In the present work, we were able to identify and characterize a source of in-plane optical anisotropies (IOAs) occurring in asymmetric double quantum wells (DQWs), namely a reduction of the symmetry from $D_{2d}$ to $C_{2v}$ as imposed by asymmetry along the growth direction. We report on reflectance anisotropy spectroscopy (RAS) of double GaAs quantum well structures coupled by a thin ($<2$ nm) tunneling barrier. Two groups of DQW systems were studied: one where both QWs have the same thickness (symmetric DQW) and another where they have different thicknesses (asymmetric DQW). RAS measures the IOAs arising from the intermixing of the heavy and light holes in the valence band when the symmetry of the DQW system is lowered from $D_{2d}$ to $C_{2v}$. If the DQW is symmetric, residual IOAs stem from the asymmetry of the QW interfaces, e.g., that associated with Ga segregation into the AlGaAs layer during the epitaxial growth process. In the case of an asymmetric DQW with QWs with different thicknesses, the AlGaAs layers (that are sources of anisotropies) are not distributed symmetrically at both sides of the tunneling barrier. Thus the system loses its inversion symmetry, yielding an increase in the RAS strength. The RAS line shapes were compared with reflectance spectra in order to assess the heavy- and light-hole mixing induced by the symmetry breakdown. The energies of the optical transitions were calculated by numerically solving the one-dimensional Schrödinger equation using a finite-difference method. Our results are useful for interpretation of the transitions occurring in both symmetric and asymmetric DQWs.

The electron-phonon interaction is fundamental to many physical and chemical processes. The ab initio computation based on modern many-body Green's function (MBGF) theory can be used to accurately calculate the properties of matters including the electron-nucleus coupling of bulk materials and nanocrystals. Yet it is hard to find reports of computed electron-phonon interaction and phonon-assisted dynamics of electrons and excitons in nanocrystals using MBGF theory without any input of semiempirical parameters. We establish a computationally executable approach for modeling electron-phonon and exciton-phonon interaction under the framework of the $GW$ Bethe-Salpeter equation in semiconductor nanoclusters. We computed both nonradiative relaxation and inelastic scattering of electrons and excitons, and uncovered that both single-phonon relaxation and multiple-phonon relaxation are significant for nonradiative relaxation of electrons and excitons in nanocrystal of ${\mathrm{Si}}_{46}$, and the two relaxations correspond to two types of physical processes that have totally different spectral line shapes, respectively. The inelastic scattering decay is a primary decay mechanism for multiexciton relaxation. The computed single-phonon and multiphonon nonradiative relaxation rates in ${\mathrm{Si}}_{46}$ are between 1 and 1000 fs for different excitonic energies, which is consistent with experimental results reported for similar nanocrystals.

The drift of electron spin helices in an external in-plane electric field in GaAs quantum wells is studied by means of time-resolved magneto-optical Kerr microscopy. The evolution of the spin distribution measured for different excitation powers reveals that, for short delay times and higher excitation powers, the spin helix drift slows down while its envelope becomes anisotropic. The effect is understood as a local decrease of the electron gas mobility due to electron collisions with nonequilibrium holes within the excitation spot and is reproduced well in the kinetic theory framework. For larger delay times, when the electrons constituting the spin helix and nonequilibrium holes are separated by an electric field, the spin helix drift accelerates and the mobility reaches its unperturbed value again.

Electronic correlations are crucial to the low-energy physics of metallic systems with localized $d$ and $f$ states; however, their effect on band insulators and semiconductors is typically negligible. Here, we measure the electronic structure of the half-Heusler compound FeVSb, a band insulator with a filled shell configuration of 18 valence electrons per formula unit ($s^2{p}^6{d}^{10}$). Angle-resolved photoemission spectroscopy reveals a mass renormalization of $m^{*}/m_{\text{bare}}=1.4$, where $m^{*}$ is the measured effective mass and $m_{\text{bare}}$ is the mass from density functional theory calculations with no added on-site Coulomb repulsion. Our measurements are in quantitative agreement with dynamical mean-field theory calculations, highlighting the many-body origin of the mass renormalization. This mass renormalization lies in dramatic contrast to other filled shell intermetallics, including the thermoelectric materials CoTiSb and NiTiSn, and has a similar origin to that in FeSi, where Hund's coupling induced fluctuations across the gap can explain a dynamical self-energy and correlations. Our work calls for a rethinking of the role of correlations and Hund's coupling in intermetallic band insulators.

The topological and thermoelectric properties of ${\left(\mathrm{Eu}\mathrm{O}\right)}_n/{\left(\mathrm{Mg}\mathrm{O}\right)}_m$(001) superlattices (SLs) are explored using density functional theory calculations including a Hubbard $U$ term together with Boltzmann transport theory. In ${\left(\mathrm{Eu}\mathrm{O}\right)}_1/{\left(\mathrm{Mg}\mathrm{O}\right)}_3$(001) SL at the lattice constant of MgO a sizable band gap of 0.51 eV is opened by spin-orbit coupling (SOC) due to a band inversion between occupied localized Eu $4f$ and empty $5d$ conduction states. This inversion between bands of opposite parity is accompanied by a reorientation in the spin texture along the contour of band inversion surrounding the $\mathrm{\Gamma }$ point and leads to a Chern insulator with $C$ = –1, also confirmed by the single edge state. Moreover, this Chern insulating phase shows promising thermoelectric properties, e.g., a Seebeck coefficient between 400 and $800\mu {\mathrm{VK}}^{-1}$. A similar SOC-induced band inversion takes place also in the ferromagnetic semimetallic ${\left(\mathrm{Eu}\mathrm{O}\right)}_2/{\left(\mathrm{Mg}\mathrm{O}\right)}_2$(001) SL. Despite the vanishing band gap, it leads to a substantial anomalous Hall conductivity with values up to –1.04 $e^2/h$ and somewhat lower Seebeck coefficient. Both cases emphasize the relation between nontrivial topological bands and thermoelectricity also in systems with broken inversion symmetry.

We theoretically investigate the Floquet generation of a second-order topological superconducting (SOTSC) phase hosting Majorana corner modes (MCMs), considering a quantum spin Hall insulator with a proximity-induced superconducting $s$-wave pairing in it. Our dynamical prescription consists of the periodic kick in time-reversal symmetry breaking the in-plane magnetic field and fourfold rotational symmetry breaking the mass term in the bulk while these Floquet MCMs are preserved by antiunitary particle-hole symmetry. The first driving protocol always leads to four zero-energy MCMs (i.e., one Majorana state per corner) as a sign of a strong SOTSC phase. Interestingly, the second protocol can result in a weak SOTSC phase, harboring eight zero-energy MCMs (two Majorana states per corner), in addition to the strong SOTSC phase. We characterize the topological nature of these phases by a Floquet quadrupolar moment and Floquet Wannier spectrum. We believe that relying on the recent experimental advancement in the driven systems and proximity - superconductivity, our schemes may be possible to test in the future.

We resolved some controversial issues on the Bloch-Grüneisen (BG) temperature in doped graphene via analytical and numerical calculations based on full inelastic electron-acoustic-phonon (EAP) scattering rate and various approximation schemes. Analytic results for BG temperature obtained by semi-inelastic (SI) approximation (which gives scattering rates in excellent agreement with the full inelastic scattering rates) are compared with those obtained by quasielastic (QE) approximation and the commonly adopted value of ${\mathrm{\Theta }}_F^{\text{LA}}=2\hslash v_{\text{LA}}{k}_F/k_B$. It is found that the commonly adopted BG temperature in graphene (${\mathrm{\Theta }}_F^{\text{LA}}$) is about 5 times larger than the value obtained by the QE approximation and about 2.5 times larger than that by the SI approximation, when using the crossing-point temperature where low-temperature and high-temperature limits of the resistivity ($\rho$) meet. The corrected analytic relation based on SI approximation agrees extremely well with the transition temperatures determined by fitting the low- and high-$T$ behavior of available experimental data of graphene's resistivity. Using the analytic expression of ${\mathrm{\Theta }}_{\text{BG}}$ we can prove that the normalized resistivity defined as $R=\rho \left(T\right)/\rho \left({\mathrm{\Theta }}_{\text{BG}}\right)$ plotted as a function of $(T/{\mathrm{\Theta }}_{\text{BG}})$ is independent of the carrier density. We also introduced a way to determine the BG temperature including the full inelastic EAP scattering rate when the deviation of electron energy from the chemical potential is taken into account by finding the maximum of $\partial \rho /\partial T$.

Due to a strong Coulomb interaction, excitons dominate the excitation kinetics in two-dimensional (2D) materials. While Coulomb scattering between electrons has been well studied, the interaction of excitons is more challenging and remains to be explored. As neutral composite bosons consisting of electrons and holes, excitons show nontrivial scattering dynamics. Here, we study exciton-exciton interaction in transition-metal dichalcogenides and related van der Waals heterostructures on microscopic footing. We demonstrate that the crucial criterion for efficient scattering is a large electron/hole mass asymmetry, giving rise to internal charge inhomogeneities of excitons and emphasizing their cobosonic substructure. Furthermore, both exchange and direct exciton-exciton interactions are boosted by enhanced exciton Bohr radii. We also predict an unexpected temperature dependence that is usually associated with phonon-driven scattering, and we reveal an orders of magnitude stronger interaction of interlayer excitons due to their permanent dipole moment. The developed approach can be generalized to arbitrary material systems and will help to study strongly correlated exciton systems, such as moire super lattices.

The ferrimagnetic spinel ${\mathrm{MnCr}}_2{\mathrm{S}}_4$ shows a variety of magnetic-field-induced phase transitions owing to bond frustration and strong spin-lattice coupling. However, the site-resolved magnetic properties at the respective field-induced phases in high magnetic fields remain elusive. Our soft x-ray magnetic circular dichroism studies up to 40 T directly evidence element-selective magnetic-moment reorientations in the field-induced phases. The complex magnetic structures are further supported by entropy changes extracted from magnetocaloric-effect measurements. Moreover, thermodynamic experiments reveal an unusual tetracritical point in the $H\text{−}T$ phase diagram of ${\mathrm{MnCr}}_2{\mathrm{S}}_4$ due to strong spin-lattice coupling.

Quantum magnets display a wide variety of collective excitations, including spin waves (magnons), coherent singlet-triplet excitations (triplons), and pairs of fractional spins (spinons). These modes differ radically in nature and properties, and in all conventional analyses any given material is interpreted in terms of only one type. We report inelastic neutron scattering measurements on the spin-1/2 antiferromagnet $\mathrm{Se}\mathrm{Cu}{\mathrm{O}}_3$, which demonstrate that this compound exhibits all three primary types of spin excitation. ${\mathrm{Cu}}_1$ sites form strongly bound dimers while ${\mathrm{Cu}}_2$ sites form a network of spin chains, whose weak three-dimensional (3D) coupling induces antiferromagnetic order. We perform quantitative modeling to extract all of the relevant magnetic interactions and show that magnons of the ${\mathrm{Cu}}_2$ system give a lower bound to the spinon continua, while the ${\mathrm{Cu}}_1$ system hosts a band of high-energy triplons at the same time as frustrating the 3D network.

When nanometric, noncoplanar spin textures with scalar spin chirality (SSC) are coupled to itinerant electrons, they endow the quasiparticle wave functions with a gauge field, termed Berry curvature, in a way that bears analogy to relativistic spin-orbit coupling (SOC). The resulting deflection of moving charge carriers is termed the geometrical (or topological) Hall effect. Previous experimental studies modeled this signal as a real-space motion of wave packets under the influence of a quantum-mechanical phase. In contrast, we here compare the modification of Bloch waves themselves and of their energy dispersion due to SOC and SSC. Using the canted pyrochlore ferromagnet ${\mathrm{Nd}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ as a model compound, our transport experiments and first-principles calculations show that SOC impartially mixes electronic bands with equal or opposite spin, while SSC is much more effective for opposite-spin band pairs.

Noncentrosymmetric superconductors have recently attracted much attention, since the lack of inversion symmetry mixes spin-singlet and -triplet pairing states, which may allow the realization of topological superconductivity. In this Letter, we study the electronic properties of the family of inversion-broken $\mathrm{Ce}T{X}_3$ heavy-fermion superconductors, finding topological nodal lines as well as Dirac and Weyl points, which are renormalized closer to the Fermi energy by correlations. We find that the Weyl nodal lines have a substantial effect on the Fermi surface spin structure of the normal state and lead to line nodes in the superconducting phase.

We performed a point contact spectroscopy study in an intermediate-valence heavy-fermion (HF) compound $\mathrm{Eu}{\mathrm{Ni}}_2{\mathrm{P}}_2$. Above the Kondo temperature $T_{\mathrm{K}}$, the differential conductance spectra show a broad peak due to the Kondo resonance at the Eu site. Below $T_{\text{K}}$, the broad peak splits into two peaks that can be reproduced by the summation of two Fano curves with the same parameters, indicating the emergence of the indirect hybridization gap in an Anderson lattice. With decreasing temperature, the separation between the two peaks increases and saturates at very low temperatures. The results reveal the evolution of the electronic structure in the HF state due to the development of the lattice coherence.

When a system is thermally coupled to only a small part of a larger bath, statistical fluctuations of the temperature (more precisely, the internal energy) of this “sub-bath” around the mean temperature defined by the larger bath can become significant. We show that these temperature fluctuations generally give rise to $1/f$-like noise power spectral density from even a single two-level system. We extend these results to a distribution of fluctuators, finding the corresponding modification to the Dutta-Horn relation. Then we consider the specific situation of charge noise in silicon quantum dot qubits and show that recent experimental data [E. J. Connors et al., Phys. Rev. B 100, 165305 (2019)] can be modeled as arising from as few as two two-level fluctuators, and accounting for sub-bath size improves the quality of the fit.

Barium zirconate ($\mathrm{BaZr}{\mathrm{O}}_3$) has attracted increasing attention due to its distinguished dielectric property and high chemical stability. As one kind of perovskite oxide, $\mathrm{BaZr}{\mathrm{O}}_3$ often exhibits a delicate interplay of lattice distortions and strain. In this work, the effect of strain on the ferroelectric and antiferrodistortive distortions in $\mathrm{BaZr}{\mathrm{O}}_3$ thin films is investigated at finite temperature by using density functional theory calculations and the first-principles based effective Hamiltonian method. It is found that due to the delicate balance between antiferrodistortive and ferroelectric distortions, polarization in $\mathrm{BaZr}{\mathrm{O}}_3$ thin films at absolute zero kelvin can be activated only under an in-plane tensile strain larger than 3.0%. The rotation of oxygen octahedra (antiferrodistortive distortion) is along the out-of-plane direction under the compressive strain and small tensile strain, whereas it changes to the in-plane direction when the tensile strain is larger than 0.3%. When the tensile strain increases to 4.1%, both rotation of oxygen octahedra and polarization are along the [110] direction. Furthermore, the temperature dependence of ferroelectric and antiferrodistortive distortions under different strains is investigated by using an effective Hamiltonian model. The strain-temperature phase diagram is calculated to identify the different distortions under different strains and temperatures.

Absorption and scattering of the impinging electromagnetic waves are the two fundamental operations describing the energy exchange of any organic or inorganic particle with its environment. In the case of virion cells, substantial extinction power (counting both absorbing and scattering effects) is a prerequisite for performing a variety of coupling actions against the viral particles, and thus is a highly sought-after feature. By considering realistic dispersion for the dielectric permittivity of proteins and a core-shell modeling allowing for rigorous formulation via Mie theory, we report optical extinction resonances for corona virions at mid-infrared range that are not significantly perturbed by changes in the object's size or the background host. Our findings indicate the optimal regime for interaction of photonic radiation with viral particles, and may assist towards the development of equipment for thermal damage, disintegration, or neutralization of coronavirus cells.

We have studied the superconducting gap structure of ${\mathrm{LaPt}}_2{\mathrm{Si}}_2$ by measuring the temperature dependence of the London penetration depth shift $\mathrm{\Delta }\lambda \left(T\right)$ and point contact spectroscopy of single crystals. $\mathrm{\Delta }\lambda \left(T\right)$ shows an exponential temperature dependence at low temperatures, and the derived normalized superfluid density ${\rho }_s\left(T\right)$ can be well described by a single-gap $s$-wave model. The point-contact conductance spectra can also be well fitted by an $s$-wave Blonder-Tinkham-Klapwijk model, where the gap value shows a typical BCS temperature and magnetic field dependence consistent with type-II superconductivity. These results suggest fully gapped superconductivity in ${\mathrm{LaPt}}_2{\mathrm{Si}}_2$, with moderately strong electron-phonon coupling.

Multiterminal Josephson junctions constitute engineered topological systems in arbitrary synthetic dimensions defined by the superconducting phases. Microwave spectroscopy enables the measurement of the quantum geometric tensor, a fundamental quantity describing both the quantum geometry and the topology of the emergent Andreev bound states in a unified manner. In this work we propose an experimentally feasible and scalable multiterminal setup of $N$ quantum dots connected to $N+1$ superconducting leads which allows us to deterministically study nontrivial topology in terms of the Chern number of the noninteracting ground state. An important result is that the nontrivial topology in a linear chain appears beyond a threshold value of the nonlocal proximity-induced pairing potential which represents the novel theoretical key ingredient of our proposal. Moreover, we generalize the microwave spectroscopy scheme to the multiband case and show that the elements of the quantum geometric tensor of the noninteracting ground state can be experimentally accessed from the measurable oscillator strengths at low temperature.

Antiferromagnetic (AF) spin fluctuations are commonly believed to play a key role in electron pairing of cuprate superconductors. In electron-doped cuprates, a paradox still exists about the interplay among different electronic states in quantum perturbations, especially between superconducting and magnetic states. Here, we report a systematic transport study of cation-optimized ${\mathrm{La}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_{4\pm \delta }$ $(x=0.10)$ thin films in high magnetic fields. We find an AF quantum phase transition near 60 T, where the Hall number jumps from $n_{\mathrm{H}}=-x$ to $n_{\mathrm{H}}=1-x$, resembling the change in $n_{\mathrm{H}}$ at the AF boundary $(x_{\mathrm{AF}}=0.14)$ tuned by Ce doping. In the AF region a spin-dependent state manifesting anomalous positive magnetoresistance is observed, which is closely related to superconductivity. Once the AF state is suppressed by magnetic field, a polarized ferromagnetic state is predicted, reminiscent of the recently reported ferromagnetic state at the quantum end point of the superconducting dome by Ce doping. The magnetic field that drives phase transitions in a manner similar to but distinct from doping thereby provides a unique perspective to understand the quantum criticality of electron-doped cuprates.