We study the spin relaxation of charge carriers in graphene in the presence of spin-polarized charged impurities by calculating the time evolution of an initially polarized state. The spin relaxation time shows completely different energy behavior for short-ranged and long-ranged spin scatterers and can be used to identify the dominant source of spin scattering. Our results agree well with recent experimental findings and indicate that their spin relaxation is likely caused by long-ranged scatterers.

The density-matrix-renormalization-group method and the Hartree-Fock approximation with the charge-density-wave instability are used to study a formation and condensation of excitonic bound states in the generalized Falicov-Kimball model. In particular, we examine effects of various factors, such as the $f\text{-electron}$ hopping, the local and nonlocal hybridizations, as well as the increasing dimension of the system on the excitonic momentum distribution $N\left(q\right)$ and especially on the number of zero-momentum excitons $N_0=N(q=0)$ in the condensate. It is found that the negative values of the $f\text{-electron}$ hopping integrals $t_f$ support the formation of a zero-momentum condensate, whereas the positive values of $t_f$ have the fully opposite effect. The opposite effects on the formation of the condensate exhibit also the local and nonlocal hybridizations. The first one strongly supports the formation of the condensate, whereas the second one destroys it completely. Moreover, it was shown that the zero-momentum condensate remains robust with increasing dimension of the system.

The realization of synthetic gauge fields has attracted a lot of attention recently in relation to periodically driven systems and the Floquet theory. In ultracold atom systems in optical lattices and photonic networks, this allows one to simulate exotic phases of matter such as quantum Hall phases, anomalous quantum Hall phases, and analogs of topological insulators. In this paper, we apply the Floquet theory to engineer anisotropic Haldane models on the honeycomb lattice and two-leg ladder systems. We show that these anisotropic Haldane models still possess a topologically nontrivial band structure associated with chiral edge modes. Focusing on (interacting) boson systems in $s$-wave bands of the lattice, we show how to engineer through the Floquet theory, a quantum phase transition (QPT) between a uniform superfluid and a Bose-Einstein condensate analog of Fulde-Ferrell-Larkin-Ovchinnikov states, where bosons condense at nonzero wave vectors. We perform a Ginzburg-Landau analysis of the QPT on the graphene lattice, and compute observables such as chiral currents and the momentum distribution. The results are supported by exact diagonalization calculations and compared with those of the isotropic situation. The validity of high-frequency expansion in the Floquet theory is also tested using time-dependent simulations for various parameters of the model. Last, we show that the anisotropic choice for the effective vector potential allows a bosonization approach in equivalent ladder (strip) geometries.

We investigate the cluster size convergence of the energy and observables using two forms of density matrix embedding theory (DMET): the original cluster form (CDMET) and a new formulation motivated by the dynamical cluster approximation (DCA-DMET). Both methods are applied to the half-filled one- and two-dimensional Hubbard models using a sign-problem free auxiliary-field quantum Monte Carlo impurity solver, which allows for the treatment of large impurity clusters of up to 100 sites. While CDMET is more accurate at smaller impurity cluster sizes, DCA-DMET exhibits faster asymptotic convergence towards the thermodynamic limit. We use our two formulations to produce new accurate estimates for the energy and local moment of the two-dimensional Hubbard model for $U/t=2,4,6$. These results compare favorably with the best data available in the literature, and help resolve earlier uncertainties in the moment for $U/t=2$.

The coefficient $\alpha$ of the Rashba spin-orbit interaction is calculated in an asymmetric quantum well consisting of ${\mathrm{Ga}}_{0.47}{\mathrm{In}}_{0.53}\mathrm{As}$ (well), ${\mathrm{Al}}_{0.48}{\mathrm{In}}_{0.52}\mathrm{As}$ (left barrier), and ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{\mathrm{As}}_y{\mathrm{Sb}}_{1-y}$ (right barrier) as a function of the external electric field perpendicular to the well $E_z^{\mathrm{ex}}$ which is controlled by the gate voltage. This coefficient $\alpha$, which depends on the band offset, can be tuned to be zero by adjusting the Al fraction $x$ in the right barrier layer to the optimum value $x_0$ in the case where the wave function vanishes at the left heterointerface. Such a composition-adjusted asymmetric quantum well is proposed as a structure in which the magnitude of $\alpha$ can be switched by changing the polarity of $E_z^{\mathrm{ex}}$. The calculation shows that, when $|x-x_0|<0.01$, the on/off $\left|\alpha \right|$ ratio $>40$ for a large enough $|E_z^{\mathrm{ex}}|$ ($|E_z^{\mathrm{ex}}|>{10}^7$ V/m for a well width of 20 nm), which results in the on/off spin-relaxation-rate ratio exceeding ${10}^3$ in the Dyakonov-Perel mechanism.

We study the electronic and magnetic structures of quasi-one-dimensional interfaces along the zigzag direction in two-dimensional GaN/SiC heterostructures using first-principles calculations. Four representative heterostructures with six inequivalent interfaces are discussed in detail. Our results indicate that a bulk electric field will develop only when both interfaces feature no gap states and the total net charge at interfaces are of opposite sign. All the geometries studied exhibit an intriguing quasi-one-dimensional conductor character, of which three show finite nonzero magnetic moment. Furthermore, the magnetic moment in one of the systems can be tuned by applying an electric field along the normal direction of monolayer indicating a strong magnetoelectric coupling. Our analysis shows that the magnetization at the interfaces is closely related to the density of states at the Fermi level due to the Stoner instability, and the ribbon-width-dependent magnetization for different geometries implies the existence of an in-bulk electric field.

Formation and growth of CsBr clusters embedded in unsupported Ar clusters was studied using synchrotron radiation photoelectron spectroscopy. The development of the core-level electronic structure for cluster sizes between a few and a few hundred atoms contained information about the local coordination of the constituent particles. The experimental results indicate that a gradual structural phase transition from NaCl structure to CsCl structure for CsBr clusters takes place at around 160 atoms per cluster.

It is well known that a closed loop of magnetic dipoles can give rise to the rather elusive toroidal moment. However, artificial structures required to generate the necessary magnetic moments in metamaterials are typically optically large, complex to make, and easily compromised by the kinetic inductance at high frequencies. Instead of using magnetic dipoles, we propose a minimal model based on just three aligned discrete electric dipoles in which the occurrence of resonant toroidal modes is guaranteed by symmetry. The advantage of this model is its simplicity and the same model supports toroidal moments from the microwave regime up to optical frequencies as exemplified by a three-antenna array and a system consisting of three nanosized plasmonic particles. Both the microwave and high-frequency configurations exhibit nonradiating “anapoles.” Experiments in the microwave regime confirm the theoretical predictions.

Based on ab initio density functional calculations, we studied the stability and electronic properties of two-dimensional indium iodide (InI). The calculated results show that monolayer and few-layer InI can be as stable as its bulk counterpart. The stability of the monolayer structure is further supported by examining the electronic and dynamic stability. The interlayer interaction is found to be fairly weak ($\sim 160$ meV/atom) and mechanical exfoliation to obtain monolayer and few-layer structures will be applicable. A direct band gap of 1.88 eV of the bulk structure is obtained from the hybrid functional method, and is comparable to the experimental one ($\sim 2.00$ eV). The electronic structure can be tuned by layer stacking and external strain. The size of the gap is a linear function of an inverse number of layers, suggesting that we can design few-layer structures for optoelectronic applications in the visible optical range. In-plane tensile or hydrostatic compressive stress is found to be useful not only in varying the gap size to cover the whole visible optical range, but also in inducing a semiconductor-metal transition with an experimentally accessible stress. The present result strongly supports the strategy of broadening the scope of group-V semiconductors by looking for isoelectronic III-VII atomic-layered materials.

The damping $\left(\mathrm{\Gamma }a\right)$ of the transverse acoustic (TA) phonon in single crystals of the relaxor $\mathrm{KT}{\mathrm{a}}_{1-x}\mathrm{N}{\mathrm{b}}_x{\mathrm{O}}_3$ with $x=0.15–0.17$ was studied by means of high resolution inelastic cold neutron scattering near the (200) Brillouin Zone (BZ) point where diffuse scattering is absent, although it is present near (110). In a wide range of temperatures centered on the phase transition, $T=195\mathrm{K}÷108\mathrm{K}$, the TA phonon width (damping) exhibits a step increase around momentum $q=0.07$, goes through a shallow maximum at $q=0.09–0.12$, and remains high above and up to the highest momentum studied of $q=0.16$. These experimental results are explained in terms of a resonant interaction between the TA phonon and the collective or correlated reorientation through tunneling of the off-center $\mathrm{N}{\mathrm{b}}^{+5}$ ions. The observed TA damping is successfully reproduced in a simple model that includes an interaction between the TA phonon and a dispersionless localized mode (LM) with frequency ${\omega }_L$ and damping ${\mathrm{\Gamma }}_L({\mathrm{\Gamma }}_L<{\omega }_L)$, itself coupled to the transverse optic (TO) mode. Maximum damping of the TA phonon occurs when its frequency is ${\omega }_a\approx {\omega }_L$. The values of ${\omega }_L$ and ${\mathrm{\Gamma }}_L$ are moderately dependent on temperature, but the oscillator strength, $M^2$, of the resonant damping exhibits a strong maximum in the range $T\sim 120\mathrm{K}÷150\mathrm{K}$ in which neutron diffuse scattering near the (110) BZ point is also maximum and the dielectric susceptibility exhibits the relaxor behavior. The maximum value of $M$ appears to be due to the increasing number of polar nanodomains. In support of the proposed model, the observed value of ${\omega }_L\approx 0.7\mathrm{THz}$ is found to be similar to the estimate previously obtained by Girshberg and Yacoby [J. Phys.: Condens. Matter 24, 015901 (2012)]. Alternatively, the TA phonon damping can be successfully fitted in the framework of an empirical Havriliak-Negami (HN) relaxation model that includes a strong resonancelike transient contribution.

In the morphotropic phase boundary region where tetragonal and rhombohedral phases coexist in the ferroelectric solid solution ${\mathrm{PbZr}}_{1-x}{\mathrm{Ti}}_x{\mathrm{O}}_3$(PZT), large strains can be induced at the interface due to the lattice misfit of the two structures. We show that for bulk PZT the misfit strains between tetragonal and rhombohedral phases can lead to an adaptive monoclinic structure in the morphotropic phase boundary (MPB) region, similar to the effects of misfit strains between a crystal and substrate in epitaxial ferroelectric thin films. We use Landau theory to sixth order in polarization to provide insight into factors controlling the occurrence of the monoclinic phase in the MPB region.

We are able to detect clear signatures of dephasing—a distinct trait of many-body localization (MBL)—via the dynamics of two-site entanglement, quantified through the concurrence. Using the protocol implemented by M. Schreiber et al. [Science 349, 842 (2015)], we show that in the MBL phase the average two-site entanglement decays in time as a power law, while in the Anderson localized phase it tends to a plateau. The power-law exponent is not universal and displays a clear dependence on the interaction strength. This behavior is also qualitatively different from the ergodic phase, where the two-site entanglement decays exponentially. All the results are obtained by means of time-dependent density matrix renormalization-group simulations and further corroborated by analytical calculations on an effective model. Two-site entanglement has been measured in cold atoms: our analysis paves the way for the first direct experimental test of many-body dephasing in the MBL phase.

In this paper the charge density wave (CDW) and magnetic ordering for the high-pressure phase of ${\mathrm{TbGe}}_{2.85}$ with the ${\mathrm{AuCu}}_3$ structure have been studied by means of electrical resistivity and neutron powder diffraction at pressures up to 3 and 5 GPa, respectively. The electrical resistivity measurements showed that the charge density wave transition temperature $T_{\mathrm{CDW}}$ decreases with increasing the external pressure and the CDW transition in ${\mathrm{TbGe}}_{2.85}$ is suppressed at $P\ge 2.6\mathrm{GPa}$. The Néel temperature $T_N$ is approximately independent of the pressure. The neutron powder diffraction at low temperatures and high pressures reveals the appearance of the second magnetic commensurate phase at $P=1.2\mathrm{GPa}$ with wave-vector $\mathbf{k}=(1/2,0,0)$. The calculation of the electronic structure of the ${\mathrm{TbGe}}_3$ (${\mathrm{AuCu}}_3$-type structure) compound was also performed. The high value of the electronic states at the Fermi level confirms the instabilities of the stoichiometric compound ${\mathrm{TbGe}}_3$.

The ab initio electronic structure calculations of the ${\mathrm{Ni}}_2\mathrm{MnGa}$ alloy indicate that the orthorhombic 4O structure exhibits the lowest energy compared to all known martensitic structures. The 4O structure is formed by nanotwin double layers, i.e., oppositely oriented nanotwins consisting of two (101) lattice planes of nonmodulated martensitic structure. It exhibits the lowest occupation of density of states at the Fermi level. The total energy 1.98 meV/atom below the energy of nonmodulated martensite is achieved within structural relaxation by shifting Mn and Ga atoms at the nanotwin boundaries. The same atomic shift can also be found in other martensitic nanotwinned or modulated structures such as 10M and 14M, which indicates the importance of the nanotwin double layer for the stability of these structures. Our discovery shows that the nanotwinning or modulation is a natural property of low-temperature martensitic phases in Ni-Mn-Ga alloys.

Ordering and disordering processes in complex oxides strongly influence their physicochemical properties when they are submitted to severe conditions, such as high temperature, high pressure, or irradiation. This paper examines the role played by the local atomic order on the structural stability of ${\mathrm{Y}}_2\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7$ pyrochlore submitted to ion irradiation by combining experimental and atomistic computation studies. X-ray absorption spectroscopy at the Ti K edge, molecular dynamics simulations, and calculations using a tight-binding variable-charge model show that the short-range order around Ti atoms in ${\mathrm{Y}}_2\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7$ is strongly modified by irradiation. Strong local distortions around Ti defects occur due to a decrease of the Ti coordination number. These local atomic rearrangements trigger the overall amorphization of the compound. These results show that the local short-range order influences the long-range structural stability of complex oxides, thus providing a key feature for the control of the functional properties of these materials.

Aluminum oxide hydroxide, AlOOH, is a prototypical hydrous mineral in the geonomy. The study of the high-pressure phase evolution of AlOOH is of fundamental importance in helping to understand the role of hydrous minerals in the water storage and transport in Earth, as in other planets. Here, we have systematically investigated the high-pressure phase diagram of AlOOH up to 550 GPa using the efficient crystal structure analysis by particle swarm optimization (CALYPSO) algorithm in conjunction with first principles calculations. We predict a peculiar monoclinic phase (space group $P{2}_1/c$, 16 atoms/cell, $Z=4$ ) as the most stable phase for AlOOH above 340 GPa. The occurrence of this new phase results in the breakup of symmetric linear O-H-O hydrogen bonds into asymmetric, bent O-H-O linkages and in sevenfold coordinated metal cations. The new $P{2}_1/c$ phase turns out to be a universal high-pressure phase in group 13 oxide hydroxides, and stable for both compressed GaOOH and InOOH. The formation of the new phase in all compounds is favored by volume reduction due to denser packing.

We investigate nonadiabatic effects for the vibrational stretch mode of the CO molecule adsorbed on the top site of the Cu(100) surface. By studying the long-wavelength ($\mathbf{q}\approx 0$) imaginary and real parts of the density functional theory based phonon self-energy due to the electron-phonon coupling ${\mathrm{\Pi }}_{\lambda }$ we obtain the phonon linewidth and the frequency renormalization of the CO stretch mode, respectively. To simulate electronic scattering processes that lead to further damping of the phonon modes we include a phenomenological damping in the phonon self-energy, as well as in the single-electron spectral function that enters ${\mathrm{\Pi }}_{\lambda }$, through the momentum distribution function. For the specific case of electron-impurity scattering we explicitly show how this process opens the indirect intraband channel and broadens the linewidth of the CO stretch mode. To emphasize the importance of accounting for electronic scattering processes we compare the phonon linewidths in the clean noninteracting limit (infinite electron lifetime) and when electronic scattering processes are phenomenologically included (finite electron lifetime) with available experimental data. We find that the agreement with experiments is improved in the latter case.

The transfer of conserved charges through insulating matter via smooth deformations of the Hamiltonian is known as quantum adiabatic, or Thouless, pumping. Central to this phenomenon are Hamiltonians whose insulating gap is controlled by a multidimensional (usually two-dimensional) parameter space in which paths can be defined for adiabatic changes in the Hamiltonian, i.e., without closing the gap. Here, we extend the concept of Thouless pumps of band insulators by considering a larger, three-dimensional parameter space. We show that the connectivity of this parameter space is crucial for defining quantum pumps, demonstrating that, as opposed to the conventional two-dimensional case, pumped quantities depend not only on the initial and final points of Hamiltonian evolution but also on the class of the chosen path and preserved symmetries. As such, we distinguish the scenarios of closed/open paths of Hamiltonian evolution, finding that different closed cycles can lead to the pumping of different quantum numbers, and that different open paths may point to distinct scenarios for surface physics. As explicit examples, we consider models similar to simple models used to describe topological insulators, but with doubled degrees of freedom compared to a minimal topological insulator model. The extra fermionic flavors from doubling allow for extra gapping terms/adiabatic parameters—besides the usual topological mass which preserves the topology-protecting discrete symmetries—generating an enlarged adiabatic parameter space. We consider cases in one and three spatial dimensions, and our results in three dimensions may be realized in the context of crystalline topological insulators, as we briefly discuss.

Interstitial boron-related defects in silicon subjected to irradiation with 5 MeV electrons at a temperature of 80 K are investigated by Fourier-transform infrared absorption spectroscopy. This study demonstrates the radiation-enhanced annealing of interstitial boron during irradiation. We have revealed the interaction, which occurs in the course of irradiation, of diffusing interstitial boron atoms with one another and with interstitial oxygen. The local vibrational modes associated with these defects are identified, and the thermal stability of the defects is determined.