Experimental investigation of the weak antilocalization (WAL) effect on the Hall resistivity is quite rare, because the WAL is known to have no influence on the Hall effect in a single-band system. We challenge this view in a system that has both electrons and holes by deriving a two-band model modified by the WAL effect and by applying it to the low-field magnetoresistance (MR) and Hall resistance (HR) of $\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{S}{\mathrm{b}}_x$ single crystals $(0\le x\le 17.0\%)$. Simultaneous occurrence of a dip in MR and nonlinearity in HR suggests that the $\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{S}{\mathrm{b}}_x$ is a rare three-dimensional system, in which WAL and two distinct charge carriers interplay. The modified two-band model describes all the main features of MR and HR that are not captured by the conventional theory. From the quantitative analysis based on the modified theory, the values of key parameters, such as carrier density and mobility of electrons and holes, are estimated. The modified two-band model provides a solid framework for understanding electrical transport phenomena of a material with strong spin-orbit interaction and multiple distinct charge carriers.

We consider periodically driven potential impurities coupled to the surface states of a two-dimensional topological insulator. The problem is addressed by means of two models, of which the first model is an effective continuum Hamiltonian for the surface states, whereas the Kane-Mele lattice model is our second approach. While both models result in drastic changes in the local density of electron states with increasing amplitude and frequency of the driving field, the linearly low-energy local density of electron states remains in the continuum model, however, with an increased Fermi velocity. The spectrum of the continuum model remains gapless under the emergence of new impurity resonances near the Fermi energy. The Kane-Mele lattice model represents a finite size system, with edge states appearing at the boundary of the system. We, thus, consider the impurity at two different positions, one at the boundary and one at the center of the lattice. In the former case, a reduction and broadening of the low-energy local density of electron states result with increasing amplitude of the driving field. On the other hand, there are no new resonances emerging in the spectrum. In the latter case, the spectrum is gapped both in the absence of the impurity as well as for weak amplitudes of the driving field, while the gap tends to fill up with impurity states with increasing amplitude.

In the field of quantum magnetism, the advent of numerous spin-orbit assisted Mott insulating compounds, such as the family of Kitaev materials, has led to a growing interest in studying general spin models with nondiagonal interactions that do not retain the SU(2) invariance of the underlying spin degrees of freedom. However, the exchange frustration arising from these nondiagonal and often bond-directional interactions for two- and three-dimensional lattice geometries poses a serious challenge for numerical many-body simulation techniques. In this paper, we present an extended formulation of the pseudofermion functional renormalization group that is capable of capturing the physics of frustrated quantum magnets with generic (diagonal and off-diagonal) two-spin interaction terms. Based on a careful symmetry analysis of the underlying flow equations, we reveal that the computational complexity grows only moderately, as compared to models with only diagonal interaction terms. We apply the formalism to a kagome antiferromagnet which is augmented by general in-plane and out-of-plane Dzyaloshinskii-Moriya (DM) interactions, as argued to be present in the spin liquid candidate material herbertsmithite. We calculate the complete ground state phase diagram in the strength of in-plane and out-of-plane DM couplings, and discuss the extended stability of the spin liquid of the unperturbed kagome antiferromagnet in the presence of these couplings.

In this study, we show the divacant substitution of Fe impurities atom produces the formation of an electron spin channel along the armchair direction of graphene. This spin channel creates a directional dependent spin exchange between impurities. Using density functional theory, we simulated the electronic and magnetic properties for a supercell of graphene with spatial variation of the Fe atoms along either the armchair or zigzag directions. Overall, we find that the exchange interaction between the two Fe atoms fluctuates from ferromagnetic to antiferromagnetic as a function of the spatial distance in the armchair direction. Given the induced magnetic moment and increased density of states at the Fermi level by the surrounding carbon atoms, we conclude that an RKKY-like interaction may characterize the exchange interactions between the Fe atoms. Furthermore, we examined the same interactions for Fe atoms along the zigzag direction in graphene and found no evidence for an RKKY interaction as this system shows a standard exchange between the transition-metal impurities. Therefore, we determine that the spin channel produced through Fe-substitution in graphene induces a directional-dependent spin interaction, which may provide stability to spintronic and multifunctional devices and applications for graphene.

Nitrogen-vacancy (NV) centers in diamond have been used in recent years for a wide range of applications, from nanoscale NMR to quantum computation. These applications depend strongly on the efficient readout of the NV center's spin state, which is currently limited. Here we suggest a method of reading the NV center's spin state, using the weak optical transition in the singlet manifold. We numerically calculate the number of photons collected from each spin state using this technique, and show that an order of magnitude enhancement in spin-readout signal-to-noise ratio is expected, making single-shot spin readout within reach. Thus, this method could lead to an order of magnitude enhancement in sensitivity for ubiquitous NV-based sensing applications, and remove a major obstacle from using NVs for quantum information processing.

$\mathrm{F}{\mathrm{e}}_4\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_9$ was recently reported to be a new magnetoelectric material with two distinct dielectric anomalies located at $T_N\approx 90\mathrm{K}$ for an antiferromagnetic transition and $T_{\mathrm{str}}\approx 77\mathrm{K}$ of unknown origin, respectively. By analyzing low-temperature neutron-powder-diffraction data, here we determined its magnetic structure below $T_N$ and uncovered the origin of the second dielectric anomaly as a structural phase transition across $T_{\mathrm{str}}$. In the antiferromagnetically ordered state below $T_N$, both Fe1 and Fe2 magnetic moments lying within the weakly and strongly buckled honeycomb layers are arranged in a fashion that the three nearest neighbors are directed oppositely. Upon cooling below $T_{\mathrm{str}}$, the symmetry of crystal structure is lowered from trigonal $P-3c1$ to monoclinic $C2/c$, in which a weak sliding of the metal octahedral planes introduces a monoclinic distortion of $\sim 1.7^{\circ }$. The magnetic structure is preserved in the low-temperature monoclinic phase, and the Fe magnetic moment increases from $2.1\left(1\right){\mu }_B$ at 95 K to $3.83\left(4\right){\mu }_B$ at 10 K assuming an equal moment configuration at Fe1 and Fe2 sites. The magnetic point group and linear magnetoelectric tensor at each temperature region are determined. From a symmetry-related tensor analysis, the microscopic origins of the magnetoelectric effects between $T_N$ and $T_{\mathrm{str}}$ are proved to be spin-current and $d-p$ hybridization mechanisms.

We study analytically the dynamics of vortices in strongly coupled exciton-photon fields in the presence of energy detuning. We derive equations for the vortex core velocity and mass, where they mainly depend on Rabi coupling and the relative distance between the vortex cores in photon and exciton fields, and as a result, core positions oscillate in each field. We use Magnus force balanced with a Rabi-induced force to show that the core of the vortex behaves as an inertial-like particle. Our analysis reveals that the core is lighter on the periphery of the beam and therefore it is faster in that region. While detuning induces oscillations in population imbalance of components through the relative phase between coupled fields, in the presence of topological charges detuning can control the orbital dynamics of the cores. Namely, it causes the vortex core to move in larger or smaller orbits with different velocities and changes angular momentum and energy content of the vortex field.

The current-induced magnetization dynamics is an important subject in spintronics research. It contains many interesting physical phenomena and has a high potential for spintronic applications. Most magnetic memory devices are based on the magnetization switching operation. Thus, finding the switching mechanism, as well as the exact understanding of it, is an essential implement for spintronic devices. Here, we show that the current can switch the magnetization via chiral buckling. To destabilize the uniform magnetization, a critical current value is needed. The Dzyaloshinskii-Moriya interaction (DMI) produces a shift in this critical current and the sign of this shift also depends on the domain polarity. Thus, a certain current can select a certain domain polarity. During the switching process, a fluctuation of magnetization is exponentially amplified by the current that forms the magnetization buckling. We explain these phenomena by linear stability analysis and show that two distinct switching operations are possible depending on the DMI.

We report twisted magnetization phases, i.e., exchange springs, across interfaces in a semiconducting NdN/GdN superlattice and compare the exchange springs to those reported previously across SmN/GdN interfaces. Like SmN, NdN has an orbital moment larger than and opposing its spin magnetic moment, resulting in exchange-Zeeman competition that in turn precipitates the exchange springs. In contrast to SmN, NdN has a large net magnetic moment, adding more complexity to the exchange-spring behavior. We investigate how different temperatures and applied magnetic fields influence the twisted magnetization using complementary magnetometry and element specific x-ray magnetic circular dichroism. The exchange spring can be driven into two different phases, with either the hard NdN or soft GdN entering the twisted phase. This tunable twisted magnetization in ferromagnetic semiconductors could open up a pathway to control tunneling magnetoresistance in spintronics devices, encouraging further investigation of these new types of exchange springs.

Terbium gallium garnet (TGG), ${\mathrm{Tb}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$, is well known for its applications in laser optics but also exhibits complex low-temperature magnetism that is not yet fully understood. Its low-temperature magnetic order is determined by means of time-of-flight neutron powder diffraction. It is found to be a multiaxial antiferromagnet with magnetic ${\mathrm{Tb}}^{3+}$ ions forming six sublattices of magnetic moments aligned parallel and antiparallel to the $\langle 100\rangle$ crystallographic directions of the cubic unit cell. The structure displays strong easy-axis anisotropy with respect to a twofold axis of symmetry in the local orthorhombic environment of the ${\mathrm{Tb}}^{3+}$ sites. The crystal-field splitting within the single-ion ground-state manifold is investigated by inelastic neutron scattering on powder samples. A strong temperature dependence of the quasidoublet ground state is observed and revised parameters of the crystal-field Hamiltonian are given. The results of bulk magnetic susceptibility and magnetization measurements are in good agreement with values based on the crystal-field model down to 20 K, where the onset of magnetic correlations is observed.

The structure of liquid gallium (Ga) is a long-standing issue in the studies of metallic liquids. In this paper we investigate pure Ga melt in the temperature range of 307–1356 K by high-temperature ${}^{69}\mathrm{Ga}$ and ${}^{71}\mathrm{Ga}$ NMR. A structural crossover at around 1000 K is revealed through the analyses of NMR observables including the Knight shift and the spin-lattice relaxation time, which may result from the temperature-induced structural evolution of the covalently bonded clusters in pure Ga melt. However, structure and diffusivity anomalies around 450 and 730 K at ambient pressure, which may be induced by the liquid-liquid critical point, are not observed.

We have used the stochastic series expansion quantum Monte Carlo method to study interacting hard-core bosons on the square lattice with pair-hopping processes supplementing the standard single-particle hopping. Such pair hopping arises in effective models for frustrated quantum magnets. Our goal is to investigate the effects of the pair-hopping process on the commonly observed superfluid, insulating (Mott), and supersolid ground-state phases in the standard hard-core boson model with various interaction terms. The model is specifically motivated by the observation of finite dispersion of two-magnon bound states in neutron-diffraction experiments ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$. Our results show that the pair hopping has different effects on Mott phases at different filling fractions, “melting” them at different critical pair-hopping amplitudes. Thus, it appears that pair hopping may play an important role in determining which out of a potentially large number of Mott phases (stabilized by details of the charge-diagonal interaction terms) actually survive the totality of quantum fluctuations present.

Dopants, even single atoms, can influence the electrical and magnetic properties of materials. Here we demonstrate the opportunity for detecting the magnetic response of an embedded magnetic impurity in a nonmagnetic host material. We combine a depth sectioning approach with electron magnetic circular dichroism in scanning transmission electron microscopy to compute the depth-resolved magnetic inelastic-scattering cross section of single Co impurity buried in the host crystal of GaAs. Our calculations suggest that the magnetic dichroic signal intensity is sensitive to the depth and lateral position of the electron probe relative to the magnetic impurity. Additionally, a more precise dichroic signal localization can be achieved via choosing higher-collection-angle ($\beta$) apertures. Quantitative evaluation of the inelastic-scattering cross section and signal-to-noise ratio indicates that the magnetic signal from a single Co atom is on the verge of being detectable with today's state-of-the-art instrumentation.

We investigate the chemical substitution of group 5 into $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ (“122”) iron arsenide, in the effort to understand why Fe-site hole doping of this compound (e.g., using group 5 or 6) does not yield bulk superconductivity. We find an increase in the $c$-lattice parameter of the $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ with the substitution of V, Nb, or Ta; the reduction in $c$ predicts the lack of bulk superconductivity [Konzen and Sefat, J. Phys.: Condens. Matter 29, 083001 (2017)] that is confirmed here through transport and magnetization results. However, our spectroscopy measurements find a coexistence of antiferromagnetic and local superconducting nanoscale regions in V-122, observed in a transition-metal hole-doped iron arsenide. In $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$, there is a complex connection between local parameters such as composition and lattice strain, average lattice details, and the emergence of bulk quantum states such as superconductivity and magnetism.

We describe the use of coupled-cluster theory as an impurity solver in dynamical mean-field theory (DMFT) and its cluster extensions. We present numerical results at the level of coupled-cluster theory with single and double excitations (CCSD) for the density of states and self-energies of cluster impurity problems in the one- and two-dimensional Hubbard models. Comparison to exact diagonalization shows that CCSD produces accurate density of states and self-energies at a variety of values of $U/t$ and filling fractions. However, the low cost allows for the use of many bath sites, which we define by a discretization of the hybridization directly on the real frequency axis. We observe convergence of dynamical quantities using approximately 30 bath sites per impurity site, with our largest 4-site cluster DMFT calculation using 120 bath sites. We suggest that coupled-cluster impurity solvers will be attractive in ab initio formulations of dynamical mean-field theory.

We perform large-scale projector determinant quantum Monte Carlo simulations to study the insulating states of the half-filled SU(6) Hubbard model on the square lattice. The transition from the antiferromagnetic state to the valence bond solid state occurs as increasing the Hubbard $U$. In contrast, in the SU(2) and SU(4) cases antiferromagnetism persists throughout the entire interaction range. In the SU(6) case, antiferromagnetism starts to develop in the weak interacting regime based on the Slater mechanism of Fermi surface nesting. As $U$ passes a crossover value $U^{*}/t\approx 9$, the single-particle gap scales linearly with $U$, marking the onset of Mott physics. In the Mott regime, antiferromagnetism becomes to be suppressed as $U$ increases and vanishes after $U$ passes the critical value $U_{\mathrm{AF},\mathrm{c}}/t=13.3\pm 0.05$. The critical exponents are obtained via critical scalings as ${\nu }_{\mathrm{AF}}=0.60\pm 0.02$ and ${\eta }_{\mathrm{AF}}=0.44\pm 0.03$. As $U$ further increases, the valence bond solid ordering appears exhibiting the anomalous dimension ${\eta }_{\mathrm{VBS}}=0.98\pm 0.01$.

A recent measurement of the optical conductivity in graphene [Gallagher, Yang, Lyu, Tian, Kou, Zhang, Watanabe, Taniguchi, and Wang, Science 364, 158 (2019)] offers a possibility of experimental determination of microscopic time scales describing scattering processes in the electronic fluid. In this paper, I report a theoretical calculation of the optical conductivity in graphene at arbitrary doping levels, within the whole “hydrodynamic” temperature range, and for arbitrary nonquantizing magnetic fields. The obtained results are in good agreement with the available experimental data.

We find that the first-order quantum phase transitions (QPTs) are characterized by intrinsic jumps of relevant operators while the continuous ones are not. Based on such an observation, we propose a bond reversal method where a quantity $\mathcal{D}$, the difference of bond strength (DBS), is introduced to judge whether or not a QPT is of first order. This method is first applied to an exactly solvable spin-$1/2$ XXZ Heisenberg chain and a quantum Ising chain with a longitudinal field where distinct jumps of $\mathcal{D}$ appear at the first-order transition points for both cases. We then use it to study the topological QPT of a cross-coupled ($J_{\times }$) spin ladder where the Haldane–rung-singlet transition switches from being continuous to exhibiting a first-order character at $J_{\times ,I}\simeq 0.30\left(2\right)$. Finally, we study a recently proposed one-dimensional analogy of a deconfined quantum critical point connecting two ordered phases in a spin-$1/2$ chain. We rule out the possibility of a weakly first-order QPT because the DBS is smooth when crossing the transition point. Moreover, we affirm that such a transition belongs to the Gaussian universality class with a central charge $c=1$.

In the search for stable topological semimetals with clean band profiles, we have screened all the $3d$ metal-intercalated transition metal dichalcogenides (3dI-TMDCs) by performing hybrid-functional-based ab initio calculations. Two classes of topological materials featuring 12 Weyl points (WPs) in the $k_z=0$ plane (without spin-orbit interactions) are identified: (a) ferromagnetic Weyl semimetals $\mathrm{V}{T}_3{X}_6$ and (b) spinless Weyl semimetals $\mathrm{Mn}{T}_3{X}_6$ (nonmagnetic), where $T=\text{Nb}$, Ta and $X=\text{S}$, Se. ${\mathrm{VNb}}_3{\mathrm{S}}_6$, prototypical of class (a), is half metallic with only two bands crossing at the Fermi level to form WPs. If spin-orbit interactions are included, ${\mathrm{VNb}}_3{\mathrm{S}}_6$ is a Weyl semimetal with only two WPs for magnetization $\mathbf{m}$ in the basal plane, whereas it is a ferromagnetic topological insulator for $\mathbf{m}$ normal to the plane. ${\mathrm{MnNb}}_3{\mathrm{S}}_6$ in the nonmagnetic phase is essentially a spinless version of ${\mathrm{VNb}}_3{\mathrm{S}}_6$ featuring an equally clean band profile. Interestingly, both materials exhibit a quasinodal line connecting the WPs, which we explain in terms of the quasimirror symmetry and the orbital nature of the bands. 3dI-TMDCs are chemically and thermally stable stoichiometric compounds containing no toxic elements and are a viable platform for the study of topological condensed-matter physics.