The dephasing mechanisms of $p$-shell and $s$-shell excitons in an InAs self-assembled quantum dot ensemble are examined using two-dimensional coherent spectroscopy (2DCS). 2DCS provides a comprehensive picture of how the energy level structure of dots affects the exciton dephasing rates and recombination lifetimes. We find that at low temperatures, dephasing of $s$-shell excitons is lifetime limited, whereas $p$-shell excitons exhibit significant pure dephasing due to scattering between degenerate spin states. At elevated temperatures, quadratic exciton-phonon coupling plays an important role in both $s$-shell and $p$-shell exciton dephasing. We show that multiple $p$-shell states are also responsible for stronger phonon dephasing for these transitions.

We study both experimentally and theoretically the fundamental interplay of exciton localization and polarization in semiconducting single-walled carbon nanotubes. From Stark spectroscopy of individual carbon nanotubes at cryogenic temperatures, we identify localized excitons as permanent electric dipoles with dipole moments of up to $1e\AA$. Moreover, we demonstrate field-effect doping of localized excitons with an additional charge which results in defect-localized trions. Our findings, in qualitative agreement with theoretical calculations, not only provide fundamental insight into the microscopic nature of localized excitons in carbon nanotubes, they also signify their potential for sensing applications and may serve as guidelines for molecular engineering of exciton-localizing quantum dots in other atomically thin semiconductors including transition metal dichalcogenides.

We present a linear response calculation for twisted bilayer graphene. The calculation is performed for both the continuum and tight-binding models, with the aim of assessing the validity of the former. All qualitatively important features previously reported by us [Stauber et al., Phys. Rev. Lett. 120, 046801 (2018)] for the Drude matrix in the continuum model are also present in the tight-binding calculation, with increasing quantitative agreement for decreasing twist angle. These features include the chiral longitudinal magnetic moment associated with plasmonic modes, and the anomalous counterflow around the neutrality point, better interpreted as a paramagnetic response. We have addressed the differences between Drude and equilibrium response, and we showed that orbital paramagnetism is the equilibrium response to a parallel magnetic field over a substantial doping region around the neutrality point. Chirality also causes the equilibrium response to exhibit a nontrivial current structure associated with the nonvertical character of interlayer bonds in the tight-binding calculation.

The list of two-dimensional Dirac systems with a tilt in their Dirac cone spectrum is expanding, and now, in addition to the organic system $\alpha {(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{I}}_3$, it includes the two-dimensional $8Pmmn$-borophene sheet, which allows for controlled doping by the gate voltage. We analytically calculate the polarization function of tilted Dirac cone for an arbitrary tilt parameter, $0\le \eta <1$, and arbitrary doping. This enables us to find two interesting plasmonic effects solely caused by the tilt. (i) In addition to the standard plasmon oscillations, a strong enough tilt induces an additional linearly dispersing overdamped branch of plasmons, which is strongly Landau damped due to overlap with a large density of intraband free particle-hole (PH) excitations. (ii) There appears a kink in the plasmon dispersion for any nonzero tilt parameter. The kink appears when the plasmon branch enters the interband continuum of PH excitations. This kink becomes most manifest for wave vectors perpendicular to the tilt direction and fades away by approaching the tilt direction. Experimental measurements of the wave vector and energy of the plasmonic kink, when combined with our analytic formula for the kink energy scale, allow for a direct experimental estimation of the tilt parameter. Furthermore, we find that, for a fixed chemical potential $\mu$, the screening is significantly enhanced by bringing the tilt $\eta$ close to 1.

Spin-orbit (SO) Mott insulators are regarded as a new paradigm of magnetic materials, whose properties are largely influenced by SO coupling and featured by highly anisotropic bond-dependent exchange interactions between the spin-orbital entangled Kramers doublets, as typically manifested in $5d$ iridates. Here, we propose that a very similar situation can be realized in cuprates when the ${\mathrm{Cu}}^{2+}$ ions reside in a tetrahedral environment, as in spinel compounds. Using first-principles electronic structure calculations, we construct a realistic model for the diamond lattice of the ${\mathrm{Cu}}^{2+}$ ions in ${\mathrm{CuAl}}_2{\mathrm{O}}_4$ and show that the magnetic properties of this compound are largely controlled by anisotropic compass-type exchange interactions that dramatically modify the magnetic ground state by lifting the spiral spin-liquid degeneracy and stabilizing a commensurate single-$\mathit{q}$ spiral.

Raman and infrared spectroscopies are used as local probes to study the dynamics of the Nd-O bonds in the weakly multiferroic ${\mathrm{NdMn}}_2{\mathrm{O}}_5$ system. The temperature dependence of selected Raman excitations reveals the splitting of the Nd-O bonds in ${\mathrm{NdMn}}_2{\mathrm{O}}_5$. The ${\mathrm{Nd}}^{3+}$ ion crystal field (CF) excitations in ${\mathrm{NdMn}}_2{\mathrm{O}}_5$ single crystals are studied by infrared transmission as a function of temperature, in the $1800-8000{\mathrm{cm}}^{-1}$ range, and under an applied magnetic field up to 11 T. The frequencies of all ${}^4{I}_j$ CF levels of ${\mathrm{Nd}}^{3+}$ are determined. We find that the degeneracy of the ground-state Kramers doublet is lifted (${\mathrm{\Delta }}_0\sim 7.5{\mathrm{cm}}^{-1}$) due to the ${\mathrm{Nd}}^{3+}\text{−}{\mathrm{Mn}}^{3+}$ interaction in the ferroelectric phase, below $T_C\sim 28\mathrm{K}$. The ${\mathrm{Nd}}^{3+}$ magnetic moment $m_{\mathrm{Nd}}\left(T\right)$ and its contribution to the magnetic susceptibility and the specific heat are evaluated from ${\mathrm{\Delta }}_0\left(T\right)$ indicating that the ${\mathrm{Nd}}^{3+}$ ions are involved in the magnetic and the ferroelectric ordering observed below $\sim 28\mathrm{K}$. The Zeeman splitting of the excited CF levels of the ${\mathrm{Nd}}^{3+}$ ions at low temperature is also analyzed.

We present an approach for carrying out nonadiabatic molecular dynamics simulations of systems in which nonadiabatic transitions arise from the coupling between the classical atomic motions and a quasicontinuum of electronic quantum states. Such conditions occur in many research areas, including chemistry at metal surfaces, radiation damage of materials, and warm-dense-matter physics. The classical atomic motions are governed by stochastic Langevin-like equations, while the quantum electron dynamics is described by a master equation for the populations of the electronic states. These working equations are obtained from a first-principles derivation. Remarkably, unlike the widely used Ehrenfest and surface-hopping methods, the approach naturally satisfies the principle of detailed balance at equilibrium and therefore can describe the evolution to thermal equilibrium from an arbitrary initial state. A practical algorithm is cast in the form of the widely used fewest-switches surface-hopping algorithm but with switching probabilities that are not specified ad hoc like in the standard algorithm but are instead derived.

Lead-based perovskite relaxor ferroelectrics are widely used as materials for numerous applications due to their extraordinary dielectric, piezoelectric, and electrostrictive properties. While the mechanisms of relaxor behavior are disputable, the importance of quenched (static) random electric fields created at nanoscale by the disordered heterovalent cations has been well recognized. Meanwhile, an increasing amount of scientific and technological efforts has been concentrated on lead-free perovskites, in particular, solid solutions of classical ferroelectric $\mathrm{BaTi}{\mathrm{O}}_3$ (BT), which better meet ecological requirements. Among BT-based solutions the homovalent systems are elaborately studied where strong random electric fields are absent, while the solubility limit of heterovalent solutions is typically too low to fully reveal the peculiarities of relaxor behavior. In this paper, we prepare a perovskite solid solution system $\left(1-x\right)\mathrm{B}{\mathrm{a}}^{2+}\mathrm{T}{\mathrm{i}}^{4+}{\mathrm{O}}_3-x\mathrm{D}{\mathrm{y}}^{3+}\mathrm{F}{\mathrm{e}}^{3+}{\mathrm{O}}_3$ ($0\le x\le 0.3$) and study it as a model heterovalent lead-free system. We determine crystal structure, ferroelectric, and dielectric properties of ceramics in a wide range of temperatures and concentrations, construct a phase diagram, and find and analyze the concentration-induced crossover from normal ferroelectric to relaxor behavior. We demonstrate that quenched random electric fields of moderate strength promote the ferroelectric-to-relaxor crossover, but do not change qualitatively the peculiarities of relaxor behavior, while strong enough fields destroy the relaxor state, so that the material becomes an ordinary linear dielectric. The experimental results are compared with the predictions of known theories of relaxor ferroelectricity.

Refrigeration systems based on the compression of greenhouse gases are environmentally threatening and cannot be scaled down to on-chip dimensions. In the vicinity of a phase transition, caloric materials present large thermal responses to external fields, which makes them promising for developing alternative solid-state cooling devices. Electrocaloric effects are particularly well suited for portable refrigeration applications; however, most electrocaloric materials operate best at nonambient temperatures or require the application of large electric fields. Here, we predict that modest electric fields can yield giant room-temperature electrocaloric effects in multiferroic ${\mathrm{BiCoO}}_3$ (BCO) thin films. Depending on the orientation of the applied field, the resulting electrocaloric effect is either direct (heating) or inverse (cooling), which may enable the design of enhanced refrigeration cycles. We show that spin-phonon couplings and phase competition are the underlying causes of the disclosed caloric phenomena. The dual electrocaloric response of BCO thin films can be effectively tuned by means of epitaxial strain, and we anticipate that other control strategies such as chemical substitution are also possible.

Quenched randomness can lead to robust nonequilibrium phases of matter in periodically driven (Floquet) systems. Analyzing transitions between such dynamical phases requires a method capable of treating the twin complexities of disorder and discrete time-translation symmetry. We introduce a real-space renormalization group approach, asymptotically exact in the strong-disorder limit, and exemplify its use on the periodically driven interacting quantum Ising model. We analyze the universal physics near the critical lines and multicritical point of this model, and demonstrate the robustness of our results to the inclusion of weak interactions.

Intermediate band semiconductors hold the promise to significantly improve the efficiency of solar cells but only if the intermediate impurity band is metallic. We apply a recently developed first principles method to investigate the origin of electron localization in Ti doped Si, a promising candidate for intermediate band solar cells. We compute the critical Ti concentration and compare it against the available experimental data. Although Anderson localization is often overlooked in the context of intermediate band solar cells, our results show that in Ti doped Si it plays a more important role in the metal insulator transition than Mott localization. To this end we have devised a way to gauge the relative strengths of these two localization mechanisms that can be applied to study localization in doped semiconductors in general. Our findings have important implications for the theory of intermediate band solar cells.

The Ising antiferromagnet on a face-centered cubic (fcc) lattice with nearest-neighbor interaction only is well known to exhibit a macroscopic (exponential in the system size $L$) ground-state degeneracy. With increasing temperature, this degeneracy is expected to be lifted and the model undergoes a first-order phase transition. For a model with an exponential degeneracy in the whole low-temperature phase, it was recently found that the finite-size scaling behavior is governed by leading correction terms $\sim L^{-2}$ instead of $\sim L^{-3}$ as usual. To test the conjecture that such a transmuted behavior may effectively persist also for the fcc antiferromagnet up to some crossover system size, we have performed parallel multicanonical Monte Carlo simulations for lattices of linear size $L\le 18$ with periodic boundary conditions and determined various inverse pseudo phase transition temperatures, as well as the extremal values of the specific heat and the energetic Binder parameter. We indeed find that, for the simulated lattice sizes, the conjectured transmuted finite-size scaling ansatz fits the data better than the standard ansatz. On this basis, we extrapolate for the transition temperature an estimate of $T_0=1.735047\left(46\right)$.

We present measurements of pure spin current absorption on lateral spin valves. By varying the width of the absorber we demonstrate that spin current absorption measurements enable one to characterize efficiently the spin transport properties of ferromagnetic elements. The analytical model used to describe the measurement takes into account the polarization of the absorber. The analysis of the measurements thus allows the determination of the polarization and the spin diffusion length of a studied material independently, contrary to most experiments based on lateral spin valves where those values are entangled. We report the spin transport parameters of some of the most important materials used in spinorbitronics ($\mathrm{C}{\mathrm{o}}_{60}\mathrm{F}{\mathrm{e}}_{40}$, $\mathrm{N}{\mathrm{i}}_{81}\mathrm{F}{\mathrm{e}}_{19}$, Co, Pt, and Ta), at room and low (10 K) temperatures.

Magnetic excitations in a heavily electron-doped antiferromagnet, ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$, have been investigated using powder inelastic neutron scattering. Unlike other parent compounds of the iron-based superconductors, the magnetic excitation gap in ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$ was not detected down to the lowest measured temperature of 4 K. This result can be understood as a result of quasi-isotropy within the $ab$ plane, which is consistent with the band calculation result that the $d_{xy}$ orbital plays the dominant role in the magnetism of ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{H}}_{0.5}$. In addition, the intensities of the magnetic excitations in this phase are much stronger than those in nondoped LaFeAsO. Even in the paramagnetic phase, the magnetic excitation in ${\mathrm{LaFeAsO}}_{0.5}{\mathrm{D}}_{0.5}$ persists. These results corroborate recent studies showing that the electron doping enhances the localized nature in this system.

The density of low-energy particle-hole excitations is nonanalytic in a singular Fermi liquid, but it is altered on entering a superconducting state in which, in the pure limit, it vanishes asymptotically at the chemical potential and in general is analytic. The single-particle excitations in the superconducting states are then quasiparticles so that a form of Landau theory may be constructed for thermodynamic and transport properties in the superconducting state. In this theory, the renormalization of measurable properties due to quasiparticle interactions, such as specific heat, compressibility, magnetic susceptibility, superfluid density, etc., changes in a temperature dependent fashion from the noninteracting theory. This is illustrated by showing the renormalization of these quantities and the relation between the parameters introduced to account for their temperature dependence. When the renormalizations in the normal state are large or singular, temperature dependence of properties in the superconducting states are then in general not useful for identifying the nodal character or symmetry of the superconducting state except for measurements at very low temperatures, the upper limits of which are specified. The results obtained are expected to be useful in interpreting the experimental results for the temperature dependence of various properties in the superconducting state born of singular Fermi liquids.

Parity control of superconducting islands hosting Majorana zero modes (MZMs) is required to operate topological qubits made from proximitized semiconductor nanowires. We test this control by studying parity effects in hybrid InAs-Al single-Cooper-pair transistors (SCPTs) to evaluate the feasibility of this material system. In particular, we investigate the gate-charge modulation of the supercurrent and observe a consistent $2e$-periodic pattern indicating a general lack of low-energy subgap states in these nanowires at zero magnetic field. In a parallel magnetic field, an even-odd pattern develops with a gate-charge spacing that oscillates as a function of field demonstrating that the modulation pattern is sensitive to the presence of a single bound state. In addition, we find that the parity lifetime of the SCPT decreases exponentially with magnetic field as the bound state approaches zero energy. Our work shows that aluminum is the preferred superconductor for future topological qubit experiments and highlights the important role that quasiparticle traps and superconducting gap engineering would play in these qubits. Moreover, we demonstrate a means by which bound states can be detected in devices with superconducting leads.

We report experimental measurements of critical disorder in weakly disordered, one-dimensional photonic crystals. We measure the configurationally averaged transmission at various degrees of weak disorder. We extract the density of states (DoS) after fitting the transmission with theoretical profiles, and identify the Lifshitz tail realized by weak disorder. We observe the vanishing of Van Hove singularities and the flattening of the DoS with increasing disorder in our system. Systematic variation of disorder strength allows us to study the behavior of Lifshitz exponent with the degree of disorder. This provides a direct handle to the critical disorder in the one-dimensional crystal, at which the transport behavior of the system is known to change. The contradictory behavior at very weak disorder in the DoS variation at the band edge and the midgap are seen to resolve into synchronous behavior beyond the critical disorder. The experimentally measured transmission is shown to carry a clear signature of the critical disorder, which is in very good agreement with the theoretically expected disorder.

We report on the high-temperature evolution of far- and mid-infrared reflectivity and emissivity spectra of ambient orthorhombic $\mathrm{ErMn}{\mathrm{O}}_3$ from 12 K to sample decomposition above 1800 K. At low temperatures the number of phonons agrees with the predictions for orthorhombic space group ${D_{2h}}^{16}\text{−}Pbnm$ ($Z=4$) and coexists with a paramagnon spin resonance and rare-earth crystal-field transitions. Increasing the temperature, a number of vibrational bands undergo profile broadening and softening approaching the orbital disordered phase where the orthorhombic O´ lower-temperature cooperative phase coexists with cubic-orthorhombic O. $\mathrm{O}\text{−}\mathrm{ErMn}{\mathrm{O}}_3$ undergoes a first-order order-disorder transition into the perovskite cubic phase at $T_{\mathrm{cubic}}\sim 1329\mathrm{K}\pm 20\mathrm{K}$ where the three triple degenerate phonons allowed by the space group $Pm\text{−}3m$ ($Z=1$) are identified. At about 800 K, a quantitative small polaron analysis of the orthorhombic midinfrared real part optical conductivity shows that antisymmetric and symmetric breathing modes sustain the strongest electron-phonon interactions. Above $T_{\mathrm{cubic}}$ the bipolaron fingerprint profile is the midinfrared dominant and only feature. Its appearance correlates with the localized screening of the highest vibrational mode reststrahlen band. We propose that the longitudinal optical mode macroscopic field screening is a consequence of dynamically sharing δ disproportioned $e_g$ electrons hovering over the Jahn-Teller distorted octahedral dimer ${\left\{\mathrm{Mn}{\left(Q_{\mathrm{JT}}\right)}^{3+\delta }\left[\mathrm{Mn}\left(Q_{\mathrm{JT}}\right){\left[3\right]}^{-\delta }\right]{\mathrm{O}}_{6/2}\right\}}_2$. A thermal driven insulator-metal transition is detected with onset ∼1600 K. We also address the occurrence of an inhomogeneity induced terahertz band result of heating the samples in dry air, triggering $\mathrm{M}{\mathrm{n}}^{3+}\text{−}\mathrm{M}{\mathrm{n}}^{4+}$ double exchange, under the presence of $\mathrm{M}{\mathrm{n}}^{4+}$ smaller ions stabilizing the orthorhombic lattice.

We characterize asymmetric growth of magnetic bubble domains in perpendicularly magnetized Co/Ni multilayers grown on ${\mathrm{Pt}}_x{\mathrm{Ir}}_{1-x}$ seed layers by application of perpendicular and in-plane magnetic fields. Using a refined model of domain wall creep that incorporates contributions from the anisotropic elastic energy, $\varepsilon$, and a chirality-dependent prefactor, $v_0$, we elucidate factors that govern the mobility of Dzyaloshinskii domain walls as a function of seed-layer composition. The interfacial Dzyaloshinskii-Moriya interaction (DMI) magnitude is found to decrease monotonically with $x_{Ir}$, which is independently confirmed by Brillouin light scattering. Moreover, the persistence of significant asymmetry in velocity curves across the full composition range supports previous assertions that a chirality-dependent attempt frequency akin to chiral damping could play a critical role in the observed trends. This work helps resolve fundamental questions about the factors governing Dzyaloshinskii domain-wall creep and demonstrates varying Pt-Ir seed-layer composition as a method to tune DMI.