Nuclear spins in the proximity of electronic spin defects in solids are promising platforms for quantum information processing due to their ability to preserve quantum states for a remarkably long time. Here we report a comprehensive ab initio study of the nuclear spin dynamics in solid-state systems. First, we characterize spin exchange-dependent oscillations of the Hahn-echo signal of the single nuclear spins in isonuclear spin baths pointing at a new sensing modality of dynamical-decoupling spectroscopy. Using first-principles simulations, we then quantify the enhancement in the coherence of nuclear spins as a function of distance and state of the electron spin and validate our results with experimental data for the nitrogen vacancy in diamond. Finally, we describe how hybridization of the electronic states suppresses the coherence time of strongly coupled nuclear spins and how dynamical changes of the electron spin state may deteriorate nuclear coherence. The computational framework developed in our work is general and can be broadly applied to predict the dynamical properties of nuclear spins in a wide variety of systems. Overall, our results elucidate many pitfalls that should be avoided to preserve the nuclear spin state in solid-state systems.

Motivated by the recent experimental realization of the half-quantized Hall effect phase in a three-dimensional (3D) semimagnetic topological insulator [M. Mogi et al., Nat. Phys. 18, 390 (2022)], we propose a scheme for realizing the half-quantized Hall effect and axion insulator in experimentally mature 3D topological insulator heterostructures. Our approach involves optically pumping and/or magnetically doping the topological insulator surface, such as to break time reversal and gap out the Dirac cones. By toggling between left and right circularly polarized optical pumping, the sign of the half-integer Hall conductance from each of the surface Dirac cones can be controlled, such as to yield half-quantized ($0+1/2$), axion ($-1/2+1/2=0$), and Chern ($1/2+1/2=1$) insulator phases. We substantiate our results based on detailed band structure and Berry curvature numerics on the Floquet Hamiltonian in the high-frequency limit. Our paper showcases how topological phases can be obtained through mature experimental approaches such as magnetic layer doping and circularly polarized laser pumping and opens up potential device applications such as a polarization chirality-controlled topological transistor.

The broadening and polarization of excitonic luminescence in monolayer transition metal dichalcogenides largely suffer from inhomogeneity and temperature – an unresolved problem to date. In this work, through few-layer graphene encapsulation of monolayer $\mathrm{W}{\mathrm{S}}_2$, we reduce the interexcitonic energy separation, which then can have a narrow resonance with a specific phonon mode of our choice. The resulting single-step exciton relaxation with the resonating phonon mode significantly suppresses the inhomogeneous broadening, allowing us to achieve the narrowest exciton linewidth of $1.06$ meV (which translates to $0.19$ meV after deconvolution with the excitation laser linewidth). The single-phonon resonance helps to achieve a high quantum efficiency despite graphene encapsulation. The technique is powerful in tuning the exciton polarization during relaxation by choosing a specific resonating phonon mode. For example, the valley coherence (polarization) improves from $\sim 68\%(\sim 40\%)$ to $\sim 90\%(\sim 75\%)$ on resonance with $2A_1^{{}^{\prime }}$ and $A_1^{{}^{\prime }}$ modes, respectively. We further demonstrate a strong polarization reversal on resonance with a chiral phonon mode. Strikingly, the above features remain robust against temperature (up to 200 K) and sample age (a few months in ambient condition). The findings will lead to clean excitonic measurements without requiring cryogenic cooling.

We analyze theoretically the origin of the spin Nernst and thermal Hall effects in ${\mathrm{FePS}}_3$ as a realization of a two-dimensional antiferromagnet (2D AFM). We find that a strong magnetoelastic coupling, hybridizing magnetic excitations (magnons) and elastic excitations (phonons), combined with time-reversal symmetry breaking, results in Berry curvature hotspots in the region of anticrossing between the two distinct hybridized bands. Furthermore, a large spin Berry curvature emerges due to interband transitions between two magnonlike bands, where a small energy gap is induced by magnetoelastic coupling between such bands that are energetically distant from anticrossings of hybridized bands. These nonzero Berry curvatures generate topological transverse transport (i.e., the thermal Hall effect) of hybrid excitations, dubbed magnon-polarons, as well as of the spin (i.e., the spin Nernst effect) carried by them, in response to an applied longitudinal temperature gradient. We investigate the dependence of the spin Nernst and thermal Hall conductivities on the applied magnetic field and temperature, unveiling a very large spin Nernst conductivity even at zero magnetic field. Our results suggest the ${\mathrm{FePS}}_3$ AFM, which is already available in 2D form experimentally, as a promising platform to explore the topological transport of magnon-polaron quasiparticles at terahertz frequencies.

We consider magnetotransport on a helical edge of a quantum spin Hall insulator, in the presence of bulk midgap states side coupled to the edge. In the presence of a magnetic field, the midgap levels are spin split, and hybridization of these levels with the itinerant edge states leads to backscattering, and the ensuing increase in the resistance. We show that there is a singular cusplike contribution to the positive magnetoresistance stemming from resonant midgap states weakly coupled to the edge. The singular behavior persists for both coherent and incoherent edge transport regimes. We use the developed theory to fit the experimental data for the magnetoresistance for monolayer ${\mathrm{WTe}}_2$ at liquid helium temperatures. The results of the fitting suggest that the cusplike behavior of the resistance in weak magnetic fields observed in experiments on monolayer ${\mathrm{WTe}}_2$ with long edge channels might indeed be explained by hybridization of the helical edge states with spin-split bulk midgap states. In particular, the dependence of the magnetoresistance on the direction of the external magnetic field is well described by the incoherent edge transport theory, at the same time being quite distinct from the one expected for a magnetic-field-induced edge gap.

In a recent breakthrough experiment [Nature (London) 614, 445 (2023)], signatures of Majorana zero modes have been observed in tunnel spectroscopy for a minimal Kitaev chain constructed from coupled quantum dots. However, as Ising anyons, Majoranas' most fundamental property of non-Abelian statistics is yet to be detected. Moreover, the minimal Kitaev chain is qualitatively different from topological superconductors in that it supports Majoranas only at a sweet spot. Therefore, it is not obvious whether non-Abelian characteristics such as braiding and fusion can be demonstrated in this platform with a reasonable level of robustness. In this work, we theoretically propose a protocol for detecting the Majorana fusion rules in an artificial Kitaev chain consisting of four quantum dots. In contrast with the previous proposals for semiconductor-superconductor hybrid nanowire platforms, here we do not rely on mesoscopic superconducting islands, which are difficult to implement in quantum dot chains. To show the robustness of the fusion protocol, we discuss the effects of three types of realistic imperfections on the fusion outcomes, e.g., diabatic errors, dephasing errors, and calibration errors. We also propose a fermion parity readout scheme using quantum capacitance. Our work will shed light on future experiments on detecting the non-Abelian properties of Majorana modes in a quantum dot chain.

We investigate the effect of quantum noise on the measurement-induced quantum phase transition in monitored random quantum circuits. Using the efficient simulability of random Clifford circuits, we find that the transition is broadened into a crossover and that the phase diagram as a function of projective measurements and noise exhibits several distinct regimes. We show that a mapping to a classical statistical mechanics problem accounts for the main features of the random circuit phase diagram. The bulk noise maps to an explicit permutation symmetry-breaking coupling; this symmetry is spontaneously broken when the noise is switched off. These results have implications for the realization of entanglement transitions in noisy quantum circuits.

We consider a configuration of three stacked graphene monolayers with equal consecutive twist angles $\theta$. Remarkably, in the chiral limit when interlayer coupling terms between $\text{AA}$ sites of the moiré pattern are neglected we find four perfectly flat bands (per valley and spin) at a sequence of magic angles which are exactly equal to the twisted bilayer graphene (TBG) magic angles divided by $\sqrt{2}$. Therefore, the first magic angle for equal-twist trilayer graphene (eTTG) in the chiral limit is ${\theta }_{*}\approx 1.{05}^{\circ }/\sqrt{2}\approx 0.{74}^{\circ }$. We prove this relation analytically and show that the Bloch states of the eTTG's flat bands are nonlinearly related to those of TBG. Additionally, we show that at the magic angles, the upper and lower bands must touch the four exactly flat bands at the Dirac point of the middle graphene layer. Finally, we explore the eTTG's spectrum away from the chiral limit through numerical analysis.

Two-dimensional (2D) transition metal carbides and nitrides (MXene) host a plethora of interests, e.g., energy storage and conversions, and intriguing quantum phenomena ranging from electronic topological properties to superconductivity. However, topological magnetism is still a missing piece in MXenes. Here, via first-principles calculations, we validate the existence of significant Dzyaloshinskii–Moriya interaction (DMI) in 2D multiferroic MXenes, where the magnitude of DMI scales with the strength of electric dipole of the MXenes. Moreover, using atomistic spin model simulations, we demonstrate that room-temperature (RT) magnetic skyrmions can be spontaneously stabilized at monolayer ${\mathrm{Co}}_2\mathrm{C}{\mathrm{F}}_2$. The chirality of DMI can be tuned via ferroelectricity effect and, consequently, the skyrmions. We further show that a proper van der Waals substrate can effectively enhance the magnetic anisotropy of ${\mathrm{Co}}_2\mathrm{C}{\mathrm{F}}_2$ through interlayer charge transfer, while the tunable DMI and RT skyrmions can also be maintained. These results prove that MXenes and their heterostructure can be promising candidates in fabricating ultrathin skyrmionic devices and pave the way for the RT electrically controllable skyrmions toward spintronic applications.

Interfacial charge transfer in oxide heterostructures gives rise to a rich variety of electronic and magnetic phenomena. Designing heterostructures where one of the thin-film components exhibits a metal-insulator transition opens a promising avenue for controlling such phenomena both statically and dynamically. In this work, we utilize a combination of depth-resolved soft x-ray standing-wave and hard x-ray photoelectron spectroscopies in conjunction with polarization-dependent x-ray absorption spectroscopy to investigate the effects of the metal-insulator transition in ${\mathrm{LaNiO}}_3$ on the electronic and magnetic states at the ${\mathrm{LaNiO}}_3/{\mathrm{CaMnO}}_3$ interface. We report a direct observation of the reduced effective valence state of the interfacial Mn cations in the metallic superlattice with an above-critical ${\mathrm{LaNiO}}_3$ thickness (6 unit cells, u.c.) facilitated by the charge transfer of itinerant $\mathrm{Ni}3d{e}_g$ electrons into the interfacial ${\mathrm{CaMnO}}_3$ layer. Conversely, in an insulating superlattice with a below-critical ${\mathrm{LaNiO}}_3$ thickness of $2\mathrm{u}.\mathrm{c}.$, a homogeneous effective valence state of Mn is observed throughout the ${\mathrm{CaMnO}}_3$ layers due to the blockage of charge transfer across the interface. The ability to switch and tune interfacial charge transfer enables precise control of the emergent ferromagnetic state at the ${\mathrm{LaNiO}}_3/{\mathrm{CaMnO}}_3$ interface and, thus, has far-reaching consequences on the future strategies for the design of next-generation spintronic devices.

Single crystals of In-doped ${\mathrm{PbBi}}_2{\mathrm{Te}}_4$ are synthesized via a conventional solid-state method. Chemical analysis and Hall measurements indicate that In replaces Pb, introducing $n$-type carriers, creating ${\mathrm{Pb}}_{1-x}{\mathrm{In}}_x{\mathrm{Bi}}_2{\mathrm{Te}}_4$. A superconducting transition is observed with a maximum transition temperature around 2.06 K for ${\mathrm{Pb}}_{1-x}{\mathrm{In}}_x{\mathrm{Bi}}_2{\mathrm{Te}}_4$. Field-dependent transport measurements reveal type-II superconductivity and yield a maximum upper critical field around 1.55 T. Thermodynamic data indicates bulk superconductivity in the BCS weak-coupling limit. Our findings establish an ambient-pressure superconducting system in the ${AM}_2{X}_4$ family, and doped ${\mathrm{PbBi}}_2{\mathrm{Te}}_4$ as a promising platform for the study of topological superconductivity.

The dynamics of the $S=3/2$ kagomé antiferromagnet chromium jarosite, ${\mathrm{KCr}}_3$(OD)${}_6$(${\mathrm{SO}}_4$)${}_2$, was studied using high-resolution neutron time-of-flight spectroscopy on a polycrystalline sample with a nearly stoichiometric magnetic lattice [$2.8\left(2\right)\%$ Cr vacancies]. Neutron spectroscopy reveals diffuse spin wave excitations in the ordered phase with an incomplete gap and significant finite-lifetime broadening as well as a pronounced kagomé zero mode. Using linear spin wave theory, we estimate the exchange couplings. The system is highly two dimensional with the leading nearest-neighbor coupling being $J_1=0.881$ meV. Above $T_N$, diffuse excitations from the classical spin liquid regime dominate. We model the $Q$ response and energy response separately and show that in both the ordered phase and the classical spin liquid regime they are strongly coupled.

We propose a ferromagnetic planar heterojunction ${\mathrm{MoSe}}_2/{\mathrm{WSe}}_2/{\mathrm{MoSe}}_2$, and theoretically explore the quantum transport modulated by off-resonant circular polarized light. We find that fully spin- and valley-polarized transport can be realized in both P (parallel) and AP (antiparallel) magnetization configurations. Specifically, the spin polarization can be reversed by switching the helicity of circular polarized light, while the valley polarization can be significantly modulated by changing magnetization configurations, and it's easier to reverse the valley polarization in the antiparallel configuration than that in the parallel configuration. Moreover, we demonstrate that the tunneling magnetoresistance (TMR) in the planar heterojunction can be modulated due to the spin-valley-dependent effective potential induced by optical modulation and band offset. Especially, the negative TMR is realized in strongly asymmetric magnetization configuration, which is contributed by specific spin- and valley-polarized states, and the sign of TMR can be switched by adjusting optical modulation intensity. This work may shed light on promising applications of spin-valley or TMR devices based on monolayer transition-metal dichalcogenide (TMDC) planar heterojunctions.

Higher-order topological phases have attracted much attention due to the rich topological modes localized at multidimensional boundaries. Here we propose valley higher-order Weyl semimetal phases in honeycomb lattices and suggest its realization in sonic crystals and discuss its possible candidates in other systems. Such topological semimetals host three-dimensional Weyl points in the bulk, two-dimensional Fermi-arc surface states on the surfaces, and one-dimensional hinge-arc states on hinges perpendicular to the hexagonal plane. All these states exhibit valley-contrasting physics in different dimensions (bulk, surfaces, and hinges) in a single physical system. Enriched by the emergent valley degree-of-freedom, these valley higher-order Weyl semimetals have unconventional properties that are appealing for the topological manipulation of wave propagation and hence the material properties and potential applications.

Four-dimensional (4D) topological insulators, which are impossible in real materials, have attracted much attention by virtue of the recent progress achieved in quantum simulations of higher-dimensional systems. In this paper, we employ the supercell approximation to investigate the disorder effects on a system that supports the 4D topological insulator phases characterized by quantized second Chern numbers and the normal insulator phase. We demonstrate that the 4D topological insulator phases are robust against weak disorders. Moreover, we reveal that disorder can transform a normal insulator to a 4D topological insulator with an emergent second Chern number, referred to as a 4D topological Anderson insulator. An effective-medium theory based on the Born approximation further confirms the numerical conclusions.

Minimally twisted bilayer graphene in the presence of an interlayer bias develops a triangular network of valley chiral modes that propagate along the $AB/BA$ interfaces and scatter at the $AA$ regions. The low-energy physics of the resulting network can be captured by means of a phenomenological scattering network model, allowing to calculate the energy spectrum and the magnetoconductance in a straightforward way. Although there is in general a good agreement between microscopic and phenomenological models, there are some aspects that have not been captured so far with the latter, in particular, the appearance of flatbands in the energy spectrum associated to a localized density of states at the $AA$ regions. To bring both approaches closer together, we modify the previous energy-independent phenomenological model and add the possibility to scatter to a set of discrete energy levels at the $AA$ regions, yielding an $S$ matrix with energy-dependent parameters. Furthermore, we investigate the impact of Coulomb repulsion in these regions on a mean-field level and discuss possible effects of decoherence due to elastic and inelastic cotunneling events.

In this paper, two nonsymmorphic two-dimensional structures of CdTe and ZnTe are modeled, and using state-of-the-art density functional theory with the group theory of solids, their symmetry-enforced electronic properties are studied. The in-plane ferroelectricity coupled with strong spin-orbit coupling induces a unidirectional out-of-plane Rashba spin-orbit field that can host a momentum-independent uniform spin configuration known as persistent spin texture (PST) at the Brillouin zone center. PST in these structures is found to be robust against external perturbations such as strain, structural distortion, and independent of layer thickness. These unprecedented intrinsic spin transport properties hold utmost importance in spintronics, as the experimental stringent condition of equal Rashba and Dresselhaus constants [Phys. Rev. Lett. 90, 146801 (2003)] is eliminated. The calculated persistent spin helix wavelength of $<\sim 5$ nm paves the way for developing next-generation nanosized nonballistic spin field-effect transistors compared with micrometer-sized GaAs/AlGaAs quantum wells. Further, these materials exhibit finite spin Hall conductivity at the band edges and hence can be used in ferromagnet-free spin Hall transistors. Although CdTe and ZnTe systems have been widely studied for photocatalysis and solar cell applications over the past few decades, their potential application in spintronic devices has not been explored. Mono/few layers of CdTe and ZnTe synthesized from (110) facets of bulk zinc-blende crystals [Nat. Commun. 3, 1057 (2012)] satisfy all symmetry operations of the nonsymmorphic space group and hence can be considered ideal materials to verify our theoretical results experimentally.

We study operator correlations of a spinful Luttinger liquid after introducing a non-Hermitian interaction quench, yielding supersonic modes and dominant superconducting correlations as signatures of the nonunitary dynamics as well as spin-charge separation. A comparative analysis with the Hermitian counterpart, i.e., when the quench is Hermitian, shows a significant difference in the behavior of the model. We derive exact expressions for different operator correlations and show that the superconducting correlations decay slower than the charge and spin density wave correlations, especially within the short-time limit, and at the long-time limit all the operator correlations merge differed only by phase factors in the case of non-Hermitian interaction quench whereas they do not merge in the case of Hermitian interaction quench. In both cases known Luttinger liquid universality is retained at the long-time limit. We also analyze how the dynamics of operator correlations vary in the presence of anisotropy in the quenching parameters.

The understanding of extreme near-field heat transport across vacuum nanogaps between polar dielectric materials remains an open question. In this work, we present an investigation of heat transport across vacuum nanogaps between magnesium oxide (MgO) surfaces by nonequilibrium molecular dynamics (NEMD) simulation, which naturally accounts for the nonlocal dielectric response from both acoustic and optical branches as well as phonon tunneling. A consistent comparison is also made with the continuum fluctuational electrodynamics theory using both local and nonlocal dielectric functions computed by equilibrium molecular dynamics with the anharmonic damping properly included. As a result, the direct NEMD result shows significant deviations from the continuum theory even up to a gap size of a few nanometers. The lattice anharmonicity is demonstrated to have a large impact on the energy transmission and thermal conductance, in contrast to its moderate effect reported for metallic vacuum nanogaps. The present work thus provides further insight into the physics of heat transport in the extreme near-field regime between polar materials, and puts forward a methodology to account for anharmonic effects.