We used continuous wave photoluminescence (cw-PL) and time-resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the $\mathrm{ZnTe}/(\mathrm{Zn},\mathrm{Mn})\mathrm{Se}$ system the holes are confined in the nonmagnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic (Zn,Mn)Se matrix. On the other hand, in the $(\mathrm{Zn},\mathrm{Mn})\mathrm{Te}/\mathrm{ZnSe}$ system, the holes are confined in the magnetic (Zn,Mn)Te QDs, while the electrons remain in the surrounding nonmagnetic ZnSe matrix. The magnetic polaron formation energies $E_{\mathrm{MP}}$ in both systems were measured from the temporal redshift of the band-edge emission. The magnetic polaron exhibits distinct characteristics depending on the location of the Mn ions. In the $\mathrm{ZnTe}/(\mathrm{Zn},\mathrm{Mn})\mathrm{Se}$ system the magnetic polaron shows conventional behavior with $E_{\mathrm{MP}}$ decreasing with increasing temperature $T$ and increasing magnetic field $B$. In contrast, $E_{\mathrm{MP}}$ in the $(\mathrm{Zn},\mathrm{Mn})\mathrm{Te}/\mathrm{ZnSe}$ system has unconventional dependence on temperature $T$ and magnetic field $B$; $E_{\mathrm{MP}}$ is weakly dependent on $T$ as well as on $B$. We discuss a possible origin for such a striking difference in the MP properties in two closely related QD systems.

Solving the initial value problem for semiclassical equations that describe two-dimensional electrons with the Dirac spectrum, we found that collective excitations of the electrons are composed of a few distinct components of the oscillations. There always exist sustained plasma oscillations with the well-known plasmon frequency ${\omega }_{\text{pl}}\left(k\right)$. Additionally, there are oscillations with a “carrier frequency” $\omega =v_{F}k$ slowly decaying in time, according to a power law ($v_F$ and $k$ are the Fermi velocity and wave vector). The reason for the onset of these oscillations has a fundamental character related to the branching of the polarization function of the Dirac electrons. A strongly anisotropic initial disturbance of the electron distribution generates an additional component of the undamped oscillations in the form of an electron unidirectional beam, which are van Kampen's modes in the Dirac plasma.

We present a procedure for the separation of the intrinsic, side-jump, and skew-scattering contributions to the spin Hall conductivity within the ab initio Kubo-Středa approach. Furthermore, two distinct contributions to the side-jump mechanism, either independent of the vertex corrections or solely caused by them, are quantified as well. This allows for a detailed analysis of individual microscopic contributions to the spin Hall effect. The efficiency of the proposed method is demonstrated by a first-principles study of dilute metallic alloys based on Cu, Au, and Pt hosts.

We study the fate of double-Weyl fermions in three-dimensional systems in the presence of long-range Coulomb interactions. By employing the momentum-shell renormalization-group approach, we find that the fixed point of noninteracting double-Weyl fermions is unstable against Coulomb interactions and flows to a nontrivial stable fixed point with anisotropic screening effect. Moreover, experimentally measurable quantities such as specific heat obtain unusual logarithmic corrections. We also discuss implications of our results to three-dimensional materials ${\mathrm{HgCr}}_2{\mathrm{Se}}_4$ and ${\mathrm{SrSi}}_2$, candidate materials of hosting double-Weyl fermions.

We study lithium systems over a range of a number of atoms, specifically atomic anion, dimer, metallic cluster, and body-centered-cubic crystal, using the fixed-node diffusion Monte Carlo method. The focus is on analysis of the fixed-node errors of each system, and for that purpose we test several orbital sets in order to provide the most accurate nodal hypersurfaces. The calculations include both core and valence electrons in order to avoid any possible impact by pseudopotentials. To quantify the fixed-node errors, we compare our results to other highly accurate calculations, and wherever available, to experimental observations. The results for these Li systems show that the fixed-node diffusion Monte Carlo method achieves accurate total energies, recovers $96–99\%$ of the correlation energy, and estimates binding energies with errors bounded by $0.1 \mathrm{eV}/\mathrm{at}$.

We investigate the spectroscopic fingerprints of electron-plasmon coupling in integrated (PES) and angle-resolved photoemission spectroscopy (ARPES). To account for electron-plasmon interactions at a reduced computational cost, we derived the plasmonic polaron model, an approach based on the cumulant expansion of many-body perturbation theory that circumvents the calculation of the $GW$ self-energy. Through the plasmonic polaron model, we predict the complete spectral functions and the effects of electron-plasmon coupling for Si, Ge, GaAs, and diamond. Si, Ge, and GaAs exhibit well-defined plasmonic polaron band structures, i.e., broadened replica of the valence bands redshifted by the plasmon energy. Based on these results, (i) we assign the structures of the plasmon satellite of silicon (as revealed by PES experiments) to plasmonic Van Hove singularities occurring at the $L$, $\mathrm{\Omega }$, and $X$ high-symmetry points and (ii) we predict the ARPES signatures of electron-plasmon coupling for Si, Ge, GaAs, and diamond. Overall, the concept of plasmonic polaron bands emerges as a new paradigm for the interpretation of electronic processes in condensed matter, and the theoretical approach presented here provides a computationally affordable tool to explore its effect in a broad set of materials.

Anisotropy in properties of materials is important in materials science and solid-state physics. Measurement of the full resistivity tensor of crystals using the standard four-point method with bar shaped samples requires many measurements and may be inaccurate due to misalignment of the bars along crystallographic directions. Here an approach to extracting the resistivity tensor using van der Pauw measurements is presented. This reduces the number of required measurements. The theory of the van der Pauw method is extended to extract the tensor from parallelogram shaped samples with known geometry. Methods to extract the tensor for both known and unknown principal axis orientation are presented for broad applicability to single crystals. Numerical simulations of errors are presented to quantify error sources. Several benchmark experiments are performed on isotropic graphite samples to verify the internal consistency of the developed theory, test experimental precision, and characterize error sources. The presented methods are applied to a $\mathrm{RuS}{\mathrm{b}}_2$ single crystal at room temperature and the results are discussed based on the error source analysis. Temperature resolved resistivities along the $a$ and $b$ directions are finally reported and briefly discussed.

We investigate the low-energy electronic transport across grain boundaries in graphene ribbons and infinite flakes employing two distinct techniques. Using the recursive Green's-function method, we compute the electronic transmittance across different types of grain boundaries in graphene ribbons and flakes. We use the charge and current density spatial distributions to enhance our understanding of their electronic transport properties. We find that electronic transport depends both on the grain boundaries' microscopic details and on their orientation. In addition, we employ the transfer-matrix formalism to analytically study the electronic transport across a class of zigzag grain boundaries with periodicity 3. We find that these grain boundaries give rise to intervalley scattering.

It was shown recently [J. L. Lado and J. Fernández-Rossier, Phys. Rev. Lett. 113, 027203 (2014)] that edge magnetic moments in graphene-like nanoribbons are strongly influenced by the intrinsic spin-orbit interaction. Due to this interaction an anisotropy comes about which makes the in-plane arrangement of magnetic moments energetically more favorable than that corresponding to the out-of-plane configuration. In this paper we raise both the edge magnetism problem and the differential conductance and shot noise Fano factor issues, in the context of finite-size flakes within the Coulomb blockade (CB) transport regime. Our findings elucidate the following problems: (i) modification of CB diamonds by the appearance of in-plane magnetic moments and (ii) modification of CB diamonds by the intrinsic spin-orbit interaction.

In this work we performed 2.5-dimensional (2.5D) dislocation dynamics simulations coupling climb with the glide dislocation motion to model the creep behavior of olivine, one of the main component of the Earth's upper mantle. In particular, we present an application of this method to determine the creep strain rate in a material with high lattice resistance, such as olivine. We show that by including the climb mechanism we reach steady state creep conditions. Moreover, we find that a creep power law with a stress exponent close to 3 can be extracted from our simulations and we provide a model based on Orowan's law to predict the creep strain rates in the high temperature and low stress regime. The model presented is relevant to describe the plastic flow of olivine in the Earth's mantle deformation conditions and can be useful to derive the high temperature creep behavior of other materials.

The two-dimensional polymer structure and lattice dynamics of the superionic conductor ${\mathrm{Li}}_4{\mathrm{C}}_{60}$ are investigated by neutron diffraction and spectroscopy. The peculiar bonding architecture of this compound is confirmed through the precise localization of the carbon atoms involved in the intermolecular bonds. The spectral features of this phase are revealed through a combination of ab initio lattice dynamics calculations and inelastic neutron scattering experiments. The neutron scattering observables are found to be in very good agreement with the simulations which predict a partial charge transfer from the Li atoms to the ${\mathrm{C}}_{60}$ cage. The absence of a well-defined band associated with the Li atoms in the experimental spectrum suggests that this species is not ordered even at the lowest temperatures. The calculations predict an unstable Li sublattice at a temperature of $\sim 200$ K, which we relate to the large ionic diffusivity of this system: Low-frequency optic modes of the Li ions couple to the soft structure of the polymer.

Hybrid quantum systems combining circuit QED with spin-doped solids are an attractive platform for distributed quantum information processing. There, the magnetic ions serve as coherent memory elements and reversible conversion elements of microwave to optical qubits. Among many possible spin-doped solids, erbium ions offer the unique opportunity for a coherent conversion of microwave photons into the telecom C band at $1.54\mu \mathrm{m}$ employed for long distance communication. In our work, we perform a time-resolved electron spin resonance study of an ${\mathrm{Er}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ spin ensemble at millidegrees Kelvin temperatures and demonstrate multimode storage and retrieval of up to 16 coherent microwave pulses. The memory efficiency is measured to be ${10}^{-4}$ at a coherence time of $T_2=5.6\mu \mathrm{s}$. We observe a saturation of the spin coherence time below 50 mK due to full polarization of the surrounding electronic spin bath.

Magnetic properties of the $S=\frac{1}{2}$ antiferromagnet $\alpha \text{−}{\mathrm{Cu}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ have been studied using magnetization, quantum Monte Carlo (QMC) simulations, and neutron diffraction. Magnetic susceptibility shows a broad peak at $\sim 50\mathrm{K}$ followed by an abrupt increase indicative of a phase transition to a magnetically ordered state at $T_N=33.4\left(1\right)$ K. Above $T_N$, a fit to the Curie-Weiss law gives a Curie-Weiss temperature of $\mathrm{\Theta }=-73\left(1\right)$ K suggesting the dominant antiferromagnetic coupling. The result of the QMC calculations on the helical-honeycomb spin network with two antiferromagnetic exchange interactions $J_1$ and $J_2$ provides a better fit to the susceptibility than the previously proposed spin-chain model. Two sets of the coupling parameters $J_1:J_2=1:0.45$ with $J_1=5.79\left(1\right)$ meV and $0.65:1$ with $J_2=6.31\left(1\right)$ meV yield equally good fits down to $\sim T_N$. Below $T_N$, weak ferromagnetism due to spin canting is observed. The canting is caused by the Dzyaloshinskii-Moriya interaction with an estimated $bc$-plane component $\left|D_p\right|\simeq 0.14J_1$. Neutron diffraction reveals that the $S=\frac{1}{2}{\mathrm{Cu}}^{2+}$ spins antiferromagnetically align in the $F{d}^{\prime }{d}^{\prime }2$ magnetic space group. The ordered moment of 0.93(9) ${\mu }_B$ is predominantly along the crystallographic $a$ axis.

With sintered needles aligned and a magnetic field applied transverse to its easy axis, the rare-earth ferromagnet ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ becomes a room-temperature realization of the random field Ising model. The transverse field tunes the pinning potential of the magnetic domains in a continuous fashion. We study the magnetic domain reversal and avalanche dynamics between liquid helium and room temperatures at a series of transverse fields using a Barkhausen noise technique. The avalanche size and energy distributions follow power-law behavior with a cutoff dependent on the pinning strength dialed in by the transverse field, consistent with theoretical predictions for Barkhausen avalanches in disordered materials. A scaling analysis reveals two regimes of behavior: one at low temperature and high transverse field, where the dynamics are governed by the randomness, and the second at high temperature and low transverse field, where thermal fluctuations dominate the dynamics.

Massless Dirac fermions occur as low-energy modes in several quasi-two-dimensional condensed matter systems such as graphene, the surface of bulk topological insulators, and in layered organic semiconductors. When the rotational symmetry in such systems is reduced either by an in-plane electric field or an intrinsic tilt of the Dirac cones, the allowed dipolar optical transitions evolve from a few selected transitions into a wide fan of interband transitions. We show that the Lorentz covariance of the low-energy carriers allows for a concise analysis of the emerging magneto-optical properties. We predict that infrared absorption spectra yield quantitative information on the tilted Dirac cone structure in organic compounds such as $\alpha -{\left(\text{BEDT-TTF}\right)}_2{\mathrm{I}}_3$.

Superfluidity, which is the ability of a liquid or gas to flow with zero viscosity, is one of the most remarkable implications of collective quantum coherence. In equilibrium systems such as liquid ${}^4\mathrm{He}$ and ultracold atomic gases, superfluid behavior conjugates diverse yet related phenomena, such as a persistent metastable flow in multiply connected geometries and the existence of a critical velocity for frictionless flow when hitting a static defect. The link between these different aspects of superfluid behavior is far less clear in driven-dissipative systems displaying collective coherence, such as microcavity polaritons, which raises important questions about their concurrency. With a joint theoretical and experimental study, we show that the scenario is particularly rich for polaritons driven in a three-fluid collective coherent regime, i.e., a so-called optical parametric oscillator. On the one hand, the spontaneous macroscopic coherence following the phase locking of the signal and idler fluids has been shown to be responsible for their simultaneous quantized flow metastability. On the other hand, we show here that the pump, signal, and idler have distinct responses when hitting a static defect; while the signal displays modulations that are barely perceptible, the ones appearing in the pump and idler are determined by their mutual coupling due to nonlinear and parametric processes.

A first-principles, excited-state analysis is carried out to identify ways of producing silicon quantum dots with low excitonic reorganization energy. These focus on the general strategy of either reducing or constraining exciton-phonon coupling, and four approaches are explored. The results can be implemented in quantum dot solids to mitigate polaronic effects and increase the lifetime of coherent excitonic superpositions. It is demonstrated that such designs can also be used to alter the shape of the spectral density for reorganization so as to reduce the rates of both decoherence and dissipation. The results suggest that it may be possible to design quantum dot solids that support partially coherent exciton transport.

Orbital interaction plays an important role in topological insulators. Using first-principles calculations, we demonstrate that hydrogenation can change two-dimensional (2D) trivial insulator/semimetal-like Pb, Mo, and W to a $Z_2$ topological insulator with giant gaps by filtering unwanted orbitals such as the $p_z$ orbital in Pb, and the $d_{3z^2-r^2}$ in Mo and W. For PbH, the large intrinsic spin-orbit coupling (SOC) confined in the in-plane orbitals $p_{x,y}$ results in a bulk gap of 1.07 eV. For the case of MoH and WH, hydrogenation results in a novel $s{d}^4$ hybridization with a Dirac cone formed by the out-of-plane orbital $d_{yz/xz}$ orbitals. Furthermore, due to the electron-electron interaction and the strong SOC in the $4d$ and $5d$ elements, the bulk band gap is significantly enhanced with a value of 1.28 eV for MoH and 0.30 eV for WH. The strong correlation effect also induces several topological phase transitions in WH like the Dirac semimetal and Mott insulating phase, while MoH remains a topological insulator even with large correlation effect. We propose that these new strongly correlated 2D materials can be realized experimentally by using substrate such as the boron nitride sheet, which can exhibit various novel properties such as the Dirac semimetal phase with large spin-orbit energy splitting as well as the quantum spin Hall property.

The Kondo zero-bias anomaly of Co adatoms probed by scanning tunneling microscopy is known to depend on the height of the tip above the surface, and this dependence is different on different low index Cu surfaces. On the (100) surface, the Kondo temperature first decreases then increases as the tip approaches the adatom, while on the (111) surface it is virtually unaffected. These trends are captured by combined density functional theory and numerical renormalization-group calculations. The adatoms are found to be described by an $S=1$ Anderson model on both surfaces, and ab initio calculations help identify the symmetry of the active $d$ orbitals. We correctly reproduce the Fano line shape of the zero-bias anomaly for Co/Cu(100) in the tunneling regime but not in the contact regime, where it is probably dependent on the details of the tip and contact geometry. The line shape for Co/Cu(111) is presumably affected by the presence of surface states, which are not included in our method. We also discuss the role of symmetry, which is preserved in our model scattering geometry but most likely broken in experimental conditions.

Quantum walks, whose dynamics is prescribed by alternating unitary coin and shift operators, possess topological phases akin to those of Floquet topological insulators, driven by a time-periodic field. While there is ample theoretical work on topological phases of quantum walks where the coin operators are spin rotations, in experiments a different coin, the Hadamard operator, is often used instead. This was the case in a recent photonic quantum walk experiment, where protected edge states were observed between two bulks whose topological invariants, as calculated by the standard theory, were the same. This hints at a hidden topological invariant in the Hadamard quantum walk. We establish a relation between the Hadamard and the spin rotation operator, which allows us to apply the recently developed theory of topological phases of quantum walks to the one-dimensional Hadamard quantum walk. The topological invariants we derive account for the edge state observed in the experiment; we thus reveal the hidden topological invariant of the one-dimensional Hadamard quantum walk.