InP-based colloidal quantum dots are promising for optoelectronic devices such as light-emitting diodes and lasers. Understanding and optimizing their emission process is of scientific interest and essential for large-scale applications. Here we present a study of the exciton recombination dynamics in InP QDs with various shells: ZnS, ZnSe, and (Zn,Cd)Se with different amounts of Cd (5, 9, 12%). Phonon energies extracted from Raman spectroscopy measurements at cryogenic temperatures (4–5 K) are compared with exciton emission peaks observed in fluorescence line narrowing spectra. This allowed us to determine the position of both the bright $F=\pm 1$ state and the lowest dark $F=\pm 2$ state. We could identify the phonon modes involved in the radiative recombination of the dark state and found that acoustic and optical phonons of both the core and the shell are involved in this process. The Cd content in the shell increases electron wave-function delocalization, and thereby enhances the exciton-phonon coupling through the Fröhlich interaction.

First-principles calculations were used to study the Fermi surface of body-centered cubic vanadium at elevated temperatures. Supercell calculations accounted for effects of thermal atom displacements on band energies, and band unfolding was used to project the spectral weight of the electron states into the Brillouin zone of a standard bcc unit cell. An electronic topological transition (ETT, or Lifshitz transition) occurred near the $\mathrm{\Gamma }$ point with increasing temperature, but the large thermal smearings from the atomic disorder and the Fermi-Dirac distribution reduced the effect of this ETT on the electron-phonon interactions. The phonon dispersions showed thermal stiffening of their Kohn anomalies near the $\mathrm{\Gamma }$ point and of the longitudinal N phonon mode. In general the effects of the ETT were overcome by the thermal smearing of the Fermi surface that reduces the spanning vector densities for anomalous phonon modes.

Motivated by the recent observation and argument on a large half-quantum vortex (HQV) pair connected by a Kibble-Lazarides-Shafi wall in superfluid ${}^3\mathrm{He}$ in nematic aerogels, we numerically study to what extent a huge HQV pair can intrinsically occur in the polar-distorted $B$ (PdB) phase of superfluid ${}^3\mathrm{He}$. First, the “impurity”-scattering model used in a previous study is extended to a form interpolating the weakly and strongly anisotropic cases, and it is found that, within the Ginzburg-Landau (GL) approach, Anderson's theorem is satisfied in the strongly anisotropic case. By taking account of the Fermi-liquid (FL) corrected gradient terms and solving numerically the resulting GL free energy, the anisotropy dependence of the vortex structure minimizing the free energy is examined within the weak-coupling approximation. It is found that, close to the transition between the polar and PdB phases, an interplay of the strong anisotropy and the FL correction makes possible the emergence of a large HQV pair in the PdB phase, and that, nevertheless, such a large pair easily shrinks upon deeply entering the PdB phase, indicating that a pinning effect due to the aerogel structure is necessary in order to keep a large pair size there. The obtained result indicates the validity of the London limit for describing the vortex structure, and a consistency with the picture based on the NMR measurement is discussed.

Recently, one analog of the Anderson's theorem for the $s$-wave superconductor has attracted much interest in the context of the $p$-wave polar pairing state of superfluid ${}^3\mathrm{He}$ in a model aerogel in the limit of strong uniaxial anisotropy. We discuss to what extent the theorem is satisfied in the polar phase in real aerogels by examining the normal to polar transition temperature $T_c$ and the low temperature behavior of the superfluid energy gap under an anisotropy of a moderate strength and comparing the obtained results with experimental data. The situation in which the Anderson's theorem clearly breaks down is briefly discussed.

We study the formation dynamics of spinor polariton condensates trapped in ring-shaped confining potentials created by excitonic reservoirs. We consider in detail the interplay of the effective spin-orbit interaction provided by transverse electric and transverse magnetic photonic modes splitting (TE-TM splitting) and exciton Zeeman splitting provided by an external magnetic field. We demonstrate that tuning of the trap size obtained by shaping of the external nonresonant and depolarized pumping allows formation of pairs of half-vortices of topological charges $\pm 1/2$ in both spin components. Further, we show that the probabilities of the realizations of four possible vortex configurations strongly depend on the value of the magnetic field. For certain values of the field, the probability of the formation of a vortex with desired topological charge reaches 90%, which opens the possibility of on-demand control of angular momentum of quantum fluids of light with a magnetic field.

In this study, we used the thermally driven spin current to investigate the spin frustrations and spin fluctuations in spin-glass (SG) $\mathrm{C}{\mathrm{u}}_{1-x}\mathrm{M}{\mathrm{n}}_x$ alloys. Tuning the $\mathrm{C}{\mathrm{u}}_{1-x}\mathrm{M}{\mathrm{n}}_x$ composition results in a transition of the alloys from the SG state to the antiferromagnetic state; these states have different spin-freezing temperatures $\left(T_f\right)$. Most spins randomly freeze at temperatures lower than the $T_f$ of the alloy. For each alloy composition, we obtained a temperature-dependent inverse spin Hall voltage with a peak at $T_p$. Crucially, $T_p$ had nearly identical composition dependence as that of $T_f,$, with $T_p$ being nine times larger than $T_f$. Similar behavior was captured using the SG insulator, amorphous ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$. These results indicated that the maximum spin fluctuation in both conducting and insulating SGs occurred at temperatures considerably higher than the $T_f$ of each. In addition, we demonstrated the importance of the effective number of valence electrons in tailoring the spin Hall angle in binary alloys.

Using ab initio calculations, we have analyzed the influence of anharmonic effects on the electronic structure and the phonon-dispersion relations of body-centered-cubic (bcc) niobium (Nb) and investigated the temperature dependence of the Kohn anomaly in this metal. A comparison of the results obtained in the framework of the temperature-dependent effective potential method with those derived within the quasiharmonic approximation demonstrates the importance of the explicit treatment of the finite-temperature effects upon the theoretical description of bcc Nb lattice dynamics. In agreement with experimental results, the inclusion of anharmonic vibrations in our calculations leads to the disappearance of the Kohn anomaly for the acoustic mode in a vicinity of the $\mathrm{\Gamma }$ point with increasing temperature. Moreover, the calculated phonon self-energy indicates that the origin of the temperature dependence is related to the change of the electronic structure. We have calculated the temperature dependence of the electronic spectral function and analyzed the Fermi surface of Nb. A significant temperature-induced smearing of the electronic states has been identified as the origin of the disappearance of the Kohn anomaly in Nb at elevated temperature.

In recent years, second-order topological insulators have been proposed as a new class of topological insulators. Second-order topological insulators are materials with gapped bulk and surfaces, but with topologically protected gapless states at the intersection of two surfaces. These gapless states are called hinge states. In this paper, we give a general proof that any insulators with inversion symmetry and gapped surface in class A always have hinge states when the ${\mathbb{Z}}_4$ topological index ${\mu }_1$ is ${\mu }_1=2$. We consider a three-dimensional insulator the boundary conditions of which along two directions change by changing the hopping amplitudes across the boundaries. We study behaviors of gapless states through continuously changing boundary conditions along the two directions, and reveal that the behaviors of gapless states result from the ${\mathbb{Z}}_4$ strong topological index. From this discussion, we show that gapless states inevitably appear at the hinge of a three-dimensional insulator with gapped surfaces when the strong topological index is ${\mathbb{Z}}_4=2$ and the weak topological indices are ${\nu }_1={\nu }_2={\nu }_3=0$.

We study the transport properties of two-dimensional electron systems with strong Rashba spin-orbit coupling (SOC) doped with interacting magnetic impurities. Interactions between magnetic impurities cause the formation of magnetic clusters with temperature-dependent mean sizes distributed randomly on the surface of the system. Treating magnetic clusters as scattering centers, by employing a generalized relaxation time approximation we obtain the nonequilibrium distribution functions of Rashba electrons in both regimes of above and below the band-crossing point (BCP) and present the explicit forms of the conductivity in terms of effective relaxation times. We demonstrate that the combined effects of SOC and magnetic clusters cause the system to be anisotropic and the magnetoresistance strongly depends on both the clusters' mean size and spin, the strengths of SOC, and the location of Fermi energy with respect to the BCP. Our results show that there are many contrasts between the transport properties of the system in the two regimes of above and below the BCP. By comparing the anisotropic magnetoresistance (AMR) of the two-dimensional Rashba systems with the surface AMR of three-dimensional magnetic topological insulators, we also point out the differences between these systems.

Recent experimental observations of apparently hydrodynamic electronic transport have generated much excitement. However, the understanding of the observed nonlocal transport (whirlpool) effects and parabolic (Poiseuille-like) current profiles has largely been motivated by a phenomenological analogy to classical fluids. This is due to difficulty in incorporating strong correlations in quantum mechanical calculation of transport, which has been the primary angle for interpreting the apparently hydrodynamic transport. Here we demonstrate that even free-fermion systems, in the presence of (inevitable) disorder, exhibit nonlocal conductivity effects such as those observed in experiments because of the fermionic system's long-range entangled nature. On the basis of explicit calculations of the conductivity at finite wave vector, $\sigma \left(\mathbf{q}\right)$, for selected weakly disordered free-fermion systems, we propose experimental strategies for demonstrating distinctive quantum effects in nonlocal transport at odds with the expectations of classical kinetic theory. Our results imply that the observation of whirlpools or other “hydrodynamic” effects does not guarantee the dominance of electron-electron scattering over electron-impurity scattering.

We present a general method to optimize the evaluation of Feynman diagrammatic expansions which uses an automated symbolic assignment of momentum/energy conserving variables to each diagram. With this symbolic representation, we utilize the pole structure of each diagram to sort the Feynman diagrams into groups that are likely to contain nearly equal or nearly canceling diagrams, and we show that for some model parameters this cancellation is exact. This allows for a potentially massive cancellation during the numerical integration of internal momenta variables, leading to an optimal suppression of the “sign problem” and hence reducing the computational cost. Although we define these groups using a frequency space representation, the equality or cancellation of diagrams within the group remains valid in other representations such as imaginary time used in standard diagrammatic Monte Carlo. As an application of the approach, we apply this method, combined with algorithmic Matsubara integration (AMI) [A. Taheridehkordi, S. H. Curnoe, and J. P. F. LeBlanc, Phys. Rev. B 99, 035120 (2019)] and Monte Carlo methods, to the Hubbard model self-energy expansion on a 2D square lattice, which we evaluate and compare with existing benchmarks.

Twisted bilayer graphene (TBG) hosts nearly flat bands with narrow bandwidths of a few meV at certain magic twist angles. Here we show that in twisted gapped Dirac material bilayers, or massive twisted bilayer graphenes (MTBG), isolated nearly flat bands below a threshold bandwidth $W_c$ are expected for continuous small twist angles up to a critical ${\theta }_c$ depending on the flatness of the original bands and the interlayer coupling strength. Narrow bandwidths of $W\lesssim 30\mathrm{meV}$ are expected for $\theta \lesssim 3^{\circ }$ for twisted Dirac materials with intrinsic gaps of $\sim 2\mathrm{eV}$ that finds realization in monolayers of gapped transition metal dichalcogenides (TMDC), silicon carbide (SiC) among others, and even narrower bandwidths in hexagonal boron nitride (BN) whose gaps are $\sim 5\mathrm{eV}$, while twisted graphene systems with smaller gaps of a few tens of meV, e.g., due to the alignment with hexagonal boron nitride, show the vestiges of the magic angle behavior in the bandwidth evolution. The phase diagram of finite valley Chern numbers of the isolated moire bands expands with increasing difference between the sublattice selective interlayer tunneling parameters. The valley contrasting circular dichroism for interband optical transitions is constructive near $0^{\circ }$ and destructive near ${60}^{\circ }$ alignments and can be tuned through electric field and gate-driven polarization of the mini valleys. Combining massive Dirac materials with various intrinsic gaps, Fermi velocities, and interlayer tunneling strengths suggests optimistic prospects of increasing ${\theta }_c$ and achieving correlated states with large $U/W$ effective interaction versus bandwidth ratios.

The reported temperature variations of ${\mathrm{CeB}}_6$'s magnetic entropy are inconsistent with the fourfold degeneracy of the crystal field ground state. This old question is here addressed through new specific heat measurements and an improved description, in the cage context, of both the phonons and crystal field contributions to the specific heat. The antiferromagnetic transition is characterized as first order and its latent heat determined. From the phonons' dispersion for a cage compound, the lattice specific heat contribution is derived from the ${\mathrm{LaB}}_6$ data. Once corrected for the first-order transition and lattice contributions, the magnetic entropy displays the characteristic plateau of the quadruplet crystal field ground state, but at temperatures in excess of 30 K. Below 30 K, as the ordering temperature is approached, the magnetic entropy is substantially reduced. This anomalous temperature dependence is consistent with a crystal field ground state split by the rare-earth movement, a phenomenon specific to rare-earth cage compounds.

Cation distribution in transition-metal oxides is closely tied to their physical properties. Here we employed resonant x-ray-diffraction technique and quantitatively evaluated the cation distribution in inverse spinel $\mathrm{NiC}{\mathrm{o}}_2{\mathrm{O}}_4$ films that were epitaxially grown by pulsed laser deposition. Our results reveal that oxygen partial pressure during growth $P_{\mathrm{O}2}$ is a key parameter determining cation distribution in the films, with a larger $P_{\mathrm{O}2}$ leading to an increase in the Ni concentration occupying the octahedral $\left(O_h\right)$ sites. We further show that the $P_{\mathrm{O}2}$-dependent $O_h$-site Ni concentration impacts the magnetic properties of the films: Films having the cation distribution close to the stoichiometric exhibit ferrimagnetism with a transition temperature higher than 400 K and enhanced perpendicular magnetic anisotropy. On the other hand, the reductions in the $O_h$-site Ni concentration leads to the deterioration of the ferrimagnetic properties such as magnetization, transition temperature, and anisotropy energy. Our study demonstrates that the ferrimagnetic properties and the anisotropy can be tuned through the cation distribution, which would provide an additional degree of freedom in designing spintronic devices based on spinel oxides including $\mathrm{NiC}{\mathrm{o}}_2{\mathrm{O}}_4$.

The magnetic ground state of the $B$-site ordered double perovskite $\mathrm{S}{\mathrm{r}}_2\mathrm{DyRu}{\mathrm{O}}_6$ has been investigated using muon spin rotation and relaxation (µSR), neutron powder diffraction (NPD), and inelastic neutron scattering (INS), in addition to heat capacity and magnetic susceptibility (AC and DC) measurements. A clear signature of a long-range ordered magnetic ground state has been observed in the heat capacity data, which exhibits two sharp anomalies at 39.5 and 36 K found as well in the magnetic data. Further supporting evidence consistent with long-range magnetic ordering comes from a sharp drop in the muon initial asymmetry and a peak in the relaxation rate at 40 K, along with a weak anomaly near 36 K. Based on temperature dependent NPD, the low temperature magnetic structure contains two interpenetrating lattices of $\mathrm{D}{\mathrm{y}}^{3+}$ and ${\mathrm{Ru}}^{5+}$, forming an antiferromagnetic ground state below 39.5 K with magnetic propagation vector $k=\left(0,0,0\right)$. The magnetic moments of $\mathrm{D}{\mathrm{y}}^{3+}$ and ${\mathrm{Ru}}^{5+}$ at 3.5 K are pointing along the crystallographic $b$ axis with values of ${\mu }^{\mathrm{Dy}}=4.92\left(10\right){\mu }_{\mathrm{B}}$ and ${\mu }^{\mathrm{Ru}}=1.94\left(7\right){\mu }_{\mathrm{B}}$, respectively. The temperature dependence of the ${\mathrm{Ru}}^{5+}$ moments follows a mean field type behavior, while that of the $\mathrm{D}{\mathrm{y}}^{3+}$ moments exhibits a deviation indicating that the primary magnetic ordering is induced by the order of the $4d$ electrons of ${\mathrm{Ru}}^{5+}$ rather than that of the proper $4f\mathrm{D}{\mathrm{y}}^{3+}$ electrons. The origin of the second anomaly observed in the heat capacity data at 36 K must be connected to a very small spin reorientation as the NPD studies do not reveal any clear change in the observed magnetic Bragg peaks’ positions or intensities between these two transitions. INS measurements reveal the presence of crystal field excitations (CEF) in the paramagnetic state with overall CEF splitting of 73.8 meV, in agreement with the point change model calculations, and spin wave excitations below 9 meV at 7 K. Above $T_N$, the spin wave excitations transform into a broad diffuse scattering indicating the presence of short-range dynamic magnetic correlations.

Topologically nontrivial superconducting phases have been engineered in topological materials by the proximity effect in contact with conventional superconductors. In this paper, by using the method of the Kronig-Penney model, we study the superconducting proximity effect in the bulk electronic states of Weyl semimetals by considering a multilayer structure consisting of Weyl-semimetal and superconductor layers. Due to the proximity effect, two Weyl nodes are decoupled into four nodes of Majorana fermions resulting in Weyl-superconductor phases or a three-dimensional extension of topological-superconductor phases. We find that mismatch of the Fermi velocity and potential barriers at the interface gap out Majorana nodes thus turning Weyl-superconductor phases with four Majorana nodes into Weyl-superconductor phases with half of the Majorana nodes and topological-superconductor phases with odd integer Chern numbers.

The boundary Green's-function (bGF) approach has been established as a powerful theoretical technique for computing the transport properties of tunnel-coupled hybrid nanowire devices. Such nanowires may exhibit topologically nontrivial superconducting phases with Majorana bound states at their boundaries. We introduce a general method for computing the bGF of spinful multichannel lattice models for such Majorana nanowires, where the bGF is expressed in terms of the roots of a secular polynomial evaluated in complex momentum space. In many cases, those roots, and thus the bGF, can be accurately described by simple analytical expressions, while otherwise our approach allows for the numerically efficient evaluation of bGFs. We show that from the behavior of the roots many physical quantities of key interest can be inferred, e.g., the value of bulk topological invariants, the energy dependence of the local density of states, or the spatial decay of subgap excitations. We apply the method to single- and two-channel nanowires of symmetry class D or DIII. In addition, we study the spectral properties of multiterminal Josephson junctions made out of such Majorana nanowires.