We investigate the new quantum phases on the extended Kane-Mele-Hubbard model of honeycomb lattice in the Hofstadter regime. In this regime, the orbital motion of the electrons can induce various topological phases with spontaneously broken symmetries when the spin-orbit coupling and electron correlations coexist. Here, we consider the interaction effects in the Kane-Mele model and discuss possible phases in the presence of magnetic field at integer fillings of electrons. In particular, focusing on the $2\pi /3$ magnetic flux per plaquette, the realization of numerous quantum phases are discussed within the mean-field framework: insulator with coplanar magnetic ordering, ferrimagnetic Chern insulator with nematic charge order, ferrimagnetic-ferrielectric Chern insulators, etc. Many of these phase transitions are also accompanied with a change in the topological invariants of the system. Based on our theoretical study, we propose topological multiferroic phases with a scope of realization in 2D van-der Waals materials and optical lattice systems where a significant interplay of magnetic field, spin-orbit coupling, and interactions can be engineered.

Incompressible even denominator fractional quantum Hall states at fillings $\nu =\pm \frac{1}{2}$ and $\nu =\pm \frac{1}{4}$ have been recently observed in monolayer graphene. We use a Chern-Simons description of multicomponent fractional quantum Hall states in graphene to investigate the properties of these states and suggest variational wave functions that may describe them. We find that the experimentally observed even denominator fractions and standard odd fractions (such as $\nu =1/3$, $2/5$, etc.) can be accommodated within the same flux attachment scheme and argue that they may arise from sublattice or chiral symmetry breaking orders (such as charge-density-wave and antiferromagnetism) of composite Dirac fermions, a phenomenon unifying integer and fractional quantum Hall physics for relativistic fermions. We also discuss possible experimental probes that can narrow down the candidate broken symmetry phases for the fractional quantum Hall states in the zeroth Landau level of monolayer graphene.

Topological insulators are promising for spintronics and related technologies due to their spin-momentum-locked edge states, which are protected by time-reversal symmetry. In addition to the unique fundamental physics that arises in these systems, the potential technological applications of these protected states have also been driving TI research over the past decade. However, most known topological insulator materials naturally contain spinful nuclei, and their hyperfine coupling to helical edge states intrinsically breaks time-reversal symmetry, removing the topological protection and enabling the buildup of dynamic nuclear spin polarization through hyperfine-assisted backscattering. Here, we calculate the scattering probabilities and nuclear polarization for edge channels containing up to 34 nuclear spins using a numerically exact analysis that exploits the symmetries of the problem to drastically reduce the computational complexity. We then show the emergence of universal scaling properties that allow us to extrapolate our findings to vastly larger and experimentally relevant system sizes. We find that significant nuclear polarization can result from relatively weak helical edge currents, suggesting that it may be an important factor affecting spin transport in topological insulator devices.

We measure microwave photocurrent in devices made from InAs/GaInSb bilayers where both the insulating bulk state and conducting edge state were observed in the inverted-band regime, consistent with the theoretical prediction for a quantum spin Hall (QSH) insulator. It has been theoretically proposed that microwave photocurrent could be a unique probe in studying the properties of QSH edge states. To distinguish a possible photoresponse between a bulk state and helical edge state, we prepare a Hall bar and Corbino disk from the same wafer. Results show that the Corbino-disk samples have a negligible photocurrent in the bulk gap, while clear photocurrent signals from the Hall-bar samples are observed. This finding suggests that the photocurrent may carry information concerning the electronic properties of the edge states.

The quantum critical regimes (QCR) of both a two-dimensional (2D) disordered spin-$\frac{1}{2}$ antiferromagnet and a 2D clean dimerized quantum Heisenberg model are studied using the first-principles nonperturbative quantum Monte Carlo simulations. In particular, the three well-known universal coefficients associated with QCR are investigated in detail. While in our investigation we find the obtained results are consistent with the related analytic predictions, non-negligible finite-temperature ($T$) effects are observed. Moreover, a striking finding in our study is that the numerical value for one of the universal coefficients we determine is likely to be different significantly from the corresponding (theoretical) result(s) established in the literature. To better understand the sources for the discrepancy observed here, apart from carrying out the associated analytic calculations not considered previously, it will be desirable as well to conduct a comprehensive examination of the exotic features of QCR for other disordered and clean spin systems than those investigated in this study.

In this paper, we present the numerical and experimental demonstration of asymmetric light transmission by a compact photonic crystal (PC) structure, which sustains semi-Dirac cone dispersion. The semi-Dirac point is obtained by exploiting plane-wave expansion method in a rectangular unit cell of a cylindrical dielectric rod, forming a PC structure. The preselected part of the corresponding PC structure is optimized via differential evolution to maximize the transmission efficiency in one direction, and to minimize in the opposite direction. In this regard, finite-difference time-domain method is integrated with the optimization algorithm to numerically design a compact PC structure with asymmetric light transmission effect. The detailed numerical investigation of the optimized PC structure is represented, and the experiments in the microwave regime are performed to verify the numerical results. The experimentally measured transmission efficiency reaches 78% for one direction, whereas in the opposite direction it reduces to 11%. The physical mechanism of asymmetric light transmission is related to differences of the projection of the wave incident from both directions into the eigenmodes of the photonic structure.

In the present work we explain the anomalous behavior of the attenuation of the longitudinal acoustic phonon in GaAs as a function of the phonon energy $\omega$ in the subterahertz domain. These attenuations along the [100] direction show a plateau between 0.6 and 1 GHz at low temperatures. We found an excellent agreement between measurements performed by some of us, and new ab initio calculations of third-order anharmonic processes. The formation of the plateau is explained by the competition between different phonon-phonon scattering processes such as Herring's mechanism, which dominates at low frequencies, saturates, and disappears. The plateau is shown to be determined by the phononic final-state phase space available at a given temperature. We predict that a change of scattering mechanism should also show up in the attenuation of silicon around 1.2–1.7 THz, and argue that the attenuation plateau is a general feature of cubic semiconductors.

We use tunneling spectroscopy to study the evolution of few-electron spin states in parallel InAs nanowire double quantum dots (QDs) as a function of level detuning and applied magnetic field. Compared to the much more studied serial configuration, parallel coupling of the QDs to source and drain greatly expands the probing range of excited-state transport. Owing to a strong confinement, we can here isolate transport involving only the very first interacting single QD orbital pair. For the (2,0)-(1,1) charge transition, with relevance for spin-based qubits, we investigate the excited (1,1) triplet, and hybridization of the (2,0) and (1,1) singlets. An applied magnetic field splits the (1,1) triplet, and due to spin-orbit induced mixing with the (2,0) singlet we clearly resolve transport through all triplet states near the avoided singlet-triplet crossings. Transport calculations, based on a simple model with one orbital on each QD, fully replicate the experimental data. Finally, we observe an expected mirrored symmetry between the one- to two-electron and two- to three-electron transitions resulting from the twofold spin degeneracy of the orbitals.

We present a microscopic theory of spin dynamics in weakly disordered graphene with uniform proximity-induced spin-orbit coupling (SOC). A time-dependent perturbative treatment is employed to derive the spin Bloch equations governing the spin dynamics at high electronic density for arbitrary ratio ${\lambda }_{\mathrm{SOC}}/\eta$, where $\eta$ is the disorder-induced quasiparticle broadening and ${\lambda }_{\mathrm{SOC}}$ is the largest spin-orbit energy scale. Rich scenarios are predicted, depending on a delicate competition between interface-induced Bychkov-Rashba and spin-valley interaction. In the motional narrowing regime of weak SOC (${\lambda }_{\mathrm{SOC}}\ll \eta$), the anisotropy ratio of out-of-plane to in-plane spin lifetimes $\zeta ={\tau }_s^{\perp }/{\tau }_s^{\parallel }$ agrees qualitatively with a toy model of spins in a fluctuating SOC field proposed recently by Cummings and co-workers Phys. Rev. Lett. 119, 206601 (2017). For well-resolved SOC (${\lambda }_{\mathrm{SOC}}\gtrsim \eta$), the spin dynamics is characterized by fast damped oscillations with spins relaxing on the timescale of a single scattering event. In this regime, qualitatively different formulas for $\zeta$ are obtained, which can be useful to model spin transport in ultraclean van der Waals heterostructures.

Exchange-only spin qubits hosted in ${}^{28}\mathrm{Si}$-based triple quantum dots do not suffer from decoherence caused by randomly fluctuating nuclear-spin ensembles and can be relatively robust against electrical noise when operated at a sweet spot. Remaining sources of decoherence are qubit relaxation, leakage out of the qubit subspace, and dephasing due to residual effects of charge noise, the latter two of which are the focus of this work. We investigate spin-orbit-mediated leakage rates to the three-spin ground state accompanied by virtual (i) tunneling, (ii) orbital excitation, and (iii) valley excitation of an electron. We find different power-law dependencies on the applied magnetic field $B$ for the three mechanisms as well as for the two leakage rates, ranging from $\propto B^5$ to $\propto B^{11}$, and identify the sweet spot as a point of minimal leakage. We also revisit the role of electrical noise at the sweet spot and show that it causes a decay of coherent qubit oscillations that follows a power law $\propto 1/t$ (as opposed to the more common exponential decay) and introduces a $\pi /2$ phase shift.

We investigate the effects of spin-orbit coupling on the optical response of materials. In particular, we study the effects of the commutator between the spin-orbit coupling part of the potential and the position operator on the optical matrix elements using density functional theory calculations within the generalized gradient approximation. By means of a formalism that separates a fully relativistic Kleinman-Bylander pseudopotential into the scalar-relativistic and spin-orbit-coupling parts, we calculate the contribution of the commutator arising from spin-orbit coupling to the squared optical matrix elements of isolated atoms, monolayer transition-metal dichalcogenides, and topological insulators. In the case of isolated atoms from H ($Z=1$) to Bi ($Z=83$), the contribution of spin-orbit coupling to the squared matrix elements can be as large as 14%. On the other hand, in the cases of monolayer transition-metal dichalcogenides and topological insulators, we find that this contribution is less than 1% and that it is sufficient to calculate the optical matrix elements and subsequent physical quantities without considering the commutator arising from spin-orbit coupling.

The known first-order orientational phase transition in pure ${\mathrm{C}}_{60}$ occurs at 260 K and is accompanied by the change from face-centered cubic $\left(Fm3m\right)$ to simple cubic unit cell $\left(Pa\overline{3}\right)$. While ${\mathrm{C}}_{60}$ molecules rotate freely above 260 K, they are fixed in inversion centers below 260 K. The phase transition in methane-intercalated ${\mathrm{C}}_{60}$, ${\left(\mathrm{C}{\mathrm{H}}_4\right)}_x\left({\mathrm{C}}_{60}\right)$, occurs at $236\pm 2\mathrm{K}$. It was studied by DSC and single crystal x-ray diffraction. Above 236 K, the intercalate adopts the face-centered cubic structure which is characterized by freely rotating ${\mathrm{C}}_{60}$ and disordered ${\mathrm{CH}}_4$ molecules. The ordered low-temperature crystal structure possesses simple cubic unit cell in the space group $Pa\overline{3}$ in which both ${\mathrm{C}}_{60}$ and ${\mathrm{CH}}_4$ molecules are fixed on threefold inversion axes. In the structure of ${\left(\mathrm{C}{\mathrm{H}}_4\right)}_{0.72}{\mathrm{C}}_{60}$ at 100 K, a ${\mathrm{C}}_{60}$ molecule is fixed by its 12 ${\mathrm{C}}_{60}$ neighbors due to donor-acceptor van der Waals interactions between electron-rich 6:6 C-C bonds and electron-deficient cage pentagons. At 200 K, the second orientation of a ${\mathrm{C}}_{60}$ molecule is present (with 7.5% occupancy) in which 6:6 C-C bonds are directed to cage hexagons. The methane molecules are situated in octahedral holes and statistically disordered over two orientations around inversion centers. The phase transition temperature of $\left(\mathrm{C}{\mathrm{H}}_4\right){\mathrm{C}}_{60}$ intercalate is lower than in ${\mathrm{C}}_{60}$ by $\sim 24\mathrm{K}$ due to the increased ${\mathrm{C}}_{60}\cdots {\mathrm{C}}_{60}$ separation. This study provides a reliable structural information about the low-temperature structure of the ${\left(\mathrm{C}{\mathrm{H}}_4\right)}_{0.7}\left({\mathrm{C}}_{60}\right)$ intercalate thus demonstrating significant differences between orientational phase transitions in pure fullerite ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{60}$ intercalated with methane. In addition, the molecular structure of ${\mathrm{C}}_{60}$ fullerene in the ${\left(\mathrm{C}{\mathrm{H}}_4\right)}_{0.7}{\mathrm{C}}_{60}$ intercalate is distinguished by a high precision which is superior to the former experimental data for ${\mathrm{C}}_{60}$ and its adducts reported in the literature.

The average effective mass of charge carriers produced by an intense ultrashort laser pulse in a transparent solid increases significantly as the excitation mechanism changes from multiphoton transitions to interband tunneling. We theoretically investigate this phenomenon for several dielectrics and semiconductors. For diamond as a representative dielectric, we present a detailed analysis of the laser-induced change of optical properties. When the concentration of free carriers is high, we find that the average effective mass controls not only the intraband charge-carrier transport but also the interband contributions to the optical response. We observe that the excitation-induced birefringence is particularly large for parameters for which the plasma response compensates for the linear response of an unperturbed solid.

Spins in molecular magnets can experience both anisotropic exchange interactions and on-site magnetic anisotropy. In this paper, we study the effect of exchange anisotropy on the molecular magnetic anisotropy both with and without on-site anisotropy. When both the anisotropies are small, we find that the axial anisotropy parameter $D_M$ in the effective spin Hamiltonian is the sum of the individual contributions due to exchange and on-site anisotropies. We find that even for axial anisotropy of about $15\%$, the low-energy spectrum does not correspond to a single-parent spin manifold but has intruder states arising from other parent spins. In this case, the low-energy spectrum cannot be described by multiplet states arising from a single approximate total spin state. We study the magnetic susceptibility, specific heat as a function of temperature, and magnetization as a function of applied field to characterize the system in this limit. We find that there is synergy between the two anisotropies, particularly for large systems with higher site spins.

Measured switching voltage-field diagrams of perpendicular magnetic tunnel junctions exhibit unexpected behavior at high voltages associated with significant heating of the storage layer. The boundaries deviate from the critical lines corresponding to the coercive field, which contrasts with the theoretically predicted behavior of a standard macrospin-based model. Combining recent experimental studies of the temperature dependence of spin polarization and perpendicular magnetic anisotropy, we are proposing a modified model. Our approach takes into account the Joule heating during the writing pulse, which reduces the spin polarization and the anisotropy, thereby reducing the spin torque efficiency and the coercive field during the switching. The numerical macrospin simulations based on this model are in agreement with our experimental measurements and consistent with the results derived from the linearization of Landau-Lifshitz-Gilbert equation.

We successfully synthesized a verdazyl-based charge-transfer salt $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$, which has $S_{\mathrm{V}}=1/2$ on the radical $o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2$ and $S_{\mathrm{Fe}}=5/2$ on the ${\mathrm{FeCl}}_4$ anion. Ab initio molecular orbital calculations indicate the formation of an $S_{\mathrm{V}}=1/2$ honeycomb lattice composed of three types of exchange interaction with two types of inequivalent sites. Further, the $S_{\mathrm{V}}=1/2$ at one site is sandwiched by $S_{\mathrm{Fe}}=5/2$ spins through antiferromagnetic (AF) interactions. The magnetic properties indicate that the dominant AF interactions between the $S_{\mathrm{V}}=1/2$ spins form a gapped singlet state, and the remaining $S_{\mathrm{Fe}}=5/2$ spins cause an AF order. The magnetization curve exhibits a linear increase up to approximately 7 T, and an unconventional 5/6 magnetization plateau appears between 7 and 40 T. We discuss the differences between the effective interactions associated with the magnetic properties of the present compound and ($o$-MePy-V)${\mathrm{FeCl}}_4$. We explain the low-field linear magnetization curve through a mean-field approximation of an $S_{\mathrm{Fe}}=5/2$ spin model. At higher field regions, the 5/6 magnetization plateau and subsequent nonlinear increase are reproduced by the $S_{\mathrm{V}}=1/2$ AF dimer, in which a particular internal field is applied to one of the spin sites. The ESR signals in the low-temperature and low-field regime are explained by conventional two-sublattice AF resonance modes with easy-axis anisotropy. These results demonstrate that exchange interactions between $S_{\mathrm{V}}=1/2$ and $S_{\mathrm{Fe}}=5/2$ spins in $\left[o\text{−}\mathrm{MePy}\text{−}\mathrm{V}\text{−}{\left(p\text{−}\mathrm{Br}\right)}_2\right]{\mathrm{FeCl}}_4$ realize unconventional magnetic properties with low-field classical behavior and field-induced quantum behavior.

We investigate the impact of introducing Majorana bound states, formed by a proximitized semiconducting nanowire in the topological regime, into a current biased capacitive Josephson junction, thereby adding delocalized states below the superconducting gap. We find that this qualitatively changes the charge dynamics of the system, diminishing the role of Bloch oscillations and causing single-particle tunneling effects to dominate. We fully characterize the resulting charge dynamics and the associated voltage and current signals. Our work reveals a rich landscape of behaviors in both the static and time-varying driving modes. This can be directly attributed to the presence of Majorana bound states, which serve as a pathway for charge transport and enable nonequilibrium excitations of the Majorana-Josephson device.

Response functions $\langle {\widehat{A}}_x\left(t\right){\widehat{B}}_y\left(0\right)\rangle$ for one-dimensional strongly correlated quantum many-body systems can be computed with matrix product state (MPS) techniques. Especially, when one is interested in spectral functions or dynamic structure factors of translation-invariant systems, the response for some range $|x-y|<\ell$ is needed. We demonstrate how the number of required time-evolution runs can be reduced substantially: (a) If finite-system simulations are employed, the number of time-evolution runs can be reduced from $\ell$ to $2\sqrt{\ell }$. (b) To go beyond, one can employ infinite MPS (iMPS) such that two evolution runs suffice. To this purpose, iMPS that are heterogeneous only around the causal cone of the perturbation are evolved in time, i.e., the simulation is done with infinite boundary conditions. Computing overlaps of these states, spatially shifted relative to each other, yields the response functions for all distances $|x-y|$. As a specific application, we compute the dynamic structure factor for ground states of bilinear-biquadratic spin-1 chains with very high resolution and explain the underlying low-energy physics. To determine the initial uniform iMPS for such simulations, infinite-system density matrix renormalization group (iDMRG) can be employed. We discuss that, depending on the system and chosen bond dimension, iDMRG with a cell size $n_c$ may converge to a nontrivial limit cycle of length $m$. This then corresponds to an iMPS with an enlarged unit cell of size $m{n}_c$.

Recently, an anomalous generation of multiple plasmons with large spectral weight transfer in the visible to ultraviolet range (energies below the band gap) has been experimentally observed in the insulatinglike phase of oxygen-rich strontium niobium oxides (${\mathrm{SrNbO}}_{3+\delta }$). Here, we investigate the ground-state and dielectric properties of ${\mathrm{SrNbO}}_{3+\delta }$ as a function of $\delta$ by means of extensive first-principle calculations. We find that in the random phase approximation by taking into account local field effects, our calculations are able to reproduce both the unconventional multiple generations of plasmons and the spectral weight transfers, consistent with experimental data. Interestingly, these unconventional plasmons can be tuned by oxygen stoichiometry as well as microscopic superstructure. This unusual predominance of local field effects in this class of materials is ascribed to the strong electronic inhomogeneity and high polarizability and paves a new path to induce multiple plasmons in the untapped visible to ultraviolet ranges of insulatinglike oxides.