In magnetism, skyrmions correspond to classical three-dimensional spin textures characterized by a topological invariant that keeps track of the winding of the magnetization in real space, a property that cannot be easily generalized to the quantum case since the orientation of a quantum spin is, in general, ill defined. Moreover, as we show, the quantum skyrmion state cannot be directly observed in modern experiments that probe the local magnetization of the system. However, we show that this novel quantum state can still be identified and fully characterized by a special local three-spin correlation function defined on neighboring lattice sites—the scalar chirality—which reduces to the classical topological invariant for large systems and which is shown to be nearly constant in the quantum skyrmion phase.

The quantum anomalous Hall (QAH) effect is sometimes observed in twisted bilayer graphene (tBG) when it is nearly aligned with an encapsulating hexagonal boron nitride (hBN) layer. We propose that the appearance or absence of the QAH effect in individual devices could be related to commensurability between the graphene/graphene and graphene/hBN moiré patterns. We identify a series of points in the ${\displaystyle ({\theta }_{\mathrm{GG}},{\theta }_{\mathrm{GBN}})}$ twist-angle space at which the two moiré patterns are commensurate, allowing moiré band theory to be applied, and we show that the band Chern numbers are in this case sensitive to a rigid in-plane hBN displacement. Given this property, we argue that the QAH effect is likely only when (i) the ${\displaystyle ({\theta }_{\mathrm{GG}},{\theta }_{\mathrm{GBN}})}$ twist-angle-pair is close enough to a commensurate point that the two moiré patterns yield a supermoiré pattern with a sufficiently long length scale, and (ii) the supermoiré has a percolating topologically nontrivial QAH phase. For twist angles far from commensurability, the hBN layer acts as a source of disorder that can destroy the QAH effect. Our proposal can explain a number of current experimental observations. Further experimental studies that can test this proposal more directly are suggested.

Using the density-functional-based $\mathrm{LDA}+U$ method and linear-response theory, we study the magnetic exchange interactions of the superconductor ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$. Our calculated nearest-neighbor exchange constant $J_1=82$ meV is large, weakly affected by doping, and is only slightly smaller than that found in the sister compound $\mathrm{Ca}\mathrm{Cu}{\mathrm{O}}_2.$ However, we find that the hole doping significantly enhances the interlayer exchange coupling as it affects the magnetic moment of the Ni-$3d_{3z^2-r^2}$ orbital. This can be understood in terms of the small hybridization of Ni-$3d_{3z^2-r^2}$ within the $\mathrm{Ni}{\mathrm{O}}_2$ plane, which results in a flat band near the Fermi level, and its large overlap along the $z$ direction. We also demonstrate that the Nd-$5d$ states appearing at the Fermi level do not affect the magnetic exchange interactions, and thus they may not participate in the superconductivity of this compound. Whereas many previous works emphasized the importance of the Ni-$3d_{x^2-y^2}$ and Nd-$5d$ orbitals, we analyze instead the solution of the Ni-$3d_{x^2-y^2}$/Ni-$3d_{3z^2-r^2}$ minimal model using dynamical mean field theory. It reveals an underlying Mott insulating state that, depending on the precise values of the intra-atomic Hund's coupling smaller or larger than 0.83 eV, selects upon doping either $S=0$ or 1 two-hole states at low energies, leading to very different quasiparticle band structures. We propose that trends upon doping in the spin excitation spectrum and the quasiparticle density of states can be a way to probe the Ni $3d^8$ configuration.

Direct measurements of photoexcited carrier dynamics in nickel are made using few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy at the nickel $M_{2,3}$ edge. It is observed that the core-level absorption line shape of photoexcited nickel can be described by a Gaussian broadening ($\sigma$) and a red shift (${\omega }_s$) of the ground-state absorption spectrum. Theory predicts and the experimental results verify that after initial rapid carrier thermalization, the electron temperature increase ($\mathrm{\Delta }T$) is linearly proportional to the Gaussian broadening factor $\sigma$, providing quantitative real-time tracking of the relaxation of the electron temperature. Measurements reveal an electron cooling time for 50 nm thick polycrystalline nickel films of $640\pm 80$ fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of ${\omega }_s\propto \mathrm{\Delta }{T}^{1.5}$. Rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4-fs photoexcitation pulse until the carriers reach a quasithermal equilibrium. Entwined with a <6 fs instrument response function, carrier thermalization times ranging from 34 fs to 13 fs are estimated from experimental data acquired at different pump fluences and it is observed that the electron thermalization time decreases with increasing pump fluence. The study provides an initial example of measuring electron temperature and thermalization in metals in real time with XUV light, and it lays a foundation for further investigation of photoinduced phase transitions and carrier transport in metals with core-level absorption spectroscopy.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) exhibit an electron-hole liquid (EHL) phase transition at unusually high temperatures. Because these materials are atomically thin, optical excitation leads to material expansion. As a result, during the EHL phase transition, the electronic band structure evolves due to both material thermal expansion and renormalization of the bands under high excitation densities. Specifically, these effects lead to indirect gap electronic band structure with a valence band maximum located at the $\mathrm{\Gamma }$ valley. In this paper, we developed a methodology for analyzing the spectral evolution of the photoluminescence of suspended $1\mathrm{L}\text{−}\mathrm{Mo}{\mathrm{S}}_2$ during the EHL phase transition by using Fermi liquid theory. The resulting analysis reveals a 23-fold increase in radiative recombination per carrier, as well as valley-specific carrier densities and intraband carrier relaxation kinetics in $1\mathrm{L}\text{−}\mathrm{Mo}{\mathrm{S}}_2$. More broadly, the results outline a methodology for predicting critical EHL parameters, shedding light onto the EHL phase transition in 2D TDMCs.

Laser-engineered exciton-polariton networks could lead to dynamically configurable integrated optical circuitry and quantum devices. Combining cavity optomechanics with electrodynamics in laser-configurable hybrid designs constitutes a platform for the vibrational control, conversion, and transport of signals. With this aim we investigate three-dimensional optical traps laser induced in quantum well embedded semiconductor planar microcavities. We show that the laser-generated and -controlled discrete states of the traps dramatically modify the interaction between photons and phonons confined in the resonators, accessing through coupling of photoelastic origin ($g_0/2\pi \sim 1.8\mathrm{MHz}$) an optomechanical cooperativity $C>1$ for milliwatt excitation. The quenching of Stokes processes and double-resonant enhancement of anti-Stokes ones involving pairs of discrete optical states in the sideband-resolved regime allow the optomechanical cooling of 180-GHz bulk acoustic waves, starting from room temperature down to $\sim 130$ K. These results pave the way for dynamical tailoring of optomechanical actuation in the extremely high frequency range (30–300 GHz) for future network and quantum technologies.

Weyl semimetals have been classified into type I and type II with respect to the geometry of their Fermi surfaces at the Weyl points. we propose another class of Weyl semimetal, whose unique Fermi surface contains two electron or two hole pockets touching at a multi-Weyl point, dubbed a type-III Weyl semimetal. Based on first-principles calculations, we first show that the quasi-one-dimensional compound ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ is a type-III Weyl semimetal with larger chiral charges. ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ can support fourfold helicoidal surface states with long Fermi arcs on the (001) surface. Angle-resolved photoemission spectroscopy measurements are in agreement with the gapless nature of ${\left({\text{TaSe}}_4\right)}_2\text{I}$ at room temperature and reveal its characteristic dispersion. In addition, our calculations show that external strain could induce transitions in ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ among the type-III, type-II, and type-I Weyl semimetals, accompanied with the Lifshitz transitions of the Fermi surfaces. Therefore, our work experimentally indicates ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ as a type-III Weyl semimetal and provides a promising platform to further investigate the physics of type-III Weyl fermions.

We propose a class of models exhibiting the instanton crystal phase. In this phase, the minimum of the free energy corresponds to a configuration with an imaginary-time-dependent order parameter in the form of a chain of alternating instantons and anti-instantons. The resulting characteristic feature of this state is that the average of the order parameter over the imaginary time vanishes. In order to study the model in a broad region of parameters of the model quantitatively, and to prove the existence of the instanton crystal phase, we develop an efficient numerical scheme, suitable for the exact treatment of the proposed models. In a certain limit, results demonstrating the existence of the instanton crystal phase are obtained also analytically. The numerical study of the model shows that there is a phase transition between the instanton crystal and the state with the imaginary-time-independent order parameter.

Dynamic polarization protocols aim to hyperpolarize a spin bath by transferring spin polarization from a well-controlled qubit such as a quantum dot or a color defect. Building on techniques from shortcuts to adiabaticity, we design fast and efficient dynamic polarization protocols in central spin models that apply to dipolarly interacting systems. The protocols maximize the transfer of polarization via bright states at a nearby integrable point, exploit the integrability-breaking terms to reduce the statistical weight on dark states that do not transfer polarization, and realize experimentally accessible local counterdiabatic driving through Floquet engineering. A master equation treatment suggests that the protocol duration scales linearly with the number of bath spins with a prefactor that can be orders of magnitude smaller than that of unassisted protocols. This work opens pathways to cool spin baths and extend qubit coherence times for applications in quantum information processing and metrology.

${\mathrm{LaCrGe}}_3$ has attracted attention as a paradigm example of the avoidance of ferromagnetic (FM) quantum criticality in an itinerant magnet. Here, we combined thermodynamic (specific heat and thermal expansion), transport, x-ray, and neutron scattering as well as $\mu \mathrm{SR}$ measurements to obtain insights on the temperature-pressure phase diagram of ${\mathrm{LaCrGe}}_3$. Consistent with previous studies of the phase diagram by transport measurements, our thermodynamic data shows clearly that the FM transition at $T_{\text{FM}}$ changes its character from second order to first order when it is suppressed to low temperatures by pressure. In addition, previous studies demonstrated that for high pressures a new phase occurs below $T_2$, which was proposed to be a long-wavelength antiferromagnetic state (${\mathrm{AFM}}_q$). In this paper, we provide evidence from our thermodynamic data that this phase transition is preceded by yet another phase transition at $T_1>T_2$. Our $\mu \mathrm{SR}$ data indicate that full magnetic volume fraction is only established below $T_2$, but that this magnetism is characterized by a short correlation length. Within the experimental resolution, our neutron-scattering data is not able to identify any magnetic Bragg peaks. Overall, the microscopic magnetic data is therefore consistent with the formation of FM clusters in the proximity of the avoided FM quantum critical point in ${\mathrm{LaCrGe}}_3$. This conclusion is at odds with the previous proposal of ${\mathrm{AFM}}_q$ order and raises questions on the role of disorder in this stochiometric compound.

The axial next-nearest-neighbor Ising model predicts a fractal (infinite) set of phases with incommensurate wave vectors that are separated by first-order phase boundaries. This complexity results from a simple frustration condition between nearest- and next-nearest-neighbor interactions along a chain of Ising spins. Using x-ray photon correlation spectroscopy (XPCS), we investigate the surprising antiferromagnetic dynamics that emerge from such a complex phase diagram over a wide range of temperatures. We present XPCS measurements of the frustrated magnetic chain compound ${\mathrm{Lu}}_2{\mathrm{CoMnO}}_6$ and Monte Carlo simulations. Incommensurate magnetic Bragg peaks slide towards commensurate “up-up-down-down” spin order with decreasing temperature and increasing time. Both simulation and experiment support a counterintuitive “upside-down” temperature dependence of the magnetic dynamics: at higher temperatures in the region of first-order phase boundaries, slower dynamics are observed where the speckle maintains its coherence. At the lowest temperatures, where part of the sample adopts commensurate order, the dynamics speed up and result in fast decoherence.

The phenomenology of quantum phase transitions concerns physics at low temperatures and energies, and corresponding solid-state experiments often reach millikelvin temperatures. However, this is a scale where in many solids the influence of nuclear spins and their hyperfine interaction is no longer negligible. This may limit the observability of electronic quantum critical phenomena. Here we discuss how continuous magnetic quantum phase transitions get influenced, modified, or destroyed by the coupling to nuclear spins. We use simple yet paradigmatic spin models for magnetic quantum criticality and determine modifications to the phase diagram, the excitation spectrum, and thermodynamics due to the presence of nuclear spins. We estimate crossover scales below which purely electronic quantum criticality is no longer observable, and discuss the distinct physics emerging at low temperatures. Our results are relevant for a variety of compounds displaying magnetic quantum phase transitions and, more generally, highlight the sensitivity of quantum critical systems to small perturbations.

Topological quantum liquids contain internal degrees of freedom that are coupled to geometric response. Yet, an explicit and microscopic identification of the geometric response remains difficult. Here, taking notable fractional quantum Hall (FQH) states as typical examples, we systematically investigate a promising protocol—the Dehn-twist deformation on the torus geometry—to probe the geometric response of correlated topological states and establish the relation between such response and universal properties of pertinent states. Based on analytical derivations and numerical simulations, we find that the geometry-induced Berry phase encodes features for a broad class of FQH states at the Laughlin, hierarchy, Halperin and non-Abelian Moore-Read fillings. Our findings conclusively demonstrate that the adiabatic Dehn-twist deformation can faithfully capture rich geometric and topological information, including the Hall viscosity and topological spin of the pertinent FQH state and the chiral central charge of the underlying edge conformal field theory. Our approach provides a powerful way to reveal topological orders of generic FQH states and address previously open questions.

In this paper we study the out-of-equilibrium phase diagram of the quantum version of Derrida's random energy model, which is the simplest model of mean-field spin glasses. We interpret its corresponding quantum dynamics in Fock space as a one-particle problem in very high dimension to which we apply different theoretical methods tailored for high-dimensional lattices: the forward-scattering approximation, a mapping to the Rosenzweig-Porter model, and the cavity method. Our results indicate the existence of two transition lines and three distinct dynamical phases: a completely many-body localized phase at low energy, a fully ergodic phase at high energy, and a multifractal “bad metal” phase at intermediate energy. In the latter, eigenfunctions occupy a diverging volume yet an exponentially vanishing fraction of the total Hilbert space. We discuss the limitations of our approximations and the relationship with previous studies.

Motivated by the recent experimental realization of twisted transition metal dichalcogenide bilayers, we study a simplified model driven by different forms of monochromatic light. As a concrete and representative example we use parameters that correspond to a twisted ${\mathrm{MoTe}}_2$ homobilayer. First, we consider irradiation with circularly polarized light in free space and demonstrate that the corresponding Floquet Hamiltonian takes the same form as the static Hamiltonian, only with a constant overall shift in quasienergy. This is in stark contrast to twisted bilayer graphene, where new terms are typically generated under an analogous drive. Longitudinal light, on the other hand, which can be generated from the transverse magnetic mode in a waveguide, has a much more dramatic effect—it renormalizes the tunneling strength between the layers, which effectively permits the tuning of the twist angle in situ. We find that, by varying the frequency and amplitude of the drive, one can induce a topological transition that cannot be obtained with the traditional form of the Floquet drive in free space. Furthermore, we find that strong drives can have a profound effect on the layer pseudospin texture of the twisted system, which coincides with multiple simultaneous band gap closings in the infinite-frequency limit. Surprisingly, these band-gap closings are not associated with topological transitions. For high but finite drive frequencies near 0.7 eV, the infinite-frequency band crossings become band gap minima of the order of ${10}^{-6}$ eV or smaller.

Recently, advances in film synthesis methods have enabled a study of extremely overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$. This has revealed a surprising behavior of the superfluid density as a function of doping and temperature, the explanation of which is vividly debated. One popular class of models posits electronic phase separation, where the superconducting phase fraction decreases with doping, while some competing phase (e.g., ferromagnetic) progressively takes over. A problem with this scenario is that all the way up to the dome edge the superconducting transition remains sharp, according to mutual inductance measurements. However, the physically relevant scale is the Pearl penetration depth ${\mathrm{\Lambda }}_{\mathrm{P}}$, and this technique probes the sample on a length scale $L$ that is much larger than ${\mathrm{\Lambda }}_{\mathrm{P}}$. In the present paper, we use local scanning superconducting quantum interference device (SQUID) measurements that probe the susceptibility of the sample on the scale $L\ll {\mathrm{\Lambda }}_{\mathrm{P}}$. Our SQUID maps show uniform landscapes of susceptibility and excellent overall agreement of the local penetration depth data with the bulk measurements. These results contribute an important piece to the puzzle of how high-temperature superconductivity vanishes on the overdoped side of the cuprate phase diagram.

Doping van der Waals layered semiconductors with magnetic atoms is a simple and effective approach to induce magnetism. However, investigation of the electrical properties of such two-dimensional semiconductors and the modulation of their magnetic order for spintronics is still lacking. Herein, we report a giant Zeeman shift from the spin-polarized state in tungsten diselenide ($\mathrm{W}{\mathrm{Se}}_2$) doped with a small amount of vanadium (V) atoms $(\sim 0.1\%)$. The Zeeman shift was measured via resonant magnetotunneling spectroscopy with a vertical graphite/V-$\mathrm{W}{\mathrm{Se}}_2$/graphite heterojunction. The $p$-type doping state near the valence band is substantially shifted under an external magnetic field by 7.8 meV/T, equivalent to a giant $g$ factor of approximately 135, an order of magnitude higher than that of other two-dimensional magnetic semiconductors. The ferromagnetic order of the spin glass state and its long-range interaction are revealed by the remanence of magnetoresistance between the zero-field cooling and field-cooling processes as well as magnetoresistance hysteresis. The ferromagnetic glass order is fully established at 50 K, whereas the long-range interaction persists at higher temperatures of up to 300 K in V-doped $\mathrm{W}{\mathrm{Se}}_2$ flakes with an approximate thickness of 5 nm. Our work sheds light on the magnetic nature of V-doped $\mathrm{W}{\mathrm{Se}}_2$ semiconductors and paves the way for future spintronics based on two-dimensional van der Waals magnetic semiconductors.

We propose a family of Abelian quantum Hall states termed the nondiagonal states, which arise at filling factors $\nu =p/2q$ for bosonic systems and $\nu =p/(p+2q)$ for fermionic systems, with $p$ and $q$ being two coprime integers. Nondiagonal quantum Hall states are constructed in a coupled wire model, which shows an intimate relation to the nondiagonal conformal field theory and has a constrained pattern of motion for bulk quasiparticles, featuring a nontrivial interplay between charge symmetry and translation symmetry. The nondiagonal state is established as a distinctive symmetry-enriched topological order. Aside from the usual $U\left(1\right)$ charge sector, there is an additional symmetry-enriched neutral sector described by the quantum double model $\mathcal{D}\left({\mathbb{Z}}_p\right)$, which relies on the presence of both the $U\left(1\right)$ charge symmetry and the $\mathbb{Z}$ translation symmetry of the wire model. Translation symmetry distinguishes nondiagonal states from Laughlin states, in a way similar to how it distinguishes weak topological insulators from trivial band insulators. Moreover, the translation symmetry in nondiagonal states can be associated with the $\mathbf{e}\leftrightarrow \mathbf{m}$ anyonic symmetry in $\mathcal{D}\left({\mathbb{Z}}_p\right)$, implying the role of dislocations as twofold twist defects. The boundary theory of nondiagonal states is derived microscopically. For the edge perpendicular to the direction of wires, the effective Hamiltonian has two components: a chiral Luttinger liquid and a generalized $p$-state clock model. Importantly, translation symmetry in the bulk is realized as self-duality on the edge. The symmetric edge is thus either gapless or gapped with spontaneously broken symmetry. For $p=2,3$, the respective electron tunneling exponents are predicted for experimental probes.

Interlayer excitons confined in bilayer heterostructures of transition metal dichalcogenides (TMDs) offer a promising route to implement two-dimensional dipolar superfluids. Here we study the experimental conditions necessary for the realisation of such a collective state. Particularly, we show that the moiré potential inherent to TMD bilayers yields an exponential increase of the excitons' effective mass. To allow for exciton superfluidity at sizable temperatures it is then necessary to intercalate a high-$\kappa$ dielectric between the monolayers confining electrons and holes. Thus, the moiré lattice depth is sufficiently weak for a superfluid phase to theoretically emerge below a critical temperature of around 10 K. Importantly, for realistic experimental parameters, interlayer excitons quasicondense in a state with finite momentum, so that the superfluid is optically inactive and flows spontaneously.

The experimentally observed correlated insulating states and quantum anomalous Hall (QAH) effect in twisted bilayer graphene (TBG) have drawn significant attention. However, up to date, the specific mechanisms of these intriguing phenomena are still open questions. Using an all-band Hartree-Fock variational method, we have explained the correlated insulating states and QAH effects at some integer fillings of the flat bands in TBG. Our results indicate that states breaking flavor (valley and spin) symmetries are energetically favored at all integer fillings. In particular, the correlated insulating states at $\pm 1/2$ filling and at the charge neutrality point are all valley polarized states which break $C_{2z}$ and time-reversal ($\mathcal{T}$) symmetries but preserve $C_{2z}\mathcal{T}$ symmetry. Such valley polarized states exhibit “moiré orbital antiferromagnetic ordering” on an emergent honeycomb lattice with compensating circulating current pattern in the moiré supercell. Within the same theoretical framework, our calculations indicate that the $C=\mp 1$ QAH states at $\pm 3/4$ fillings of the magic-angle TBG are spin and orbital ferromagnetic states, which emerge when a staggered sublattice potential is present. We find that the nonlocalness of the exchange interactions tends to enhance the bandwidth of the low-energy bands due to the exchange-hole effect, which reduces the gaps of the correlated insulator phases. The nonlocal exchange interactions also dramatically enhance the spin polarization of the system, which significantly stabilizes the orbital and spin ferromagnetic QAH state at $3/4$ filling of TBG aligned with hexagonal boron nitride (hBN). We also predict that, by virtue of the orbital ferromagnetic nature, the QAH effects at electron and hole fillings of hBN-aligned TBG would exhibit hysteresis loops with opposite chiralities.