Here, we propose valley-selective excitations via a two-color $(\omega +2\omega )$ laser field, made by superimposing two linearly polarized pulses at frequencies $\omega$ and $2\omega$. We have studied the intensity ratio between a few-cycle pulse of an $\omega$ and $2\omega$ laser, and its enhancement factor by employing time-dependent first-principles calculations. The valley polarization depends on the carrier-envelope phases (CEPs) of the pulses and the intensity ratio $I_{\omega }/I_{2\omega }$. We found that a two-color field enhances valley polarization by as much as 1.2 times larger than a single-color pulse. Maximum valley asymmetry is achieved for an intensity ratio $I_{\omega }/I_{2\omega }$ of 36 with a relative CEP of $\pi$. In our previous work, we found that the asymmetric vector potential induces valley polarization [A. Hashmi et al., Phys. Rev. B 105, 115403 (2022)]. In this paper, we find that the asymmetry of the electric field modulates the valley polarization. Our two-color scheme offers another path toward the optical control of valley pseudospins.

We derive a framework to apply topological quantum chemistry in systems subject to magnetic flux. We start by deriving the action of spatial symmetry operators in a uniform magnetic field, which extends Zak's magnetic translation groups to all crystal symmetry groups. Ultimately, the magnetic symmetries form a projective representation of the crystal symmetry group. As a consequence, band representations acquire an extra gauge invariant phase compared to the nonmagnetic theory. Thus, the theory of symmetry indicators is distinct from the nonmagnetic case. We give examples of new symmetry indicators that appear at $\pi$ flux. Finally, we apply our results to an obstructed atomic insulator with corner states in a magnetic field. The symmetry indicators reveal a topological-to-trivial phase transition at finite flux, which is confirmed by a Hofstadter butterfly calculation. The bulk phase transition provides a new probe of higher order topology in certain obstructed atomic insulators.

We revisit a phenomenological model of fermions coupled to fluctuating bosons that emerges from finite-momentum particle-particle pairs for describing the strange metal phase in the cuprates. The incoherent bosons dominate the transport properties for the resistivity and optical conductivity in the non-Fermi-liquid phase. Within the Kubo formalism, the resistivity is approximately linear in temperature with a Drude form for the optical conductivity, such that the Drude lifetime is inversely proportional to the temperature. Here, we compute the transport properties of such bosons within the memory-matrix approach that successfully captures the hydrodynamic regime. This technique emerges as the appropriate framework for describing the transport coefficients of the strange metal phase. Our analysis confirms the $T$-linear resistivity due to the umklapp scattering that we obtained for this effective model. Finally, we provide new predictions regarding the variation of the thermal conductivity with temperature and examine the validity of the Wiedemann-Franz law.

Ultracompact devices for second harmonic generation (SHG) have broad applications in various fields. In this study, we numerically demonstrate efficient SHG from a merged bound state in the continuum (BIC) based on a lithium niobate (LN) freestanding metasurface. Such merged BIC efficiently reduces the radiative loss and strongly enhances the quality factor (Q factor). We achieved 1% conversion efficiency under the $5^{\circ }$ incident angle with 2 $\mathrm{kW}/{\mathrm{cm}}^2$ input intensity, which surpasses the performances of other nanostructures supported by traditional isolated BICs. Our work paves a promising way to enhance the interactions between light and matters at nanoscales, and provide a feasible idea in designing ultracompact SHG light sources.

We study the prerequisites for realizing superconductivity in doped triangular-lattice Mott insulators by considering three distinct parent spin backgrounds, i.e., ${120}^{\circ }$ antiferromagnets, quantum spin liquid, and stripy antiferromagnets, and all possible sign combinations $({\tau }_1,{\tau }_2)$ of nearest-neighbor hopping and next-nearest-neighbor hopping $(t_1,t_2)$. Based on density matrix renormalization group calculations, we find that, with finite $t_2$ and specific sign combinations $({\tau }_1,{\tau }_2)$, the quasi-long-range superconductivity order can always be achieved, regardless of the nature of the parent spin backgrounds. Besides specific hopping signs $({\tau }_1,{\tau }_2)$, these superconductivity phases in triangular lattices are commonly characterized by short-ranged spin correlations and two charges per stripe. In the robust superconductivity phase realized at larger $t_2/t_1$, flipping the signs ${\tau }_2$ and ${\tau }_1$ gives rise to the stripe phase without strong pairing and a pseudogaplike phase without Cooper-pair phase coherence, respectively. Interestingly, the roles of the two hopping signs are switched at smaller $t_2/t_1$. Moreover, different sign combinations $({\tau }_1,{\tau }_2)$ would stabilize distinct phases including superconductivity, charge density waves, spin density waves, and pseudogaplike phases accordingly. Our findings suggest the important role of charge kinetic energy in realizing superconductivity in doped triangular-lattice Mott insulators.

The competition between noncommuting projective measurements in discrete quantum circuits can give rise to entanglement transitions. It separates a regime where initially stored quantum information survives the time evolution from a regime where the measurements destroy the quantum information. Here we study one such system—the projective transverse field Ising model—with a focus on its capabilities as a quantum error correction code. The idea is to interpret one type of measurement as an error and the other type as a syndrome measurement. We demonstrate that there is a finite threshold below which quantum information encoded in an initially entangled state can be retrieved reliably. In particular, we implement the maximum likelihood decoder to demonstrate that the error correction threshold is distinct from the entanglement transition. This implies that there is a finite regime where quantum information is protected by the projective dynamics, but cannot be retrieved by using syndrome measurements.

The recently discovered $\mathrm{U}{\mathrm{Te}}_2$ superconductor is regarded as a heavy-fermion mixed-valence system with very peculiar properties within the normal and superconducting (SC) states. It shows no signs of magnetic order, but has a strong anisotropy of magnetic susceptibility and SC critical field. In addition to a heavy-fermion-like behavior in the normal state, it exhibits a distinctive Schottky-type anomaly at $\sim 12$ K and a characteristic excitations gap $\sim 35-40$ K. Here, we show, by virtue of dynamical mean-field theory calculations with a quasiatomic treatment of electron correlations, that ab initio-derived crystal-field splitting of the $5f^2$ ionic configuration yields agreement with these experimental observations. A close analogy of the normal paramagnetic state of $\mathrm{U}{\mathrm{Te}}_2$ to that of $\mathrm{U}{\mathrm{Ru}}_2{\mathrm{Si}}_2$ in the Kondo arrest scenario is revealed. We consider the impact of anisotropic superexchange interactions within the U-U structural dimer on magnetization and development of strong orbital and magnetic moment fluctuations in $\mathrm{U}{\mathrm{Te}}_2$.

We report the results of muon spin rotation and relaxation $\left(\mu \mathrm{SR}\right)$ measurements on the recently discovered layered Cu-based superconducting material ${\mathrm{La}}_2{\left({\mathrm{Cu}}_{1-x}{\mathrm{Ni}}_x\right)}_5{\mathrm{As}}_3{\mathrm{O}}_2$ ($x=0.40$ and 0.45). Transverse-field $\mu \mathrm{SR}$ experiments on both samples show that the temperature dependence of superfluid density is best described by a two-band model. The absolute values of zero-temperature magnetic penetration depth ${\lambda }_{\mathrm{ab}}\left(0\right)$ were found to be 427(1.7) and 422(1.5) nm for $x=0.40$ and 0.45, respectively. Both compounds are located between the unconventional and the standard BCS superconductors in the Uemura plot. No evidence of time-reversal symmetry breaking in the superconducting state is suggested by zero-field $\mu \mathrm{SR}$ measurements.

The celebrated Little-Parks effect in mesoscopic superconducting rings has recently gained great attention due to its potential to probe half-quantum vortices in spin-triplet superconductors. However, despite the large number of works reporting anomalous Little-Parks measurements attributed to unconventional superconductivity, the general signatures of spin-triplet pairing in the Little-Parks effect have been less systematically investigated. Here we use Ginzburg-Landau theory to study the Little-Parks effect in a spin-triplet superconducting ring that supports half-quantum vortices; we calculate the field-induced Little-Parks oscillations of both the critical temperature itself and the residual resistance resulting from thermal vortex tunneling below the critical temperature. We observe two separate critical temperatures with a single-spin superconducting state in between and find that due to the existence of half-quantum vortices, each minimum in the upper critical temperature splits into two minima for the lower critical temperature. From a rigorous calculation of the residual resistance, we confirm that these two minima in the lower critical temperature translate into two maxima in the residual resistance below and establish the general conditions under which the two maxima can be practically resolved. In particular, we identify a fundamental trade-off between sharpening each maximum and keeping the overall magnitude of the resistance large. Our results will guide experimental efforts in designing mesoscopic ring geometries for probing half-quantum vortices in spin-triplet candidate materials on the device scale.

Topology is a key ingredient driving the emergence of quantum devices. The topological field-effect transistor (TFET) has been proposed to outperform the conventional field-effect transistor by replacing the on state with topology-protected quantized conductance, while the off state has the same normal insulating characteristics and hence bears similar drawbacks. Here, we demonstrate a proof-of-concept TFET, having both on and off quantized conductance, by switching between helical and chiral topological screw dislocation (SD) states in three-dimensional topological insulators. A pair of SDs are configured, with one acting as a channel and the other as a gate controlled by a local magnetic field. Reversible field switching is achieved with the on and off conductance of $2e^2/h$ and $e^2/h$, respectively, as shown by tight-binding quantum transport calculations. Furthermore, $\mathrm{BaBi}{\mathrm{O}}_3$ is shown as one candidate material that has the desired topological SD states, based on first-principles calculations. Our findings open a new route to high-fidelity topological quantum devices.

A well-established numerical technique to study the dynamics of spin systems in which symmetries and conservation laws play an important role is to microcanonically integrate their reversible equations of motion, obtaining thermalization through initial conditions drawn with the canonical distribution. In order to achieve a more realistic relaxation of the magnetic energy, numerically expensive methods that explicitly couple the spins to the underlying lattice are normally employed. Here we introduce a stochastic conservative thermostat that relaxes the magnetic energy while preserving the constant of motions, thus turning microcanonical spin dynamics into a conservative canonical dynamics, without actually simulating the lattice. We test the thermostat on the Heisenberg antiferromagnet in $d=3$ and show that the method reproduces the exact values of the static and dynamic critical exponents, while in the low-temperature phase it yields the correct spin wave phenomenology. Finally, we demonstrate that the relaxation coefficient of the new thermostat is quantitatively connected to the microscopic parameters of the spin-lattice coupling.

We observe an enhancement of inverse magnetocaloric effect (MCE) in $\mathrm{Sm}\mathrm{Ca}\mathrm{Co}\mathrm{Mn}{\mathrm{O}}_6$ (94%) and $\mathrm{Sm}\mathrm{Mn}{\mathrm{O}}_3$ (6%) nanocomposite due to uniform external field-induced random field (RF) to spin glass (SG) crossover. RF is generally observed in diluted antiferromagnet and dilute dipole-coupled Ising ferromagnet systems. Under the application of magnetic-field ($H$), such systems may show a crossover from RF to SG at low temperatures ($T$). Here, we report a similar type of crossover but in ferrimagnetic $\mathrm{Sm}\mathrm{Ca}\mathrm{Co}\mathrm{Mn}{\mathrm{O}}_6$ and antiferromagnetic $\mathrm{Sm}\mathrm{Mn}{\mathrm{O}}_3$ nanocomposite. A model of site dilution of the ferrimagnetic phase by the embedded antiferromagnetic regions is proposed. In this composite system, the RF appears at $T<35$ K, which shows a crossover to SG at $T<15$ K under the applied field. The enhanced inverse MCE (with an entropy change, $-\mathrm{\Delta }{S}_M\sim -0.95$ J ${\mathrm{Kg}}^{-1}{\mathrm{K}}^{-1}$ for a change in $H\sim 70$ kOe at $T\sim 7.5$ K) due to the SG state is the consequence of field-induced RF-SG crossover giving highly $H$-dependent $-\mathrm{\Delta }{S}_M$ in the SG regime, a requisite for large MCE.

Electronically ordered states that break multiple symmetries can melt in multiple stages, similarly to liquid crystals. In a partially melted phase, known as vestigial phase, a bilinear made out of combinations of the multiple components of the primary order parameter condenses. Multicomponent superconductors are thus natural candidates for vestigial order since they break both the $\text{U}\left(1\right)$-gauge and also time-reversal or lattice symmetries. Here, we use group theory to classify all possible real-valued and complex-valued bilinears of a generic two-component superconductor on a tetragonal or hexagonal lattice. While the more widely investigated real-valued bilinears correspond to vestigial nematic or ferromagnetic order, the little explored complex-valued bilinears correspond to a vestigial charge-$4e$ condensate, which itself can have an underlying $s$-wave, $d_{x^2-y^2}$-wave, or $d_{xy}$-wave symmetry. To properly describe the fluctuating regime of the superconducting Ginzburg-Landau action and thus access these competing vestigial phases, we employ both a large-$N$ and a variational method. We show that while vestigial order can be understood as a weak-coupling effect in the large-$N$ approach, it is akin to a moderate-coupling effect in the variational method. Despite these distinctions, both methods yield similar results in wide regions of the parameter space spanned by the quartic Landau coefficients. Specifically, we find that the nematic and ferromagnetic phases are the leading vestigial instabilities, whereas the various types of charge-$4e$ order are attractive albeit subleading vestigial channels. The only exception is for the hexagonal case, in which the nematic and $s$-wave charge-$4e$ vestigial states are degenerate. We discuss the limitations of our approach, as well as the implications of our results for the realization of exotic charge-$4e$ states in material candidates.

Quantum dynamics in certain kinetically-constrained systems can display a strong sensitivity to the initial condition, wherein some initial states give rise to persistent quantum revivals—a type of weak ergodicity breaking known as “quantum many-body scarring” (QMBS). Recent work [Yao, Pan, Liu, and Zhai, Phys. Rev. B 105, 125123 (2022)] pointed out that QMBS gets destroyed by tuning the system to a quantum critical point, echoing the disappearance of long-range order in the system's ground state at equilibrium. Here we show that this picture can be much richer in systems that display QMBS dynamics from a continuous family of initial conditions: As the system is tuned across the critical point while at the same time deforming the initial state, the dynamical signatures of QMBS at intermediate times can undergo an apparently smooth evolution across the equilibrium phase transition point. We demonstrate this using the PXP model—a paradigmatic model of QMBS that has recently been realized in Rydberg atom arrays as well as ultracold bosonic atoms in a tilted optical lattice. Using exact diagonalization and matrix product state methods, we map out the dynamical phase diagram of the PXP model with the quenched chemical potential. We demonstrate the existence of a continuous family of initial states that give rise to QMBS and formulate a ramping protocol that can be used to prepare such states in experiment. Our results show the ubiquity of scarring in the PXP model and highlight its intriguing interplay with quantum criticality.

Unpaired Majorana zero modes are central to topological quantum computation schemes as building blocks of topological qubits, and are therefore under intense experimental and theoretical investigation. Their generalizations to parafermions and Fibonacci anyons are also of great interest, in particular for universal quantum computation schemes. In this work, we find a different generalization of Majorana zero modes in effectively noninteracting systems, which are zero-energy bound states that exhibit a cross structure (two straight, perpendicular lines in the complex plane) composed of the complex number entries of the zero-mode wave function on a lattice, rather than a single straight line formed by complex number entries of the wave function on a lattice as in the case of an unpaired Majorana zero mode. These “cross” zero modes are realized for topological skyrmion phases under certain open boundary conditions when their characteristic momentum-space spin textures trap topological defects. They therefore serve as a second type of bulk-boundary correspondence for the topological skyrmion phases. In the process of characterizing this defect bulk-boundary correspondence, we develop recipes for constructing physically relevant model Hamiltonians for topological skyrmion phases, efficient methods for computing the skyrmion number, and introduce three-dimensional topological skyrmion phases into the literature.

The solid solution 122 compounds, $\mathrm{Eu}{\left({\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{P}}_2$, show a valence transition between divalent state and intermediate valence states at Eu, which is firmly correlated to multiple degrees of freedom in the solid such as the isostructural transition between the collapsed tetragonal (cT) and uncollapsed tetragonal (ucT) structures, $3d$ magnetism, and the formation of P-P dimers. To gain insights into the correlated behavior, we investigate the effect of high magnetic fields on the samples of $x=0.4$ and 0.5 using magnetostriction and magnetization measurements up to 60 T. The samples are in the Eu valence-fluctuating regime, where the possible structural transition from cT to ucT may be induced by the Eu valence change under the magnetic fields. For both samples, magnetostriction smoothly increases with increasing magnetic fields. The behavior is in good agreement with the calculated results using the interconfigurational fluctuation (ICF) model that describes the Eu valence change. This indicates that $\mathrm{\Delta }L$ represents the change of the Eu valence state in these compounds. Magnetization curves for both compounds show good agreement with the ICF model at high magnetic fields. In contrast, in the low-magnetic-field region, magnetization curves do not agree with the ICF model. These results indicate that the Eu valence changes manifest themselves in the magnetization curves at high magnetic fields and that the magnetism of the $3d$ electrons manifests itself in the magnetization at low magnetic fields. Hence, we conclude that the valence change occurs within the Eu valence fluctuation regime coupled with the cT structure. Thereby, we believe that the transition to ucT structure, which is firmly coupled with the divalent Eu state, does not occur within the magnetic field range of the present study. Even higher magnetic fields or pulsed magnetic fields with slower pulse durations may induce such large state changes in the present material, which is triggered by the Eu valence change under high magnetic fields.

The fractional quantum Hall effect (FQHE) in the second Landau level (SLL) likely stabilizes non-Abelian topological orders. Recently, a parton sequence has been proposed to capture many of the fractions observed in the SLL [A. C. Balram, SciPost Phys. 10, 083 (2021)]. We consider the first member of this sequence which has not yet been studied, which is a non-Abelian state that occurs at $4/11$. As yet FQHE in the SLL at this fraction has not been observed in experiments. Nevertheless, by studying its competition with other candidate FQHE states in the SLL we show that this parton state might be viable. We also make predictions for experimentally measurable properties of the parton state which can distinguish it from other topological orders.

Nonsymmorphic crystals can host characteristic double surface Dirac cones with fourfold degeneracy on the Dirac points, called wallpaper fermion, protected by wallpaper group symmetry. We clarify the charge and spin Hall effect of wallpaper fermions in the presence of the (anti)ferromagnetism. Based on a four-sublattice model, we construct the effective Hamiltonian of wallpaper fermions coupled with the ferromagnetic or antiferromagnetic moment. Both ferromagnetic and antiferromagnetic moments induce an energy gap for the wallpaper fermions, leading to quantized (spin) Hall conductivity. The ferromagnetic wallpaper fermion induces the Hall conductivity quantized into $e^2/h$, which is twice that for a single Dirac fermion on the surface of topological insulators. On the other hand, the spin Hall conductivity decays and reaches to be a finite value as the antiferromagnetic coupling increases. We also show that the results above are valid for a general model of wallpaper fermions from symmetry consideration.

We explore the magnetohydrodynamics of Dirac fermions in neutral graphene in the Corbino geometry. Based on the fully consistent hydrodynamic description derived from a microscopic framework and taking into account all peculiarities of graphene-specific hydrodynamics, we report the results of a comprehensive study of the interplay of viscosity, disorder-induced scattering, recombination, energy relaxation, and interface-induced dissipation. In the clean limit, magnetoresistance of a Corbino sample is determined by viscosity. Hence the Corbino geometry could be used to measure the viscosity coefficient in neutral graphene.