Color centers in diamond are a promising platform for quantum technologies, and understanding their interactions with the environment is crucial for these applications. We report a study of spin-lattice relaxation $\left(T_1\right)$ of the neutral charge state of the silicon vacancy center in diamond. Above 20 K, $T_1$ decreases rapidly with a temperature-dependence characteristic of an Orbach process and is strongly anisotropic with respect to magnetic-field orientation. As the angle of the magnetic field is rotated relative to the symmetry axis of the defect, $T_1$ is reduced by over three orders of magnitude. The electron spin coherence time $\left(T_2\right)$ follows the same temperature dependence but is drastically shorter than $T_1$. We propose that these observations result from phonon-mediated transitions to a low-lying excited state that are spin conserving when the magnetic field is aligned with the defect axis, and we discuss likely candidates for this excited state.

The trigonal form of selenium (t-Se) has an unusual, quasi-one-dimensional, chiral crystal structure. First-principles calculations have helped us uncover a polar, optic phonon in t-Se that exhibits a diagonal magnetoelectric coupling with the electric field of the incident THz radiation, and induces a parallel magnetic field due to its inherent chirality. We show that this phonon-mediated, magnetoelectric mechanism is predicted to cause optical rotation in t-Se in the 1–3 THz range and is quite distinct from the high frequency (>80 THz) activity due to electronic excitations that has been reported previously. In the second part of the paper, we report our experimental results based on THz time-domain spectroscopy as well as direct measurements of optical rotation that confirm this prediction in an aligned, monocrystalline array of t-Se microrods. The Se microrod array not only exhibits a large birefringence ($\mathrm{\Delta }n=1.3$) but also rotates the polarization of THz radiation by ∼3°/mm. To our knowledge, this is the only elemental solid known to rotate THz polarization, because the currently available THz rotators are based mainly on liquid crystals or metamaterials. The identification of new THz-active materials and a better understanding of the underlying physics are both clearly essential to the development of better sources, detectors and components.

In low-dimensional metallic systems, lattice distortion is usually coupled to a density-wave-like electronic instability due to Fermi-surface nesting (FSN) and strong electron-phonon coupling [M. D. Johannes and I. I. Mazin, Phys. Rev. B 77, 165135 (2008)]. However, the ordering of other electronic degrees of freedom can also occur simultaneously with the lattice distortion; thus, it challenges the aforementioned prevailing scenario. Recently, a hidden electronic reconstruction beyond FSN was revealed in a layered metallic compound ${\mathrm{BaTi}}_2{\mathrm{As}}_2\mathrm{O}$ below the structural transition temperature $T_s\sim 200$ K. The nature of this hidden electronic instability is under strong debate. Here, by measuring the local orbital polarization through ${}^{75}\mathrm{As}$ nuclear magnetic resonance experiments, we observe a $p-d$ bond order between Ti and As atoms in ${\mathrm{BaTi}}_2{\mathrm{As}}_2\mathrm{O}$ single crystal. Below $T_s$, the bond order breaks both rotational and translational symmetry of the lattice. Meanwhile, the spin-lattice relaxation measurement indicates a substantial loss of density of states and an enhanced spin fluctuation in the bond-ordered state. Further, first-principles calculations suggest that the mechanism of the bond order is due to the coupling between lattice and nematic instabilities. Our results strongly support a bond-order-driven electronic reconstruction in ${\mathrm{BaTi}}_2{\mathrm{As}}_2\mathrm{O}$ and shed light on the mechanism of superconductivity in this family.

We present an order-$N$ methodology to evaluate mobilities of charge carriers coupled with molecular vibrations using quantum dynamics based on first-principles calculations that can be applied to micron-scale soft materials. As a demonstration, we apply it to several organic semiconductors and show that the calculated intrinsic hole mobilities and their temperature dependences are quantitatively in good agreement with those obtained in experiments. We also clarified which vibrational modes dominate the transport properties. The methodology paves the way for quantitative prediction of the transport properties of various soft materials.

The only unequivocal known criterion for single-parameter scaling Anderson localization relies on the knowledge of the full conductance statistics. To date, theoretical studies have been restricted to model systems with symmetric scatterers, hence lacking universality. We present an in-depth statistical study of conductance distributions P(g), in disordered ‘micrometer-long’ carbon nanotubes using first principles simulations. In perfect agreement with the Dorokov-Mello-Pereyra-Kumar scaling equation, the computed P(g) exhibits a nontrivial, non-Gaussian, crossover to Anderson localization which could be directly compared with experiments.

We theoretically investigate the electronic and transport properties of a semi-Dirac material under the influence of an external time-dependent periodic driving field (irradiation) by means of Floquet theory. We explore the inelastic scattering mechanism between different sidebands, induced by irradiation, by using the Floquet scattering matrix approach. The scattering probabilities between the two nearest sidebands depend monotonically on the strength of the amplitude of the irradiation. The external irradiation induces a gap in the band dispersion which is strongly dependent on the angular orientation of momentum. Although the high-frequency limit indicates that the gap opening does not occur in an irradiated semi-Dirac material, a careful analysis of the full band structure beyond this limit reveals that a gap opening indeed appears for higher values of momentum (away from the Dirac point). Furthermore, the angular-dependent dynamical gap is also present and cannot be captured within the high-frequency approximation. The contrasting features of an irradiated semi-Dirac material, in comparison with irradiated graphene, can be probed via the behavior of conductance. The latter exhibits the appearance of nonzero conductance dips due to the gap opening in the Floquet band spectrum. Moreover, by considering a nanoribbon geometry of such a material, we also show that it can host a pair of edge modes which are fully decoupled from the bulk, which is in contrast to the case of a graphene nanoribbon where the edge modes are coupled to the bulk. We also investigate whether, if the nanoribbon of this material is exposed to the external irradiation, decoupled edge modes penetrate into the bulk.

We construct an algebraic description for the ground state and for the static response of the quantum Hall plateaus with a filling factor $\nu =N/(2N+1)$ in the large $N$ limit. By analyzing the algebra of the fluctuations of the shape of the Fermi surface of the composite fermions, we find the explicit form of the projected static structure (SSF) factor at large $N$ and fixed $z=(2N+1)q{\ell }_B\sim 1$, where $q$ is the wave number at which the system is probed, and ${\ell }_B$ is the magnetic length. When $z<3.8$, we obtain the exact universal formula for the projected SSF. The formula does not depend on the particular form of the Hamiltonian.

Two-dimensional heterostructures are likely to provide new avenues for the manipulation of magnetization that is crucial for spintronics or magnetoelectronics. Here, we demonstrate that optical spin pumping can generate a large effective magnetic field in two-dimensional $\mathrm{MoS}{\mathrm{e}}_2$/$\mathrm{WS}{\mathrm{e}}_2$ heterostructures. We determine the strength of the generated field by polarization-resolved measurement of the interlayer exciton photoluminescence spectrum: the measured splitting exceeding 10 millielectron volts (meV) between the emission originating from the two valleys corresponds to an effective magnetic field of ∼30 T. The strength of this optically induced field can be controlled by the excitation light polarization. Our finding opens up possibilities for optically controlled spintronic devices based on van der Waals heterostructures.

In this paper, we study the Hall conductance for a non-Hermitian Chern insulator and quantitatively describe how the Hall conductance deviates from a quantized value. We show the effects of the non-Hermitian terms on the Hall conductance from two aspects. On one hand, it broadens the density of states of each band, because of which there always exists a nonuniversal bulk contribution. On the other hand, it adds a decay term to the edge state, because of which the topological contribution also deviates from a quantized Chern number. We provide a simple formula for the topological contribution for a general two-band non-Hermitian Chern insulator, as a non-Hermitian version of the Thouless-Kohmoto-Nightingale-de Nijs formula. It shows that the derivation from quantized value increases either when the strength of the non-Hermitian term increases, or when the momentum dependence of the non-Hermitian term increases. Our results can be directly verified in synthetic non-Hermitian topological systems where the strength of the non-Hermitian terms can be controlled.

Motivated by recent progress in the realization of artificial gauge fields and SU($N$) Mott insulators using alkaline-earth-like atoms in optical lattices, we develop an unbiased SU($N$) real-space dynamical mean-field theory approach to study the effect of spin-orbit coupling and on-site Hubbard interaction $U$ on SU(3) fermionic systems. We investigate the behavior of the local magnetization, double occupancies, and the triple occupancy versus the Hubbard interaction across the metal to Mott insulator transition. We map out the magnetic phase diagram in the large-$U$ limit and show that the spin-orbit coupling can stabilize long-range orders such as ferromagnets, spirals, and stripes with different orientations in SU(3) Mott insulators.

We present details of materials synthesis, crystal structure, and anisotropic magnetic properties of single crystals of CeAlGe, a proposed type-II Weyl semimetal. Single-crystal x-ray diffraction confirms that CeAlGe forms in a noncentrosymmetric $I{4}_{1}md$ space group, in line with predictions of nontrivial topology. Magnetization, specific heat, and electrical transport measurements were used to confirm antiferromagnetic order below 5 K, with an estimated magnon excitation gap of $\mathrm{\Delta }=9.11$ K from heat capacity and hole-like carrier density of $1.4\times {10}^{20}$ ${\mathrm{cm}}^{-3}$ from Hall effect measurements. The easy magnetic axis is along the [100] crystallographic direction, indicating that the moment lies in the tetragonal $ab$ plane below 7 K. A spin-flop transition to less than 1 ${\mu }_B$/Ce is observed to occur below 30 kOe at 1.8 K in the $M\left(H\right)$ ($\mathbf{H}\parallel \mathbf{a}$) data. Small magnetic fields of 3 and 30 kOe are sufficient to suppress magnetic order when applied along the $a$ and $c$ axes, respectively, resulting in a complex $T–H$ phase diagram for $\mathbf{H}\parallel \mathbf{a}$ and a simpler one for $\mathbf{H}\parallel \mathbf{c}$.

While it is known that the charge-carrier mobility in amorphous metal oxide semiconductor thin film transistors (TFT) deviates from Arrhenius temperature dependence, we found that the Hall mobility measured in amorphous In-Ga-Zn-O (a-IGZO) follows an Arrhenius relation surprisingly well. We explain these observations by the effect of strong vertical electric field created by the gate voltage, which facilitates direct tunneling of trapped carriers into the conductive band and leads to virtually temperature independent mobility. We present a generalized Arrhenius model based on the effective temperature concept. We show that our model allows quantitative description of the temperature dependence of the mobility in a-IGZO TFTs over a broad temperature range.

The effect of an external in-plane electric field on neutral and charged exciton states in two-dimensional (2D) materials is theoretically investigated. These states are argued to be strongly bound, so that electron-hole dissociation is not observed up to high electric field intensities. Trions in the anisotropic case of monolayer phosphorene are demonstrated to be especially robust under electric fields, so that fields as high as 100 kV/cm yield no significant effect on the trion binding energy or probability density distribution. Polarizabilities of excitons are obtained from the parabolicity of numerically calculated Stark shifts. For trions, a fourth order Stark shift is observed, which enables the experimental verification of hyperpolarizability in 2D materials, as observed in the highly excited states of the Rydberg series of atoms and ions.

We present a detailed analysis of the role of native point defects in the antiferromagnetic (AFM) phases of bulk chromium nitride (CrN). We perform first-principles calculations by using the local spin-density approximation, including local interaction effects $\left(\text{LSDA}+U\right)$, to study the two lowest energy AFM models expected to describe the low-temperature phase of the material. We study the formation energies, lattice deformations, and electronic and magnetic structure introduced by native point defects. We find that, as expected, nitrogen vacancies are the most likely defect present in the material at low temperatures. Nitrogen vacancies present different charged states in the cubic AFM model, exhibiting two transition energies, which could be measured by thermometry experiments and could help identify the AFM structure in a sample. These vacancies also result in partial spin polarization of the induced impurity band, which would have interesting consequences in experiments. Other point defects have also signature electronic and magnetic structure that could be identified in scanning probe experiments.

In an ultrathin topological insulator (TI) film, a hybridization gap opens in the TI surface states, and the system is expected to become either a trivial insulator or a quantum spin Hall insulator when the chemical potential is within the hybridization gap. Here we show, however, that these insulating states are destroyed by the presence of a large and long-range-correlated disorder potential, which converts the expected insulator into a metal. We perform transport measurements in ultrathin dual-gated topological insulator films as a function of temperature, gate voltage, and magnetic field, and we observe a metalliclike nonquantized conductivity, which exhibits a weak antilocalizationlike cusp at low magnetic fields and gives way to a nonsaturating linear magnetoresistance at large fields. We explain these results by considering the disordered network of electron- and hole-type puddles induced by charged impurities. We argue theoretically that such disorder can produce an insulator-to-metal transition as a function of increasing disorder strength, and we derive a condition on the band gap and the impurity concentration necessary to observe the insulating state. We also explain the linear magnetoresistance in terms of strong spatial fluctuations of the local conductivity using both numerical simulations and a theoretical scaling argument.

We explore the properties of the SO(3) Majorana representation of spin. Based on its nonlocal nature, it is shown that there is an equivalence between the SO(3) Majorana representation and the Jordan-Wigner transformation in one and two dimensions. From the relation between the SO(3) Majorana representation and one-dimensional Jordan-Wigner transformation, we show that application of the SO(3) Majorana representation usually results in $Z_2$ gauge structure. Based on lattice Chern-Simons gauge theory, it is shown that the anticommuting link variables in the SO(3) Majorana representation make it equivalent to an operator form of compact ${\text{U(1)}}_1$ Chern-Simons Jordan-Wigner transformation in 2D. As examples of its application, we discuss two spin models, namely the quantum XY model on a honeycomb lattice and the ${90}^{\circ }$ compass model on a square lattice. It is shown that under the SO(3) Majorana representation both spin models can be exactly mapped into $Z_2$ gauge theory of spinons, with the standard form of $Z_2$ Gauss law constraint.

We have carried out inelastic neutron scattering experiments to study magnetic excitations in the ordered double perovskite ${\mathrm{Ca}}_2{\mathrm{FeReO}}_6$. We have found a well-defined magnon mode with a bandwidth of $\sim 50$ meV below the ferrimagnetic ordering temperature ($T_c\sim 520$ K), similar to the previously studied ${\mathrm{Ba}}_2{\mathrm{FeReO}}_6$. The spin excitation is gapless for most temperatures within the magnetically ordered phase. However, a spin gap of $\sim 10$ meV opens up below $\sim 150$ K, which is well below the magnetic ordering temperature but coincides with a previously reported metal-insulator transition and onset of structural distortion. The observed temperature dependence of the spin gap provides strong evidence for ordering of Re orbitals at $\sim 150$ K, in accordance with an earlier proposal put forward by K. Oikawa et al. based on neutron diffraction [J. Phys. Soc. Jpn. 72, 1411 (2003)] as well as recent theoretical work by Lee and Marianetti [Phys. Rev. B 97, 045102 (2018)]. The presence of separate orbital and magnetic ordering in ${\mathrm{Ca}}_2{\mathrm{FeReO}}_6$ suggests weak coupling between spin and orbital degrees of freedom and hints at a subdominant role played by spin-orbit coupling in describing its magnetism. In addition, we observed only one well-defined magnon band near the magnetic zone boundary, which is incompatible with simple ferrimagnetic spin waves arising from Fe and Re local moments but suggests a strong damping of the Re magnon mode.

Using density functional theory (DFT) calculations with the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06), we study the effects of hydrostatic pressure on the electronic properties of LaAs. We focus on the band crossing near the $X$ point that can make LaAs a topological semimetal, discussing results of both DFT within the generalized gradient approximation (GGA) and the HSE06 hybrid functional. We find that in DFT-GGA, under the calculated equilibrium lattice parameter, LaAs displays a crossing between the highest As $p$ band and the lowest La $d$ band near the $X$ point due to the overestimated $p\text{−}d$ band overlap. Such crossing does not occur when the band overlap is corrected in the HSE06 calculation. However, we find that the $p\text{−}d$ crossing can be induced in LaAs under hydrostatic pressure, showing a topological phase transition at $\sim 7\mathrm{GPa}$. The rocksalt crystal structure of LaAs is predicted to be stable under applied pressure up to 20 GPa, in good agreement with experimental observations.

In this Rapid Communication, a set of ${}^{209}\mathrm{Bi}$-nuclear magnetic resonance (NMR)/nuclear quadrupole resonance (NQR) measurements has been performed to investigate the physical properties of superconducting (SC) ${\mathrm{BaTi}}_2{\mathrm{Bi}}_2\mathrm{O}$ from a microscopic point of view. The NMR and NQR spectra at 5 K can be reproduced with a nonzero in-plane anisotropic parameter $\eta$, indicating the breaking of the in-plane fourfold symmetry at the Bi site without any magnetic order, i.e., “the electronic nematic state.” In the SC state, the nuclear spin-lattice relaxation rate divided by temperature, $1/T_{1}T$, does not change even below $T_{\mathrm{c}}$, while a clear SC transition was observed with a diamagnetic signal. This observation can be attributed to the strong two dimensionality in ${\mathrm{BaTi}}_2{\mathrm{Bi}}_2\mathrm{O}$. Comparing the NMR/NQR results among ${\mathrm{BaTi}}_{2}P{n}_2\mathrm{O}$ ($Pn=\text{As}$, Sb, and Bi), it was found that the normal and SC properties of ${\mathrm{BaTi}}_2{\mathrm{Bi}}_2\mathrm{O}$ were considerably different from those of ${\mathrm{BaTi}}_2{\mathrm{Sb}}_2\mathrm{O}$ and ${\mathrm{BaTi}}_2{\mathrm{As}}_2\mathrm{O}$, which might explain the two-dome structure of $T_{\mathrm{c}}$ in this system.