Due to the charge-neutral and localized nature of surface Majorana modes, detection schemes usually rely on local spectroscopy or interference through the Josephson effect. Here, we theoretically study the magnetic response of a two-dimensional cone of Majorana fermions localized at the surface of class-DIII topological superconductors. For a field parallel to the surface the Zeeman term vanishes, and the orbital term induces a Doppler shift of the Andreev levels, resulting in a tilting of the surface Majorana cone. For fields larger than a critical threshold field $H^{*}$ the system undergoes a transition from a type-I to type-II Dirac-Majorana cone. In a spherical geometry the surface curvature leads to the emergence of the Majorana spin connection in the tilting term via an interplay between orbital and Zeeman terms that generates a finite nontrivial coupling between negative- and positive-energy states. Majorana modes are thus expected to show a finite response to the applied field, which acquires a universal character in finite geometries and opens the way to detection of Majorana modes via time-dependent magnetic fields.

We present a detailed experimental and theoretical analysis of the dispersion and nonlinear Kerr frequency shifts of plasma modes in a one-dimensional Josephson junction chain containing 500 superconducting quantum interfence devices in the regime of weak nonlinearity. The measured low-power dispersion curve agrees perfectly with the theoretical model if we take into account the Kerr renormalization of the bare frequencies and the long-range nature of the island charge screening by a remote ground plane. We measured the self- and cross-Kerr shifts for the frequencies of the eight lowest modes in the chain. We compare the measured Kerr coefficients with theory and find good agreement.

The interplay between magnetism and superconductivity is one of the important subjects to investigate the pairing mechanism in novel superconductors. We have found a new coexistence between an antiferromagnetic order in the Fe sublattice and superconductivity of the FeAs-based ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ superconductor in the pressure range from 2.4 to 3.0 GPa by ${}^{57}\mathrm{Fe}$ nuclear forward scattering (NFS) using a single-crystal sample. The magnetic state in the Fe sublattice changes to a new antiferromagnetic one with superconductivity from a stripe-type antiferromagnetic one observed in the normal conducting state at 2.7 GPa. Below the superconducting transition temperature, the temperature dependence of ${}^{57}\mathrm{Fe}$ NFS spectra reveals that the new antiferromagnetic order develops with the superconductivity. This nontrivial coupling of two ordered states in ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ under pressure demonstrates a new and intriguing relationship between magnetism and superconductivity in Fe-based superconductors.

The distribution of the ratios of nearest neighbor level spacings has become a popular indicator of spectral fluctuations in complex quantum systems such as the localized and thermal phases of interacting many-body systems, quantum chaotic systems, and in atomic and nuclear physics. In contrast to the level spacing distribution, which requires the cumbersome and at times ambiguous unfolding procedure, the ratios of spacings do not require unfolding and are easier to compute. In this work, for the class of Wigner-Dyson random matrices with nearest neighbor spacing ratios $r$ distributed as $P_{\beta }\left(r\right)$ for the three ensembles indexed by $\beta =1,2,4$, their $k\mathrm{th}$ order spacing ratio distributions are shown to be identical to $P_{{\beta }^{\prime }}\left(r\right)$, where ${\beta }^{\prime }$, an integer, is a function of $\beta$ and $k$. This result is shown for Gaussian and circular ensembles of random matrix theory and for several physical systems such as spin chains, chaotic billiards, Floquet systems, and measured nuclear resonances.

We present Raman scattering results on the layered semiconducting ferromagnetic compound ${\mathrm{CrSiTe}}_3$. Four Raman-active modes, predicted by symmetry, are observed and assigned. The experimental results are supported by density functional theory calculations. The self-energies of the $A_g^3$ and the $E_g^3$ symmetry modes exhibit unconventional temperature evolution around 180 K. In addition, the doubly degenerate $E_g^3$ mode shows a clear change of asymmetry in the same temperature region. The observed behavior is consistent with the presence of the previously reported short-range magnetic order and strong spin-phonon coupling.

The vibrational properties of ${\mathrm{CrI}}_3$ single crystals were investigated using Raman spectroscopy and were analyzed with respect to the changes of the crystal structure. All but one mode are observed for both the low-temperature $R\overline{3}$ and the high-temperature $C2/m$ phase. For all observed modes the energies and symmetries are in good agreement with DFT calculations. The symmetry of a single layer was identified as $p\overline{3}1/m$. In contrast to previous studies we observe the transition from the $R\overline{3}$ to the $C2/m$ phase at 180 K and find no evidence for coexistence of both phases over a wide temperature range.

Conventional microlasing of electromagnetic waves requires (1) a high-$Q$ cavity and (2) a mechanism for directional emission. Previous theoretical and experimental work demonstrated that the two requirements can be met with deformed dielectric cavities that generate chaotic ray dynamics. Is it possible for a massless Dirac spinor wave in graphene or its photonic counterpart to exhibit a similar behavior? Intuitively, because of the absence of backscattering of associated massless spin-1/2 particles and Klein tunneling, confining the wave in a cavity for a long time seems not feasible. Deforming the cavity to generate classical chaos would make confinement even more difficult. Investigating the decay of a spin-1/2 wave from a scalar potential barrier-defined cavity characterized by an effective refractive index $n$ that depends on the applied potential and the particle energy, we uncover the striking existence of an interval of the refractive index in which the average lifetime of the massless spin-1/2 wave in the cavity can be as high as that of the electromagnetic wave for both integrable and chaotic cavities. We also find scaling laws for the ratio between the mean escape time associated with electromagnetic waves and that with massless spin-1/2 particles versus the index outside of this interval. The scaling laws hold regardless of the nature of the classical dynamics. All the results are verified numerically. The findings provide insight into the emergent field of Dirac electron optics and have potential applications in developing unconventional electronics using two-dimensional Dirac materials.

The structural properties and the magnetic ground state of $\mathrm{S}{\mathrm{r}}_{2/3}\mathrm{L}{\mathrm{n}}_{1/3}\mathrm{Fe}{\mathrm{O}}_3$ (Ln = La, Pr, Nd) samples were studied by means of synchrotron x-ray powder diffraction, neutron powder diffraction, and Mössbauer spectroscopy. All samples exhibit a metal-insulator-like transition coupled to a magnetic arrangement at a critical temperature, $T_{\mathrm{MI}}$. The diffraction techniques reveal strong structural changes at $T_{\mathrm{MI}}$ that lead to new cells with reduced symmetry at low temperature. The new symmetry of the low-temperature phase has been determined for all compounds. The space groups are P$\overline{3}$c1 for La-based compound and A2/n for the rest of samples. The high-resolution x-ray patterns detected superstructure peaks that can be accounted for by a small charge disproportionation between two nonequivalent Fe sites in the low-temperature phase explained in terms of a charge density wave that propagates along one of the body diagonals of the primitive cubic cell of these compounds. Our results clearly reveal that a full charge disproportionation of ${\mathrm{Fe}}^{4+}$ into ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{5+}$ is not produced. We have determined the magnetic ordering of these samples exhibiting an antiferromagnetic structure with a sixfold periodicity with respect to the primitive cubic structure. The magnetic group accounting for the magnetic arrangements was obtained by a symmetry analysis and it is C2/c (15.85) for all samples but with different unit cell depending on the type of ${\mathrm{FeO}}_6$ tilts. The collinear ordering of Fe moments is established perpendicular to the charge density wave (along the body diagonal of the primitive cubic cell) and also perpendicular to the unique monoclinic axis.

It was theoretically proposed that a forward-biased $p-n$ junction with a superconducting layer (P-N-S) would produce pure, polarization-entangled photons due to inherent spin-singlet pairing of electrons. However, any heterostructure interface generically induces Rashba spin-orbit coupling, which in turn generates a mixed singlet-triplet superconducting order parameter. Here we study the effect of triplet pairing on the purity of photons produced through Cooper pair recombination. A unique directional dependence of the state purity is found for a triplet superconductor with fixed $\mathit{d}$: pure, entangled photons are produced when the photon polarization axis is parallel to $\mathit{d}$. Induced triplet pairing in a singlet superconductor is shown to degrade the state purity, while induced singlet pairing in a triplet superconductor is shown to enhance the production of entangled pairs. These considerations may aid the design of functional devices to produce entangled photons.

Novel topological phenomena are anticipated for three-dimensional (3D) Dirac electrons. The magnetotransport properties of cubic ${\mathrm{Sr}}_3\mathrm{PbO}$ antiperovskite, theoretically proposed to be a 3D massive Dirac electron system, are studied. The measurements of Shubnikov–de Haas oscillations and Hall resistivity indicate the presence of a low density ($\sim 1\times {10}^{18}{\mathrm{cm}}^{-3}$) of holes with an extremely small cyclotron mass of $0.01–0.06m_e$. The magnetoresistance $\mathrm{\Delta }{\rho }_{xx}\left(B\right)$ is linear in magnetic field $B$ with the magnitude independent of temperature. These results are fully consistent with the presence of 3D massive Dirac electrons in ${\mathrm{Sr}}_3\mathrm{PbO}$. The chemical flexibility of the antiperovskites and our findings in the family member ${\mathrm{Sr}}_3\mathrm{PbO}$ point to their potential as a model system in which to explore exotic topological phases.

We analytically characterize the influence of a neighboring metal nanoparticle (MNP) on the behavioral trends of a quantum dot (QD) using a generalized nonlocal optical response (GNOR) method based approach, taking the MNP distance dependent modifications to the QD population relaxation and dephasing rates into account. The GNOR model is a recent generalization and an extension of the hydrodynamic Drude model (HDM), which goes beyond HDM by taking into account both the convection current and electron diffusion in the MNPs. It allows unified theoretical explanation of some experimentally observed plasmonic phenomena which otherwise would require ab initio analysis as the conventional local response approximation (LRA) fails to account for them. For example, it has been demonstrated in literature that the GNOR model captures size dependent resonance shifts of small MNPs which are unrevealed by the conventional LRA based methods, and it has proven to yield results displaying better agreement with the experimental observations for plasmonic experiments. Attempts to incorporate MNP nonlocal effects in the analytical characterization of vicinal excitons found in literature utilize the phenomenological hydrodynamic model and assume the absence of MNP interband effects. Moreover, they are only applicable to narrow parameter regions. In this paper we present a complete analytical characterization which overcomes these drawbacks and lends to the perusal of the system over wide continua of various parameters, enabling us to get an elevated view at a much lesser level of complexity compared to the conventional LRA based numerical methods or the conventional ab initio methods of accounting for the nonlocal effects. Our proposed GNOR based model predicts strong modifications to various QD properties such as population difference, absorption, MNP induced shifts to excitonic energy and Förster enhanced broadening, coherent plasmonic field enhancement, and quantum state purity, compared to the conventional LRA based predictions. Such modifications are prominent with small MNP radii, high QD dipole moments, small detunings (of the coherent external illumination from the bare excitonic resonance), and near parameter regions exhibiting plasmonic meta resonance (PMR)-like behavior. Moreover, our complete analytical characterization enables optimization of the large system parameter space for different applications, a luxury not fully offered by the methods currently available in literature.

Isoelectronic centers in semiconductor hosts offer convenient optical access to both heavy- and light-hole states, thus providing powerful resources for the ultrafast control of electron or hole spins. From a detailed analysis of the fine structure of excitons bound to isoelectronic defects of $C_s$ symmetry formed by two nitrogen impurities in GaAs, we demonstrate that the lowest energy states, normally exhibiting a strongly dominant heavy-hole character, can be reversed to a dominant light-hole character by the strain found in the vicinity of the emitter. This reversal suggests that strain could be used to engineer the orbital composition and ordering of excitonic states, allowing enhanced optical control over spin states.

We construct a general metric-tensor framework for treating inhomogeneous adiabatic deformations applied to crystalline insulators, by deriving an effective time-dependent Schrödinger equation in the undistorted frame. The response can be decomposed into “static” and “dynamic” terms that correspond, respectively, to the ampli- tude and the velocity of the distortion. We then focus on the dynamic contribution, which takes the form of a gauge field entering the effective Hamiltonian, in the linear-response limit. We uncover an intimate relation between the dynamic response to the rotational component of the inhomogeneous deformation and the diamagnetic response to a corresponding inhomogeneous magnetic field. We apply this formalism to the theory of flexoelectric response, where we resolve a previous puzzle by showing that the currents generated by the dynamic term, while real, generate no bound charges even at surfaces, and so may be dropped from a practical theory of flexoelectricity.

Field quantization in high curvature geometries help understanding the elastic and inelastic scattering of photons and electrons in nanostructures and probelike metallic domains. The results find important applications in high-resolution photonic and electronic modalities of scanning probe microscopy, nano-optics, plasmonics, and quantum sensing. We present a calculation of relevant photon interactions in both hyperboloidal and paraboloidal material domains. The two morphologies are compared for their plasmon dispersion properties, field distributions, and radiative decay rates, which are shown to be consistent with the corresponding quantities for the finite prolate spheroidal domains. The results are relevant to other material domains that model a nanostructure such as a probe tip, quantum dot, or nanoantenna.

Indium selenide (InSe) has attracted tremendous research interest due to its high mobility and potential applications in next-generation electronics. However, the underlying transport mechanism of carriers in thin InSe at low temperatures remains unknown. Here we report the gate voltage and temperature-dependent magnetotransport properties of $\gamma$-InSe transistor devices with Hall mobility up to $2455\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at the temperature of 1.7 K. We observe a gate-tunable weak antilocalization behavior at lower magnetic field $B$, which shows a transition to weak localization at higher $B$ region. We find that the magnetotransport data agree well with the Hikami-Larkin-Nagaoka theory. The conductivity and temperature dependence of phase-coherence length reveal that the electron-electron ($e\text{−}e$) interactions are dominated dephasing mechanism for electronic transport in $\gamma$-InSe at low temperatures. The maximum phase-coherence length is found to be 320 nm at 1.7 K, larger than that of monolayer $\mathrm{Mo}{\mathrm{S}}_2$ and few-layer black phosphorus. These results enrich the fundamental understanding of electronic transport properties of InSe.

We identify by first-principles calculations a new diamond phase in $R\overline{3}c$ ($D_{3d}^6$) symmetry, which has a 16-atom rhombohedral primitive cell, thus termed R16 carbon. This rhombohedral diamond comprises a characteristic all-$s{p}^3$ six-membered-ring bonding network, and it is energetically more stable than previously identified diamondlike six-membered-ring bonded BC8 and BC12 carbon phases. A phonon mode analysis verifies the dynamic structural stability of R16 carbon, and electronic band calculations reveal that it is an insulator with a direct band gap of 4.45 eV. Simulated x-ray diffraction patterns provide an excellent match to recently reported distinct diffraction peaks found in milled fullerene soot, suggesting a viable experimental synthesis route. These findings pave the way for further exploration of this new diamond phase and its outstanding properties.

We theoretically study magnetic and topological properties of antiferromagnetic kagome spin systems in the presence of both in- and out-of-plane Dzyaloshinskii-Moriya interactions. In materials such as the iron jarosites, the in-plane interactions stabilize a canted noncollinear “umbrella” magnetic configuration with finite scalar spin chirality. We derive expressions for the canting angle and use the resulting order as a starting point for a spin-wave analysis. We find topological magnon bands, characterized by nonzero Chern numbers. We calculate the magnon thermal Hall conductivity and propose the iron jarosites as a promising candidate system for observing the magnon thermal Hall effect in a noncollinear spin configuration. We also show that the thermal conductivity can be tuned by varying an applied magnetic field or the in-plane Dzyaloshinskii-Moriya strength. In contrast to previous studies of topological magnon bands, the effect is found to be large even in the limit of small canting.

The physical properties of the very-heavy-fermion compound ${\mathrm{YbCu}}_4\mathrm{Ni}$ were characterized through structural, magnetic, thermal, and transport studies along nearly four decades of temperature ranging between 50 mK and 300 K. At high temperature, the crystal electric field level splitting was determined with ${\mathrm{\Delta }}_1\left({\mathrm{\Gamma }}_6\right)=85\mathrm{K}$ and ${\mathrm{\Delta }}_2\left({\mathrm{\Gamma }}_8\right)\approx 200\mathrm{K}$, the latter being a quartet in this cubic symmetry. An effective magnetic moment ${\mu }_{\mathrm{eff}}\approx 3{\mu }_B$ is evaluated for the ${\mathrm{\Gamma }}_7$ ground state, while at high temperature the value for a ${\mathrm{Yb}}^{3+}$ ion is observed. At low temperature this compound shows the typical behavior of a magnetically frustrated system undergoing a change of regime at a characteristic temperature $T^{*}\approx 200\mathrm{mK}$ into of Fermi-liquid-type “plateau” of the specific heat: $C_m{/T|}_{T\to 0}$ = const. The change in the temperature dependence of the specific heat coincides with a maximum and a discontinuity in respective inductive and dissipative components of the ac susceptibility. More details about the nature of this ground state are revealed by the specific heat behavior under applied magnetic field.