After initial proof-of-principle demonstrations, optically pumped nitrogen-vacancy (NV) centers in diamond have been proposed as a noninvasive platform to achieve hyperpolarization of nuclear spins in molecular samples over macroscopic volumes and enhance the sensitivity in nuclear magnetic resonance (NMR) experiments. In this work we model the process of polarization of external samples by NV centers and theoretically evaluate their performance in a range of scenarios. We find that average nuclear spin polarizations exceeding 10% can in principle be generated over macroscopic sample volumes $(\gtrsim \mu \mathrm{l})$ with a careful engineering of the system's geometry to maximize the diamond-sample contact area. The fabrication requirements and other practical challenges are discussed. We then explore the possibility of exploiting local polarization enhancements in nano/micro-NMR experiments based on NV centers. For micro-NMR we find that modest signal enhancements over thermal polarization (by 1–2 orders of magnitude) can in essence be achieved with existing technology, with larger enhancements achievable via microstructuring of the sample/substrate interface. However, there is generally no benefit for nano-NMR where the detection of statistical polarization provides the largest signal-to-noise ratio. This work will guide future experimental efforts to integrate NV-based hyperpolarization to NMR systems.

Control of quantum coherence in a many-body system is one of the key issues in modern condensed matter, and conventional wisdom is that lattice vibration is an innate source of decoherence. Much research has been conducted to eliminate lattice effects. Challenging this wisdom, we show that lattice vibration may not be a decoherence source but an impetus of a novel coherent quantum many-body state. We demonstrate the possibility by studying the transverse-field Ising model on a chain with renormalization group and density-matrix renormalization group methods and theoretically discover a stable $\mathcal{N}=1$ supersymmetric quantum criticality with central charge $c=3/2$. Thus, we propose an Ising spin chain with strong spin-lattice coupling as a candidate to observe supersymmetry. Generic precursor conditions of novel quantum criticality are obtained by generalizing the Larkin-Pikin criterion of thermal transitions. Our work provides the perspective that lattice vibration may be a knob for exotic quantum many-body states.

The standard Ginzburg-Landau model of competing-order superconductors, applicable to various high $T_c$ cuprates, is studied. It is observed that this model possesses two distinct species of vortex, and consequently has two distinct integer valued topological charges. A simple point particle model of long-range forces between (anti)vortices of any species is developed and compared with numerical simulations of the full field theory, excellent agreement being found. Some of the results are quite counterintuitive. For example, a parameter regime exists where vortices of one species repel both vortices and antivortices of the other.

Coherent phonon excitation via femtosecond laser pulses can be used to control physical properties of matter, and enhancing coherent phonon excitation is highly relevant. Here, we report the strong enhancement of coherent phonon excitation in ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ (FGT) via the resonance Raman effect. On the basis of the femtosecond transient optical spectroscopy measurements, the $A_{1\mathrm{g}}$ coherent phonon excitation in FGT is obtained as a function of pump photon energy. Its excitation can be maximized by tuning the pump photon energy. The maximum coherent phonon excitation at the 1.574-eV pump photon energy corresponds to an electronic transition in FGT, and is a direct result of the resonance Raman effect. The $A_{1\mathrm{g}}$ coherent phonon generation follows the impulsive stimulated Raman scattering mechanism. Our work demonstrates that the resonance Raman effect can be an effective way to enhance coherent phonon excitation via electronic excitation.

Utilizing first-principles density functional theory calculations together with group theory analyses, we systematically investigate the spin-order-dependent magneto-optical effect (MOE), anomalous Hall effect (AHE), and anomalous Nernst effect (ANE) in the recently discovered two-dimensional room-temperature ferromagnet $1T\text{−}{\mathrm{CrTe}}_2$. We find that the spin prefers an in-plane direction by the magnetocrystalline anisotropy energy calculations. The MOE, AHE, and ANE display a period of $2\pi /3$ when the spin rotates within the atomic plane, and they are forbidden if a mirror plane perpendicular to the spin direction exists. By reorienting the spin from the in-plane to out-of-plane direction, the MOE, AHE, and ANE are enhanced by around one order of magnitude. Moreover, we establish the layer-dependent magnetic properties of multilayer $1T\text{−}{\mathrm{CrTe}}_2$ and predict antiferromagnetism and ferromagnetism for bilayer and trilayer $1T\text{−}{\mathrm{CrTe}}_2$, respectively. The MOE, AHE, and ANE are prohibited in antiferromagnetic bilayer $1T\text{−}{\mathrm{CrTe}}_2$ due to the existence of the space-time inversion symmetry, whereas all of them are activated in ferromagnetic trilayer $1T\text{−}{\mathrm{CrTe}}_2$ and are significantly enhanced compared to those of monolayer $1T\text{−}{\mathrm{CrTe}}_2$. Our results show that the magneto-optical and anomalous transports proprieties of $1T\text{−}{\mathrm{CrTe}}_2$ can be effectively modulated by altering spin direction and layer number.

Centrosymmetric multiband superconductors which break time-reversal symmetry generically have two-dimensional nodes, i.e., Fermi surfaces of Bogoliubov quasiparticles. We show that the coupling of the electrons to the lattice always leads to a weak-coupling instability of such a state toward spontaneous breaking of inversion symmetry at low temperatures. This instability is driven by a Cooper logarithm in the internal energy but the order parameter is not superconducting but distortional. We present a comprehensive symmetry analysis and introduce a measure that allows us to compare the strengths of competing distortional instabilities. Moreover, we discuss the instability using an effective single-band model. This framework reveals a duality mapping of the effective model which maps the distortional order parameter onto a superconducting one, providing a natural explanation for the Cooper logarithm and the weak-coupling nature of the instability. Finally, we consider the possibility of a pair-density wave state when inversion symmetry is broken. We find that it can indeed exist but does not affect the instability itself.

In this paper we continue with our analysis of the interplay between the pairing and the non-Fermi liquid behavior in a metal for a set of quantum-critical (QC) systems with an effective dynamical electron-electron interaction $V\left({\mathrm{\Omega }}_m\right)\propto 1/{\left|{\mathrm{\Omega }}_m\right|}^{\gamma }$, mediated by a critical massless boson (the $\gamma$-model). In previous papers we considered the cases $0<\gamma <1$ and $\gamma \approx 1$. We argued that the pairing by a gapless boson is fundamentally different from BCS/Eliashberg pairing by a massive boson as for the former there exists not one but an infinite discrete set of topologically distinct solutions for the gap function ${\mathrm{\Delta }}_n\left({\omega }_m\right)$ at $T=0$ ($n=0,1,2,...$), each with its own condensation energy $E_{c,n}$. Here we extend the analysis to larger $1<\gamma <2$. We argue that the discrete set of solutions survives, and the spectrum of $E_{c,n}$ get progressively denser as $\gamma$ increases towards 2 and eventually becomes continuous at $\gamma \to 2$. This increases the strength of “longitudinal” gap fluctuations, which tend to reduce the actual superconducting $T_c$ compared to the onset temperature for the pairing and give rise to a pseudogap region of preformed pairs. We also detect two features on the real axis, which develop at $\gamma >1$ and also become critical at $\gamma \to 2$. First, the density of states evolves towards a set of discrete $\delta$-functions. Second, an array of dynamical vortices emerges in the upper frequency half plane, near the real axis. We argue that these two features come about because on a real axis, the real part of the dynamical electron-electron interaction, $V^{\prime }\left(\mathrm{\Omega }\right)\propto cos(\pi \gamma /2)/{\left|\mathrm{\Omega }\right|}^{\gamma }$, becomes repulsive for $\gamma >1$, and the imaginary $V^{\prime \prime }\left(\mathrm{\Omega }\right)\propto sin(\pi \gamma /2)/{\left|\mathrm{\Omega }\right|}^{\gamma }$, gets progressively smaller at $\gamma \to 2$. We speculate that the features on the real axis are consistent with the development of a continuum spectrum of the condensation energy, for which we used ${\mathrm{\Delta }}_n\left({\omega }_m\right)$ on the Matsubara axis. We consider the case $\gamma =2$ separately in the next paper.

We investigated the precise crystal structures and electronic states of a quasi-two-dimensional molecular conductor $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ at ambient pressure. The electronic resistivity of this molecular solid shows metal-to-insulator (MI) crossover behavior at $T_{\mathrm{MI}}=50\mathrm{K}$. Our x-ray diffraction and ${}^{13}\mathrm{C}$ nuclear magnetic resonance experiments revealed that $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ maintains the inversion symmetry below $T_{\mathrm{MI}}$. First-principles calculations found a pair of anisotropic Dirac cones at a general $k$ point, with the degenerate contact points at the Fermi level. The origin of the insulating state in this system is a small energy gap of $\sim 2\mathrm{meV}$ opened by the spin-orbit interaction. The $Z_2$ topological invariants indicate that this system is a weak topological insulator. Our results suggest that $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ is a promising material for studying the bulk Dirac electron system in two dimensions.

For a long time, two-dimensional (2D) hexagonal ${\mathrm{MoS}}_2$ was proposed as a promising material for the valleytronic system. However, the limited size of growth and low carrier mobility in ${\mathrm{MoS}}_2$ restrict its further application. Very recently, a new kind of hexagonal 2D MXene, $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$, was successfully synthesized with large size, excellent ambient stability, and considerable hole mobility. In this paper, based on first-principles calculations, we predict that the valley-contrasting properties can be realized in monolayer $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ and its derivative $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{As}}_4$. Beyond the traditional two-level valleys, the valleys in monolayer $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{As}}_4$ are multiple folded, implying another valley dimension. Such multiple-folded valleys can be described by a three-band low-power Hamiltonian. This study presents the theoretical advance and the potential applications of monolayer $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{N}}_4$ and $\mathrm{Mo}{\mathrm{Si}}_2{\mathrm{As}}_4$ in valleytronic devices, especially multiple information processing.

We derive and apply a general scheme for mapping a setup consisting of a half-filled single-level quantum dot coupled to one normal metallic and two superconducting phase-biased leads onto an ordinary half-filled single impurity Anderson model with single modified tunneling density of states. The theory allows for the otherwise unfeasible application of the standard numerical renormalization group and enables us to obtain phase-dependent local spectral properties as well as phase-dependent induced pairing and Josephson current. The resulting transport properties match well with the numerically exact continuous-time hybridization-expansion quantum Monte Carlo. For weakly coupled normal electrode, the spectral properties can be interpreted in terms of normal-electrode-broadened Andreev bound states with phase-dependent position analogous to the superconducting Anderson model, which coexist in the $\pi$-like phase with a Kondo peak whose phase-dependent Kondo temperature is extracted.

We propose the realization of higher winding number topological states in a one-dimensional system by means of a non-Hermitian, nonreciprocal topoelectrical (TE) circuit lattice. The crucial element of the circuit is a directional intercell $\pi$-phase coupling which is realized by operational amplifiers (op-amps). The phase of the coupling coefficients can be modulated by the choice of capacitive or inductive hoppings between the voltage nodes. The resulting topological state has a winding number of 2 compared to its Hermitian counterpart, which can have at most a winding number of one. Furthermore, in this system the nontrivial topological eigenmodes are localized at the edges. This localization can coexist with the non-Hermitian skin effect, the latter of which is induced by having different magnitudes of the left- and right-directional couplings. In practice, the higher winding number topological state can be distinguished from the trivial phase by the much higher resonant impedance values. Furthermore, by shunting the op-amps, we can recover the Hermiticity of the system and the conventional topological phase. The experimental accessibility and unprecedented tunability of the model parameters in our TE model provide a ready platform for the realization and detection of higher winding number topological phases in one-dimensional systems.

The thermoelectric figure of merit $ZT$ comprises electronic and vibrational contributions that drastically change across phase transitions, and the most common theoretical ab initio approach to thermoelectricity fails to describe the evolution of $ZT$ across finite-temperature structural transitions in its entirety. Furthermore, while the thermoelectric behavior of bulk SnSe has been extensively studied, SnSe monolayers have been experimentally realized only recently, and the existent prediction of thermoelectricity on this two-dimensional material is unreliable because it misses its structural transition altogether. SnSe monolayers (and similar GeS, GeSe, GeTe, SnS, and SnTe monolayers) experience a temperature-induced two-dimensional $Pnm{2}_1\to P4/nmm$ structural transition precipitated by the softening of vibrational modes, and we describe their thermoelectric properties across the phase transition, using molecular dynamics data to inform both electronic and vibrational coefficients directly and within the same footing. Similar to recent experimental observations pointing to an overestimated $ZT$ past the transition temperature in bulk SnSe, we find a smaller $ZT$ on SnSe monolayers when compared to its value predicted by the standard paradigm, due to the dramatic changes in the electrical conductivity and lattice thermal conductivity as the structural transition ensues. The process described here lends a strong focus to both the vibrational and electronic evolutions throughout the structural transition, and it applies to thermoelectric materials undergoing thermally driven solid-to-solid structural phase transitions in one, two, and three dimensions.

Negative longitudinal magnetoresistance (NLMR) has been reported in a variety of materials and has attracted extensive attention as an electrotransport hallmark of topological Weyl semimetals. However, its origin is still under debate. Here, we demonstrate that the NLMR in a two-dimensional electron gas can be influenced by the measurement current. While the NLMR persists up to 130 K, its magnitude and magnetic field response become dependent on the applied current below 60 K. The tunable NLMR at low and high currents can be best attributed to quantum interference and disorder scattering effects, respectively. This work uncovers non-Ohmic NLMR in a non-Weyl material and highlights potential effects of the measurement current in elucidating electrotransport phenomena. We also demonstrate that NLMRs can be a valuable phenomenon in revealing the origins of other properties, such as negative MRs in perpendicular magnetic fields.

A fragile topological insulator (FTI) can be viewed as an almost-atomic insulator, with emergent negative charges localized at certain real-space points, even though the underlying lattice Hilbert space contains only positively charged states. Fragile topology in free-fermion systems has been fruitfully studied using modern topological band theory. However, the concept is well defined even for strongly correlated systems, and fragile states that cannot be realized within free-fermion band theory exist abundantly. In this work we propose a rather general parton construction for such correlation-enabled FTIs. In our parton construction the associated gauge symmetries are completely Higgsed, resulting in only short-range entangled states. The effective negative charges in the FTIs emerge naturally as the remnants of negatively charged partons. For spinful electrons with $SU\left(2\right)$ spin-rotation symmetry, the fragile phases can be viewed as FTIs of charge-2, spin singlet bosonic Cooper pairs. We discuss a few examples of correlated FTIs for both spinless and spinful fermions, including some “featureless Mott insulators” on the honeycomb lattice previously discussed in the literature.

We investigate the effect of a metal plate on the variable range hopping (VRH) conductivity of a two-dimensional electron-glass (EG) system. The VRH conductivity is known to have a stretched exponential dependence on temperature, with an exponent $p$ that depends on the shape of the EG's single-particle density of states (DOS). For constant DOS $p=\frac{1}{3}$ and for linear DOS $p=\frac{1}{2}$, also known as Mott's and Efros-Shklovskii's VRH, respectively. The presence of the plate causes two effects on the EG system, static and dynamic. The well-known static effect accounts for the additional screening of the Coulomb repulsion in the EG and for the partial filling of the Coulomb gap in the DOS. This in turn causes an increase of the conductivity at very low temperatures. Here, we investigate the complementary dynamical effect, which is related to the polaronic phenomena. Our main result is the dynamical suppression of the standard phonon-assisted hopping and, thus, suppression of the conductivity in a much wider range of temperatures as compared to the low-temperature static effect. The relation to experiments is discussed.

Strong intrinsic or interfacial spin-orbit coupling (SOC) can enable a thin-film superconductor to exceed the paramagnetic limit. For Rashba-type SOC, we show that the superconducting thermodynamic properties of a finite-size thin film are strongly sample-size dependent due to the creation of edge states; for example, in the case of geometrically anisotropic thin films, the critical field is found to be tunable through the direction of an externally applied in-plane magnetic field. These findings open perspectives for the development of superconducting spin-orbitronic devices.

Nonlinear phononics holds the promise for controlling properties of quantum materials on the ultrashort timescale. Using nonequilibrium dynamical mean-field theory, we solve a model for the description of organic solids, where correlated electrons couple nonlinearly to a quantum phonon mode. Unlike previous works, we exactly diagonalize the local phonon mode within the noncrossing approximation to include the full phononic fluctuations. By exciting the local phonon in a broad range of frequencies near resonance with an ultrashort pulse, we show it is possible to induce a Mott insulator-to-metal phase transition. Conventional semiclassical and mean-field calculations, where the electron-phonon interaction decouples, underestimate the onset of the quasiparticle peak. This fact, together with the nonthermal character of the photoinduced metal, suggests a leading role of the phononic fluctuations and of the dynamic nature of the state in the vibrationally induced quasiparticle coherence.

Non-Hermiticity is expected to add far more physical features to the already rich Floquet topological phases of matter. Nevertheless, a systematic approach to characterize non-Hermitian Floquet topological matter is still lacking. In this work we introduce a dual scheme to characterize the topology of non-Hermitian Floquet systems in momentum space and in real space using a piecewise quenched nonreciprocal Su-Schrieffer-Heeger model for our case studies. Under the periodic boundary condition, topological phases are characterized by a pair of experimentally accessible winding numbers that make jumps between integers and half integers. Under the open boundary condition, a Floquet version of the so-called open boundary winding number is found to be integers and can predict the number of pairs of zero and $\pi$ Floquet edge modes coexisting with the non-Hermitian skin effect. Our results indicate that a dual characterization of non-Hermitian Floquet topological matter is necessary and also feasible because the formidable task of constructing the celebrated generalized Brillouin zone for non-Hermitian Floquet systems with multiple hopping length scales can be avoided. This work hence paves a way for further studies of non-Hermitian physics in nonequilibrium systems.

Earlier neutron scattering studies suggested the coexistence of complex stripelike and zigzaglike antiferromagnetic orders in polar honeycomb lattice ${\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$, while its magnetoelectric (ME) behavior as an emergent effect is thus of high interest. Here we report our observations of two metamagnetic transitions and novel ME responses of ${\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ single crystals against high magnetic field $H$ up to ∼60 T. The $c$-axis (polar axis) spontaneous electric polarization $P_{\mathrm{spin}}$, emerging at the magnetic Néel temperature $T_N\sim 5.5\mathrm{K}$, and its remarkable response to $H$ applied along the $c$ axis ($H//c$) and $a$ axis ($H//a$), respectively, provide the clear evidence for the magnetism-driven ferroelectricity. While the magnetism exhibits the in-plane anisotropy to some extent, the magnetic field dependencies of magnetization and electric polarization in the low-field region and high-field region are distinctly different. In the low-field region where a weak spin-flop type metamagnetic transition occurs, the electric polarization response shows the parabolic dependence of magnetic field applied along both the $c$ axis and in-plane $a$ axis. The second metamagnetic transition happens when the magnetic field extends up to the high-field region where the magnetization and electric polarization response at low temperature are characterized by an extraordinarily broad plateau for the magnetic field along the $c$ axis but roughly linear dependence for field along the $a$ axis. These unusual phenomena are discussed, based on the symmetry-related local ME tensor analysis, and it is suggested that both the spin current and p-d hybridization mechanisms may contribute to the spontaneous electric polarization and ME responses. The present work demonstrates ${\mathrm{Ni}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ as a unique multiferroic and promising platform for exploring the rich spin-1 physics and ME phenomena in honeycomb lattice.

We calculate the Curie temperature of layered ferromagnets, chromium tri-iodide (${\mathrm{CrI}}_3$), chromium tri-bromide ($\mathrm{Cr}{\mathrm{Br}}_3$), chromium germanium tri-telluride ($\mathrm{Cr}\mathrm{Ge}{\mathrm{Te}}_3$), and the Néel temperature of a layered antiferromagnet iron di-chloride ($\mathrm{Fe}{\mathrm{Cl}}_2$), using first-principles density functional theory calculations and Monte Carlo simulations. We develop a computational method to model the magnetic interactions in layered magnetic materials and calculate their critical temperature. We provide a unified method to obtain the magnetic exchange parameters ($J$) for an effective Heisenberg Hamiltonian from first principles, taking into account both the magnetic ansiotropy as well as the out-of-plane interactions. We obtain the magnetic phase change behavior, in particular the critical temperature, from the susceptibility and the specific-heat, calculated using the three-dimensional Monte Carlo (metropolis) algorithm. The calculated Curie temperatures for ferromagnetic materials (${\mathrm{CrI}}_3, \mathrm{Cr}{\mathrm{Br}}_3$, and $\mathrm{Cr}\mathrm{Ge}{\mathrm{Te}}_3$), match well with experimental values. We show that the interlayer interaction in bulk $\mathrm{Cr}{\mathrm{I}}_3$ with $R\overline{3}$ stacking is significantly stronger than the $C2/m$ stacking, in line with experimental observations. We show that the strong interlayer interaction in $R\overline{3}$ $\mathrm{Cr}{\mathrm{I}}_3$ results in a competition between the in-plane and the out-of-plane magnetic easy axes. Finally, we calculate the Néel temperature of $\mathrm{Fe}{\mathrm{Cl}}_2$ to be $47\pm 8\mathrm{K}$ and show that the magnetic phase transition in $\mathrm{Fe}{\mathrm{Cl}}_2$ occurs in two steps with a high-temperature intralayer ferromagnetic phase transition and a low-temperature interlayer antiferromagnetic phase transition.