We investigate the ${\mathrm{H}}_3\mathrm{S}$ phase of sulfur hydride under high pressure $\simeq 200$ GPa by means of ab initio calculations within the framework of the density-functional theory with the PBE0 hybrid exchange-correlation $\left(E_{\mathrm{xc}}\right)$ approximation. The choice of $E_{\mathrm{xc}}$ has the largest effect on the calculated electron-phonon coupling (EPC) matrix elements; the high-pressure equation of state and phonon frequencies are only slightly modified. Mode-dependent EPC correction factors are determined from PBE0 using a frozen-phonon supercell approach, while standard density-functional perturbation theory is used to determine the EPC with PBE generalized-gradient approximation $E_{\mathrm{xc}}$. Our principle finding is that the calculated $\mathrm{PBE}0 T_c$ is enhanced by 25% compared to PBE. This is similar in magnitude, but in opposite direction, to the proposed suppression of $T_c$ by anharmonic effects [I. Errea et al., Phys. Rev. Lett. 114, 157004 (2015)]. Our calculations demonstrate the importance of considering exchange-correlation approximations for calculations of superconducting properties for this class of materials.

Time-dependent density-functional theory (TDDFT) often successfully reproduces excitation energies of finite systems, already in the adiabatic local-density approximation (ALDA). Here we show for prototypical molecular materials, i.e., oligothiophenes, that ALDA largely fails, and we explain why this is so. By comparing TDDFT with an in-depth analysis based on many-body perturbation theory, we demonstrate that correlation effects have a crucial impact on the energies and character of the optical excitations, not only for molecules of increasing length and in a crystalline environment but even for isolated small molecules. We argue that only high-level methodologies, which explicitly include correlation effects, can reproduce optical spectra of molecular materials with equal accuracy from gas phase to crystal structures.

The infinite density matrix renormalization group (iDMRG) algorithm is a highly successful numerical algorithm for the study of low-dimensional quantum systems, and is also frequently used to initialize the more popular finite DMRG algorithm. Implementations of both finite and infinite DMRG frequently incorporate support for the protection and exploitation of symmetries of the Hamiltonian. In common with other variational tensor network algorithms, convergence of iDMRG to the ground state is not guaranteed, with the risk that the algorithm may become stuck in a local minimum. In this paper, I demonstrate the existence of a particularly harmful class of physically irrelevant local minima affecting both iDMRG and to a lesser extent also infinite time-evolving block decimation (iTEBD), for which the ground state is compatible with the protected symmetries of the Hamiltonian but cannot be reached using the conventional iDMRG or iTEBD algorithms. I describe a modified iDMRG algorithm which evades these local minima, and which also admits a natural interpretation on topologically ordered systems with a boundary.

Good transparent conducting oxides (TCOs), such as ${\mathrm{In}}_2{\mathrm{O}}_3$:Sn (ITO), usually combine large optical band gaps, essential for high transparency, with relatively small fundamental band gaps due to low conduction-band minima, which favor $n$-type doping and enhance the electrical conductivity. It has been understood that the optical band gaps are wider than the fundamental band gaps because optical transitions between the band-edge states are forbidden. The mechanism blocking such transitions, which can play a crucial role in the designing of alternative TCOs, nonetheless remains obscure. Here, based on first-principles density functional theory calculations and symmetry analysis of three oxides, $M_2{\mathrm{O}}_3$ $(M=\mathrm{Al},\mathrm{Ga},\mathrm{In})$, we identify the physical origin of the gap disparities. Three conditions are necessary: (1) the crystal structure must have global inversion symmetry; (2) in order to belong to the $A_g$ or $A_{1g}$ irreducible representations, the states at the conduction-band minimum must have cation and oxygen $s$ character; (3) in order to have $g$ parity, the oxygen $p$ orbitals constituting the states near the valence-band maximum must be strongly coupled to the cation $d$ orbitals. Under these conditions, optical excitations across the fundamental gap will be forbidden. The three criteria explain the trends in the $M_2{\mathrm{O}}_3$ $(M=\text{Al},\text{Ga},\text{In})$ sequence, in particular, explaining why ${\mathrm{In}}_2{\mathrm{O}}_3$ in the bixbyite structure yields the highest figure of merit. Our study provides guidelines expected to be instrumental in the search for new TCO materials.

We study the polarization properties of light emitted by quantum dots that are embedded in chiral photonic crystal structures made of achiral planar GaAs waveguides. A modification of the electromagnetic mode structure due to the chiral grating fabricated by partial etching of the waveguide layer has been shown to result in a high circular polarization degree ${\rho }_c$ of the quantum dot emission in the absence of external magnetic field. The physical nature of the phenomenon can be understood in terms of the reciprocity principle taking into account the structural symmetry. At the resonance wavelength, the magnitude of $|{\rho }_c|$ is predicted to exceed 98%. The experimentally achieved value of $|{\rho }_c|=81\%$ is smaller, which is due to the contribution of unpolarized light scattered by grating defects, thus breaking its periodicity. The achieved polarization degree estimated removing the unpolarized nonresonant background from the emission spectra can be estimated to be as high as 96%, close to the theoretical prediction.

A remarkably long-lived spin plasmon may exist in two-dimensional electron liquids with imbalanced spin-up and spin-down population. The predictions for this interesting mode by Agarwal et al. [Phys. Rev. B 90, 155409 (2014)] are based on the random phase approximation. Here, we show how to account for spin-dependent correlations from known ground-state pair correlation functions and study the consequences on the various spin-dependent longitudinal response functions. The spin-plasmon dispersion relation and its critical wave vector for Landau damping by minority spins turn out to be significantly lower. We further demonstrate that spin-dependent effective interactions imply a rich structure in the excitation spectrum of the partially spin-polarized system. Most notably, we find a “magnetic antiresonance,” where the imaginary part of both, the spin-spin as well as the density-spin response function vanish. The resulting minimum in the double-differential cross section is awaiting experimental confirmation.

Silicene, the two-dimensional (2D) allotrope of silicon, has very recently attracted a lot of attention. It has a structure that is similar to graphene and it is theoretically predicted to show the same kind of electronic properties which have put graphene into the focus of large research and development projects worldwide. In particular, a 2D structure made from Si is of high interest because of the application potential in Si-based electronic devices. However, so far there is not much known about the silicene band structure from experimental studies. A comprehensive study is here presented of the atomic and electronic structure of the silicene phase on Ag(111) denoted as ($2\surd 3\times 2\surd 3$)R30° in the literature. Low energy electron diffraction (LEED) shows an unconventional rotated (“$2\surd 3\times 2\surd 3$”) pattern with a complicated set of split diffraction spots. Scanning tunneling microscopy (STM) results reveal a Ag(111) surface that is homogeneously covered by the (“$2\surd 3\times 2\surd 3$”) silicene which exhibits an additional quasiperiodic long-range ordered superstructure. The complex structure, revealed by STM, has been investigated in detail and we present a consistent picture of the silicene structure based on both STM and LEED. The homogeneous coverage by the (“$2\surd 3\times 2\surd 3$”) silicene facilitated an angle-resolved photoelectron spectroscopy study which reveals a silicene band structure of unprecedented detail. Here we report four silicene bands which are compared to calculated dispersions based on a relaxed ($2\surd 3\times 2\surd 3$) model. We find good qualitative agreement between the experimentally observed bands and calculated silicene bands of σ character.

Molybdenum (Mo) is a body-centered-cubic (bcc) transition metal that has widespread technological applications. Although the bcc transition elements are used as test cases for understanding the behavior of metals under extreme conditions, the melting curves and phase transitions of these elements have been the subject of stark disagreements in recent years. Here we use x-ray diffraction to examine the phase stability and melting behavior of Mo under shock loading to 450 GPa. The bcc phase of Mo remains stable along the Hugoniot until 380 GPa. Our results do not support previous claims of a shallow melting curve for molybdenum.

A full understanding of the basic processes of grain boundary migration is a fundamental prerequisite for predictive models of microstructural evolution in polycrystalline materials in processing and in service. In a detailed study of the kinetics of a $\left[111\right] \mathrm{\Sigma }7$ symmetric tilt boundary, we have previously shown that defect-free, flat grain boundaries, below their roughening temperature, can be strictly immobile in the experimental limit. Here we present the results of molecular dynamics simulations of grain boundaries containing a variety of “defects.” These simulations show that the presence of some of these defects restores the mobility of flat boundaries, even well below the roughening transition temperature. These defects fundamentally alter the mesoscale mechanism of grain boundary migration from one involving homogeneous nucleation to a heterogenous process. At the atomistic level, the crystal lattice reorients via coordinated shuffling of groups of atoms. In the case of flat boundaries, these shuffles must accumulate to form critically stable nuclei, but in the case of boundaries containing defects the shuffling of a small number of atoms at the defects can be sufficient, fundamentally altering the mechanism and kinetics of migration.

Using photoemission electron microscopy combined with x-ray magnetic circular dichroism we show that a progressive spatial confinement of a ferromagnet (FM), either through thickness variation or laterally via patterning, actively controls the domains of uncompensated spins in the antiferromagnet (AF) in exchange-biased systems. Direct observations of the spin structure in both sides of the FM/AF interface in a model system, $\mathrm{Ni}/\mathrm{Fe}{\mathrm{F}}_2$, show that the spin structure is determined by the balance between the competing FM and AF magnetic energies. Coexistence of exchange bias domains, with opposite directions, can be established in $\mathrm{Ni}/\mathrm{Fe}{\mathrm{F}}_2$ bilayers for Ni thicknesses below 10 nm. Patterning the $\mathrm{Ni}/\mathrm{Fe}{\mathrm{F}}_2$ heterostructures with antidots destabilizes the FM state, enhancing the formation of opposite exchange bias domains below a critical antidot separation of the order of a few $\mathrm{Fe}{\mathrm{F}}_2$ crystal domains. The results suggest that dimensional confinement of the FM may be used to manipulate the AF spin structure in spintronic devices and ultrahigh-density information storage media. The underlying mechanism of the uncompensated AF domain formation in $\mathrm{Ni}/\mathrm{Fe}{\mathrm{F}}_2$ may be generic to other magnetic systems with complex noncollinear FM/AF spin structures.

We investigated the thermodynamic properties of the Fe-based lightly disordered superconductor ${\mathrm{Ba}}_{0.05}{\mathrm{K}}_{0.95}{\mathrm{Fe}}_2{\mathrm{As}}_2$ in external magnetic field $H$ applied along the FeAs layers ($H\left|\right|ab$ planes). The superconducting (SC) transition temperature for this doping level is $T_c=6.6$ K. Our analysis of the specific heat $C(T,H)$ measured for $T<T_c$ implies a sign change of the superconducting order parameter across different Fermi pockets. We provide experimental evidence for the three components superconducting order parameter. We find that all three components have values which are comparable with the previously reported ones for the stoichiometric compound ${\mathrm{KFe}}_2{\mathrm{As}}_2$. Our data for $C(T,H)$ and resistivity $\rho (T,H)$ can be interpreted in favor of the dominant orbital contribution to the pair-breaking mechanism at low fields, while Pauli limiting effect dominates at high fields, giving rise to a gapless superconducting state with only the leading nonzero gap.

The relationships among charge order, spin fluctuations, and superconductivity in underdoped cuprates remain controversial. We use neutron scattering techniques to study these phenomena in ${\mathrm{La}}_{1.93}{\mathrm{Sr}}_{0.07}{\mathrm{CuO}}_4$, a superconductor with a transition temperature of $T_c=20$ K. At $T\ll T_c$ we find incommensurate spin fluctuations with a quasielastic energy spectrum and no sign of a gap within the energy range from 0.2 to 15 meV. A weak elastic magnetic component grows below $\sim 10$ K, consistent with results from local probes. Regarding the atomic lattice, we have discovered unexpectedly strong fluctuations of the ${\mathrm{CuO}}_6$ octahedra about Cu-O bonds, which are associated with inequivalent O sites within the ${\mathrm{CuO}}_2$ planes. Furthermore, we observed a weak elastic $\left(3\overline{3}0\right)$ superlattice peak that implies a reduced lattice symmetry. The presence of inequivalent O sites rationalizes various pieces of evidence for charge stripe order in underdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. The coexistence of superconductivity with quasistatic spin-stripe order suggests the presence of intertwined orders; however, the rotation of the stripe orientation away from the Cu-O bonds might be connected with evidence for a finite gap at the nodal points of the superconducting gap function.

The stress-induced martensitic transition in a Cu-Zn-Al shape memory alloy has been studied under three different driving conditions by using (i) a hard driving device which imposes a displacement rate on the sample heads, (ii) a soft driving device which imposes a loading rate on the sample, and (iii) a mixed driving in which the sample is driven using a spring. We have measured the local strain of different segments in the sample as a function of the applied force. During the transition, we have identified two regimes: one associated with nucleation of martensitic needle domains and another mainly associated with front propagation. For the hard and mixed driving conditions, local strain exhibits great variability between segments that is not present under soft driving. In addition, we have analyzed the distribution of force fluctuations. Fluctuations are essentially Gaussian during front propagation regimes, whereas fluctuations display a clear non-Gaussian tail in the nucleation regime for nonsoft driving conditions. Harder driving results in a wider non-Gaussian tail.

The eigenvalues of the transmission matrix provide the basis for a full description of the statistics of steady-state transmission and conductance and for control of transmitted waves. However, the nature of pulsed transmission of the transmission eigenchannels and their spectral correlation, which would permit control of propagation in the time domain, has not yet been discussed. Here we report the dramatic variation of the dynamic properties of transmission with incident waveform. Computer simulations show that lower-transmission eigenchannels respond more promptly to an incident pulse and are correlated over a wide frequency range. We explain these results together with the puzzlingly large dynamic range of transmission eigenvalues in terms of the way the quasinormal modes of the medium combine to form specific transmission eigenchannels. Key factors are the closeness of the illuminating waves to resonance with the modes forming an eigenchannel and the strong anticorrelation of low-transmission eigenchannels with resonant modes. In addition, the spectral range of modes comprising the transmission eigenchannel increases with decreasing transmission, which leads to an additional substantial suppression of transmission due to destructive interference.

Disorder-assisted electron-phonon scattering processes (supercollision processes) have been reported to dominate the cooling of hot carriers in graphene. Here, we determine to what extent this type of relaxation mechanism governs the hot carrier dynamics in the parent compound graphite. Electron temperature transients derived from time- and angle-resolved extreme ultraviolet photoemission spectra are analyzed based on a three-temperature model which considers electron gas, optical phonons, and acoustic phonons as coupled subsystems. In the probed fluence regime of $0.035–1.4\mathrm{mJ}/{\mathrm{cm}}^2$, we find no indications for supercollision processes being involved in the cooling of the hot carriers. The data are, by contrast, compatible with a hot phonon assisted mechanism involving anharmonic coupling between optical phonons and acoustic phonons, a process which has previously been suggested for graphite. We attribute the striking difference to the reported findings for (substrate-supported) graphene to the low defect density of highly ordered pyrolitic graphite.

The ground-state phase diagram of a Heisenberg $\text{spin}-\frac{1}{2}$ system on a two-leg ladder with rung alternation is studied by combining analytical approaches with numerical simulations. For the case of ferromagnetic leg exchanges a unique ferrimagnetic ground state emerges, whereas for the case of antiferromagnetic leg exchanges several different ground states are stabilized depending on the ratio between exchanges along legs and rungs. For the more general case of a honeycomb-ladder model for the case of ferromagnetic leg exchanges besides the usual rung-singlet and saturated ferromagnetic states we obtain a ferrimagnetic Luttinger liquid phase with both linear and quadratic low-energy dispersions and ground-state magnetization continuously changing with system parameters. For the case of antiferromagnetic exchanges along legs, different dimerized states including states with additional topological order are suggested to be realized.

We consider analytically and numerically an anisotropic $\text{spin}-\frac{1}{2}$ delta chain (sawtooth chain) with nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic interactions. For certain values of the interactions a lowest one-particle band becomes flat and there is a class of localized-magnon eigenstates which form a ground state with a macroscopic degeneracy. In this case the model depends on a single parameter which can be chosen as the anisotropy of the exchange interactions. When this parameter changes from zero to infinity the model interpolates between the one-dimensional isotropic ferromagnet and the frustrated Ising model on the delta chain. It is shown that the low-temperature thermodynamic properties in these limiting cases are governed by the specific structure of the excitation spectrum. In particular, the specific heat has one or infinite number of low-temperature maxima for the small or the large anisotropy parameter, correspondingly. Qualitative features of such behavior survive when the interaction parameters deviate from the relations providing the local magnon ground state.

We present evidence for the existence of a spontaneous instability towards an orbital loop-current phase in a multiorbital Hubbard model for the ${\mathrm{CuO}}_2$ planes in cuprates. Contrary to the previously proposed ${\theta }_{II}$ phase with intra-unit cell currents, the identified instability is towards a staggered pattern of intertwined current loops. The orbitally resolved current pattern thereby shares its staggered character with the proposal of $d$-density wave order. The current pattern will cause a Fermi surface reconstruction and the opening of a pseudogap. We argue that the pseudogap phase with time-reversal symmetry breaking currents is susceptible to further phase transitions and therefore offers a route to account for axial incommensurate charge order and a polar Kerr effect in underdoped cuprates.

We propose a theory of longitudinal resistivity in the normal phase of quasi-one-dimensional organic superconductors near their quantum critical point where antiferromagnetism borders with superconductivity under pressure. The linearized semiclassical Boltzmann equation is solved numerically, fed in by the half-filling electronic umklapp scattering vertex as derived from one-loop renormalization-group calculations for the quasi-one-dimensional electron-gas model. The momentum and temperature dependence of umklapp scattering has an important impact on spin fluctuations and on the behavior of longitudinal resistivity in the the normal phase. Resistivity is found to be linear in temperature around the quantum critical point at which spin-density-wave order joins superconductivity along the antinesting axis, to gradually evolve towards the Fermi-liquid behavior in the limit of weak superconductivity. A critical analysis of the predictions is made from a comparison with experiments performed on the ${\left(\mathrm{TMTSF}\right)}_2{\mathrm{PF}}_6$ member of the Bechgaard salt series under pressure. Fair agreement between theory and experiment is then found in the low-temperature range linked to quantum criticality while deviations from predictions become apparent at high temperature.

The transition from weakly damped coherent motion to localization in the context of the spin-boson model has been the subject of numerous studies with distinct behavior depending on the form of the phonon-bath spectral density $J\left(\omega \right)\propto {\omega }^s$. Sub-Ohmic ($s<1$) and Ohmic ($s=1$) spectral densities show a clear localization transition at zero temperature and zero bias, while for super-Ohmic ($s>1$) spectral densities this transition disappears. In this paper, we consider the influence of the phonon-bath spectral density on the nonequilibrium dynamics of a quantum dot with electron-phonon interactions described by the extended Holstein model. Using the reduced density matrix formalism combined with the multilayer multiconfiguration time-dependent Hartree approach, we investigate the dynamic response, the time scales for relaxation, as well as the existence of multiple long-lived solutions as the system-bath coupling changes from the sub- to the super-Ohmic cases. Bistability is shown to diminish for increasing powers of $s$ similar to the spin-boson case. However, the physical mechanism and the dependence on the model parameters such as the typical bath frequency ${\omega }_c$ and the polaron shift $\lambda$ are rather distinct.