In order to elucidate the charge density wave (CDW) nature in ${\mathrm{LaAuSb}}_2$, we have carried out the electrical resistivity, Hall coefficient, Seebeck coefficient, thermal conductivity, as well as ultrafast pump-probe spectroscopy measurements on single crystalline ${\mathrm{LaAuSb}}_2$. The CDW phase transition has been characterized by marked features near $T_C\simeq 88\mathrm{K}$. In particular, the obtained Hall carrier concentration and electronic thermal conductivity exhibit significant reduction as $T_C$ drops, providing strong evidence for partial gapping of the Fermi surfaces during the CDW phase transition. Furthermore, the ultrafast pump-probe spectroscopy reveals pronounced oscillatory features in the photoinduced transient reflectivity in the CDW ordered state. A characteristic oscillation mode with frequency of about 0.32 THz was extracted. The comparison of the obtained oscillation frequency with the frequencies of the amplitude mode in the well-studied CDW analogue ${\mathrm{LaAgSb}}_2$ allows us to ascribe the observation to the amplitude mode of CDW collective excitations in ${\mathrm{LaAuSb}}_2$.

In a recent Letter [T. Dornheim et al., Phys. Rev. Lett. 121, 255001 (2018)] we have presented ab initio results for the dynamic structure factor $S(\mathbf{q},\omega )$ of the uniform electron gas for conditions ranging from the warm dense matter regime to the strongly correlated electron liquid. This was achieved on the basis of exact path integral Monte Carlo data by stochastically sampling the dynamic local field correction $G(\mathbf{q},\omega )$. In this paper we introduce in detail this reconstruction method and provide several practical demonstrations. Moreover, we thoroughly investigate the associated imaginary-time density-density correlation function $F(\mathbf{q},\tau )$. The latter also gives us access to the static density-response function $\chi \left(\mathbf{q}\right)$ and static local field correction $G\left(\mathbf{q}\right)$, which are compared to standard dielectric theories like the widespread random phase approximation. In addition, we study the high-frequency limit of $G(\mathbf{q},\omega )$ and provide extensive results for the dynamic structure factor for different densities and temperatures. Finally, we discuss the implications of our findings for warm dense matter research and the interpretation of experiments.

The conformal anomaly, a quantum anomaly related to metric deformations in conformally invariant systems, has been recently shown to give rise to a special contribution to the Nernst signal, which remains finite at zero temperature and chemical potential. In this work we provide a Kubo calculation that confirms the result of this unexpected signal in the conformal limit and extends the calculation to finite temperature and chemical potential. As a result, we predict a distinctive experimental signature of the conformal anomaly in the form of a plateau behavior in the thermoelectric coefficient as a function of the chemical potential in the extreme quantum limit.

Matter manipulation with optical forces has become commonplace in a wide range of research fields and is epitomized by the optical trap. Calculations of optical forces on small illuminated particles typically neglect multiple scattering on nearby structures. However, this scattering can result in large recoil forces, particularly when the scattering includes directional near-field excitations. Near-field recoil forces have been studied in the case of electric, magnetic, and circularly polarized dipoles, but they exist for any type of directional near-field excitation. We use the force angular spectrum as a concise and intuitive analytical expression for the force on any dipole near planar surfaces, which allows us to clearly distinguish the effect due to the dipole, and due to the surface. We relate this directly to the coupling efficiency of surface or guided modes via Fermi's golden rule. To exemplify this, a near-field force transverse to the illumination is computationally calculated for a Huygens dipole near a metallic waveguide. We believe this formalism will prove insightful for various nanomanipulation systems within areas such as nanofluidics, sensing, biotechnology, and nanoassembly of nanostructures.

We theoretically propose a family of $f$-electron based magnets that realizes Kitaev-type bond-dependent anisotropic interactions. Based on ab initio calculations, we show that $A_2{\mathrm{PrO}}_3$ ($A$: alkali metals) crystallize in a monoclinic structure with honeycomb layers of edge-sharing ${\mathrm{PrO}}_6$ octahedra. Each ${\mathrm{Pr}}^{4+}$ cation has a $4f$ electron in the ${\mathrm{\Gamma }}_7$ doublet, which comprises a spin-orbital entangled Kramers pair with the effective moment $J_{\mathrm{eff}}=1/2$. By using the Wannier orbitals from the ab initio calculations, we find that the effective interactions between the $J_{\mathrm{eff}}=1/2$ moments are predominantly of antiferromagnetic Kitaev type for light alkali metals $A=\mathrm{Li}$ and Na, in stark contrast to the ferromagnetic ones in $4d$- and $5d$-electron based materials. Our finding would provide a playground for the Kitaev spin liquids that is hard to be accessed by previously discovered candidates.

We report on comprehensive de Haas–van Alphen (dHvA) and electronic band-structure studies of the superconducting skutterudites ${\mathrm{LaPt}}_4{\mathrm{Ge}}_{12}$ ($T_c=8.3$ K) and ${\mathrm{PrPt}}_4{\mathrm{Ge}}_{12}$ ($T_c=7.9$ K). Both materials show very rich spectra of dHvA oscillations with similar and only slightly varying angular-dependent frequencies. The spectral richness can partly be rationalized by the elaborated electronic band structures resulting in several Fermi surfaces built by six different bands. The effective cyclotron masses of both superconductors lie between about 0.5 and 1.1 times the free-electron mass. Although these values are small, we find moderate mass enhancements between about 2 and 4 when comparing to the calculated masses. Our results evidence the localized character of the $4f$ electrons in the Pr compound and are in line with an electron-phonon mediated multiband superconductivity, largely identical for both compounds.

The Pfaffian fractional quantum Hall (FQH) states are incompressible non-Abelian topological fluids present in a half-filled electron Landau level, where there is a balanced population of electrons and holes. They give rise to half-integral quantum Hall plateaus that divide critical transitions between integer quantum Hall (IQH) states. On the other hand, there are Abelian FQH states, such as the Laughlin state, that can be understood using partons, which are fermionic divisions of the electron. In this paper, we propose a family of incompressible paired parton FQH states at filling $\nu =\frac{1}{6}$ (modulo 1) that emerge from critical transitions between IQH states and Abelian FQH states at filling $\nu =\frac{1}{3}$ (modulo 1). These paired parton states are originated from a half-filled parton Landau level, where there is an equal amount of partons and holes. They generically support Ising-type anyonic quasiparticle excitations and carry non-Abelian Pfaffian topological orders (TO) for partons. We prove the principal existence of these paired parton states using exactly solvable interacting arrays of electronic wires under a magnetic field. Moreover, we establish a notion of particle-hole (PH) symmetry for partons and relate the PH symmetric parton Pfaffian TO with the gapped symmetric surface TO of a fractional topological insulator in three dimensions.

Triple point fermions are elusive electronic excitations that generalize Dirac and Weyl modes beyond the conventional high-energy paradigm. Yet, finding real materials naturally hosting these excitations at the Fermi energy has remained challenging. Here we show that twisted bilayer graphene is a versatile platform to realize robust triple point fermions in two dimensions. In particular, we establish that the introduction of localized impurities lifts one of the two degenerate Dirac cones, yielding triple point fermions at charge neutrality. Furthermore, we show that the valley polarization is preserved for certain impurity locations in the Moiré supercell for both weak and strong impurity potentials. We finally show that in the presence of interactions, a symmetry-broken state with local magnetization can develop out of the triple point bands, which can be selectively controlled by electrostatic gating. Our results put forward twisted bilayer graphene as a simple solid-state platform to realize triple point fermions at charge neutrality and demonstrate the nontrivial role of impurities in Moiré systems.

Weyl semimetals host linear energy dispersions around Weyl nodes, as well as monopoles of Berry curvature in momentum space around these points. These features give rise to unique transport signatures in a Weyl semimetal, such as transverse transport without an applied magnetic field, known as anomalous transport. The type-II Weyl semimetal, recently experimentally demonstrated in several materials, is classified by a tilting of the Weyl nodes. This paper provides a theoretical study on thermoelectric transport in time-reversal breaking type-II Weyl semimetals. Our results examine the balance between anomalous and nonanomalous contributions to the Nernst effect when subject to an external magnetic field. We also show how increasing scattering times have an enhancing effect on thermoelectric transport in these materials. Since a temperature-dependent chemical potential has been theoretically shown to be paramount when considering anomalous transport, we also study how similar considerations impact the Nernst thermopower in the nonanomalous case.

We theoretically investigate a single-molecule magnet (SMM) grafted to a quantum dot in contact with metallic leads and interacting with a resonant electromagnetic radiation. We explore both the explicit time-dependent behavior and the steady-state current-voltage characteristics of the device when the source lead is ferromagnetic. At zero-bias voltage, a net current is pumped through the device with the source spin current being reversed and amplified in the drain lead; this effect also persists for nonzero bias. We explain this effect in terms of spin transitions in the nanomagnet induced by the resonant radiation followed by their subsequent relaxation via spin-asymmetric charge-transfer processes. We demonstrate that the same effects are recovered in the time-averaged current when the device interacts with pulsed resonant radiation. Moreover, within the pulsed irradiation regime, appropriate choices of pulse length and wait times are shown here to allow the detection of coherent Rabi oscillations of the SMM spin states, via time-averaged spin current measurements.

We present a protocol for designing appropriately extended $\pi$ pulses that achieves tunable, thus selective, electron-nuclear spin interactions with low-driving radiation power. The latter is of great benefit when $\pi$ pulses are displayed over biological samples as it reduces sample heating. Our method is general since it can be applied to different quantum sensor devices such as nitrogen vacancy centers or silicon vacancy centers. Furthermore, it can be directly incorporated in commonly used stroboscopic dynamical decoupling techniques to achieve enhanced nuclear selectivity and control, which demonstrates its flexibility.

The vanadates ${\mathrm{VO}}_2$ and ${\mathrm{V}}_2{\mathrm{O}}_3$ are prototypical examples of strongly correlated materials that exhibit a metal-insulator transition. While the phase transitions in these materials have been studied extensively, there is a limited understanding of how the properties of these materials are affected by the presence of defects and doping. In this study we investigate the impact of native point defects in the form of Frenkel defects on the structural, magnetic, and electronic properties of ${\mathrm{VO}}_2$ and ${\mathrm{V}}_2{\mathrm{O}}_3$, using first-principles calculations. In ${\mathrm{VO}}_2$ the vanadium Frenkel pairs lead to a nontrivial insulating state. The unpaired vanadium interstitial bonds to a single dimer, which leads to a trimer that has one singlet state and one localized single-electron $S=1/2$ state. The unpaired broken dimer created by the vanadium vacancy also has a localized $S=1/2$ state. Thus, the insulating state is created by the singlet dimers, the trimer, and the two localized $S=1/2$ states. Oxygen Frenkel pairs, on the other hand, lead to a metallic state in ${\mathrm{VO}}_2$, but are expected to be present in much lower concentrations. In contrast, the Frenkel defects in ${\mathrm{V}}_2{\mathrm{O}}_3$ do not directly suppress the insulating character of the material. However, the disorder created by defects in ${\mathrm{V}}_2{\mathrm{O}}_3$ alters the local magnetic moments and in turn reduces the energy cost of a transition between the insulating and conducting phases of the material. We also find self-trapped small polarons in ${\mathrm{V}}_2{\mathrm{O}}_3$, which has implications for transport properties in the insulating phase.

We report a large spin Hall effect in the $3d$ transition metal alloy $\mathrm{N}{\mathrm{i}}_x\mathrm{C}{\mathrm{u}}_{1-x}$ for $x\in \{0.3,0.75\}$, detected via the ferromagnetic resonance of a permalloy ($\mathrm{Py}=\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}$) film deposited in a bilayer with the alloy. A thickness series at $x=0.6$, for which the alloy is paramagnetic at room temperature, allows us to determine the spin Hall ratio ${\theta }_{\mathrm{SH}}\approx 1,$ spin diffusion length ${\lambda }_{\mathrm{s}}$, spin mixing conductance $G_{\uparrow \downarrow }^{}$, and damping due to spin memory loss ${\alpha }_{\mathrm{SML}}$. We compare our results with similar experiments on Py/Pt bilayers measured using the same method. Ab initio band structure calculations with disorder and spin-orbit coupling suggest an intrinsic spin Hall effect in $\mathrm{N}{\mathrm{i}}_x\mathrm{C}{\mathrm{u}}_{1-x}$ alloys, although the experiments here cannot distinguish between extrinsic and intrinsic mechanisms.

Emergent crystals are the periodic alignment of “emergent particles,” i.e., localized collective behavior of atoms or their charges, spins, or orbits. The elasticity of these novel states of matter suffering mechanical loads can be strikingly softer than that of the underlying material, showing great strain-mediated tunability. However, the deformation and possible failure of emergent crystals under strong strain, vital for realizing related applications, are hitherto unclear. Here we theoretically study the nonlinear elasticity and structural transitions of a Skyrmion crystal (SkX) suffering uniaxial distortion. Under moderate tension, a SkX behaves like a ductile material, with an exotic negative crossover elastic stiffness and a negative emergent Poisson's ratio at appropriate conditions of magnetic field. Under strong strain, we observe at most six phase transitions, generating four novel emergent crystals that are thermodynamically metastable. We argue that this nondestructive polymorphism is a general feature for any emergent crystals exposed to mechanical loads.

Femtosecond time-resolved x-ray diffraction is employed to study the dynamics of the periodic lattice distortion (PLD) associated with the charge-density wave in ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_3$. Using a multipulse scheme we show the ability to extend the lifetime of coherent oscillations of the PLD about the undistorted structure through reexcitation of the electronic states. This suggests that it is possible to enter a regime where the symmetry of the potential energy landscape corresponds to the high-symmetry phase but the scattering pathways that lead to the damping of coherent dynamics are still controllable by altering the electronic state population. The demonstrated control over the coherence time offers different routes for the manipulation of coherent lattice states.

We study many-body localization (MBL) in a one-dimensional system of spinless fermions with a deterministic aperiodic potential in the presence of long-range interactions and long-range hopping. Based on perturbative arguments there is a common belief that MBL can exist only in systems with short-range interactions and short-range hopping. We analyze effects of power-law interactions and power-law hopping, separately, on a system which has all the single-particle states localized in the absence of interactions. Since delocalization is driven by proliferation of resonances in the Fock space, we mapped this model to an effective Anderson model on a complex graph in the Fock space and calculated the probability distribution of the number of resonances up to third order. Though the most-probable value of the number of resonances diverge for the system with long-range hopping [$t\left(r\right)\sim t_0/r^{\alpha }$ with $\alpha <2$], there is no enhancement of the number of resonances as the range of power-law interactions increases. This indicates that the long-range hopping delocalizes the many-body localized system but in contrast to this, there is no signature of delocalization in the presence of long-range interactions. We further provide support in favor of this analysis based on dynamics of the system after a quench starting from a charge density wave ordered state, level spacing statistics, return probability, participation ratio and Shannon entropy in the Fock space. We demonstrate that MBL persists in the presence of long-range interactions though long-range hopping with $1<\alpha <2$ delocalizes the system partially, with almost all the states extended for $\alpha \le 1$. Even in a system which has single-particle mobility edges in the noninteracting limit, turning on long-range interactions does not cause delocalization.

While the spin Seebeck effect has proven to be an invaluable tool for probing spin correlations in collinear magnetic insulators, a theory generalizing the heat-to-spin interconversion in noncollinear magnets is still lacking. Nonetheless, a variety of quantum magnets that are attracting an increasing attention to, e.g., their topological properties display a noncollinear spin order. Here, we establish a general framework for thermally driven spin transport at the interface between a noncollinear magnet and a normal metal. Modeling the interfacial coupling between localized and itinerant magnetic moments via an exchange Hamiltonian, we derive an expression for the spin current, driven by a temperature difference, for an arbitrary noncollinear magnetic order. Our theory reproduces previously obtained results for ferromagnetic and antiferromagnet systems.