficients. At high temperatures, the thermodynamic enhancement of interdiffusion prevails, while at low temperatures, kinetic effects dominate the concentration dependence, rationalized within MCT as the approach to its ideal-glass transition temperature $T_c$. The Darken equation relating self- and interdiffusion qualitatively reproduces the concentration dependence in both Zr-Ni and Al-Ni, but quantitatively, the kinetic contributions to interdiffusion can be slower than the lower bound suggested by the Darken equation. As temperature is decreased, the agreement with Darken's equation improves, due to a strong coupling of all kinetic modes that is a generic feature predicted by MCT.

Exchange bias and coercivity are investigated in antiferromagnetic (AFM)/ferromagnetic (FM) bilayers by steepest descent and Monte Carlo simulations. For a given range of the AFM anisotropy, a magnetic domain wall parallel to the interface is created in the AFM layer. This domain wall propagates along the AFM thickness and eventually gets pinned at the negative saturation due to the intrinsic Peierls potential of the AFM material. We demonstrate that this pinning and its associated dissipation is one possible source of coercivity in such exchange coupled bilayers. Monte Carlo simulations were also carried out to investigate the influence of thermal fluctuations, which induce a random walk of the domain wall throughout the AFM layer, leading to magnetic aftereffect. Depending on the experimental characteristic measurement time and temperature, the domain wall can eventually be expelled from the AFM layer leading to the reversal of the entire AFM spin lattice. This contribution to the exchange bias phenomenon particularly explains quite well the enhancement of coercivity in exchange coupled bilayers at low temperature, even in polycrystalline samples.

${}^{57}$Fe-enriched ${\mathrm{Fe}}_4{\mathrm{O}}_5$ samples were synthesized in a laser-heated diamond anvil cell at a pressure of about 15 GPa and a temperature of about 2000 K. Nuclear forward scattering (NFS) spectra were collected in the range 0–40 GPa and were combined with first-principles calculations to provide insights into the magnetic properties of ${\mathrm{Fe}}_4{\mathrm{O}}_5$. NFS spectra show that strong magnetic interactions persist up to 40 GPa and that they are generated by a single magnetic contribution. The hyperfine magnetic field $\left(B_{\mathrm{hf}}\right)$ and quadrupole splitting (QS) are in the ranges 51–53 T and 0.40–1.2 mm ${\mathrm{s}}^{-1}$, respectively. The QS shows an intriguing evolution with pressure, with a fast increase from 0.4 to 1.0 mm ${\mathrm{s}}^{-1}$ between 0 and 10 GPa and a slow increase up to 1.2 mm ${\mathrm{s}}^{-1}$ in the range 10–40 GPa. First-principles calculations suggest an antiferromagnetic ordering for the three sites, and similar magnetic moments in the range $\sim$3.6–3.8 μ${}_{\mathrm{B}}/$Fe. These values, typical of strongly correlated Fe magnetic systems, are in agreement with the experimental estimated average moment of $\sim$3.8 μ${}_{\mathrm{B}}/$Fe. The single contribution to the NFS spectrum and the similar calculated magnetic moments suggest that the iron atoms at the three crystallographic sites have similar electronic arrangements.

The Nernst effect was measured in the electron-doped cuprate superconductor ${\mathrm{Pr}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ (PCCO) at four concentrations, from underdoped $(x=0.13)$ to overdoped $(x=0.17)$, for a wide range of temperatures above the critical temperature $T_c$. A magnetic field $H$ up to 15 T was used to reliably access the normal-state quasiparticle contribution to the Nernst signal $N_{\mathrm{qp}}$, which is subtracted from the total signal $N$, to obtain the superconducting contribution $N_{\mathrm{sc}}$. As a function of $H$, $N_{\mathrm{sc}}$ peaks at a field $H^{☆}$ whose temperature dependence obeys $H_{c2}^{☆}ln(T/T_c$), as it does in a conventional superconductor such as ${\mathrm{Nb}}_x{\mathrm{Si}}_{1-x}$. The doping dependence of the characteristic field scale $H_{c2}^{☆}$, shown to be closely related to the upper critical field $H_{c2}$, tracks the domelike dependence of $T_c$, showing that superconductivity is weakened below the quantum critical point where the Fermi surface is reconstructed, presumably by the onset of antiferromagnetic order. Our data at all dopings are quantitatively consistent with the theory of Gaussian superconducting fluctuations, eliminating the need to invoke unusual vortexlike excitations above $T_c$, and ruling out phase fluctuations as the mechanism for the fall of $T_c$ with underdoping. We compare the properties of PCCO with those of hole-doped cuprates and conclude that the domes of $T_c$ and $H_{c2}$ versus doping in the latter materials are also controlled predominantly by phase competition rather than phase fluctuations.

We study the possibility of controlling the propagation of ballistic electrons using an electronic metamaterial with zero quantum index of refraction. This is obtained by tailoring the band structure of a semiconductor superlattice to exhibit a Dirac cone at the center of the Brillouin zone, creating an artificial quantum medium with zero refractive index for electrons. We demonstrate attractive functionalities, such as anomalous tunneling, ideal matching, electronic routing, cloaking, and focusing. We envision applications in nanoelectronics and sensing.