Despite their fundamental role in determining material properties, detailed momentum-dependent information on the strength of electron-phonon and phonon-phonon coupling (EPC and PPC, respectively) across the entire Brillouin zone has remained elusive. Here we demonstrate that ultrafast electron diffuse scattering (UEDS) directly provides such information. By exploiting symmetry-based selection rules and time resolution, scattering from different phonon branches can be distinguished even without energy resolution. Using graphite as a model system, we show that UEDS patterns map the relative EPC and PPC strength through their profound sensitivity to photoinduced changes in phonon populations. We measure strong EPC to the $K\text{−}\mathrm{point}$ TO phonon of $A_1^{\prime }$ symmetry $\left(K\text{−}{A}_1^{\prime }\right)$ and along the entire TO branch between $\mathrm{\Gamma }\text{−}K$, not only to the $\mathrm{\Gamma }\text{−}{E}_{2g}$ phonon. We also determine that the subsequent phonon relaxation of these strongly coupled optical phonons involve three stages: decay via several identifiable channels to TA and LA phonons $(1-2$ ps), intraband thermalization of the non-equilibrium TA/LA phonon populations $(30–40$ ps) and interband relaxation of the TA/LA modes (115 ps). Combining UEDS with ultrafast angle-resolved photoelectron spectroscopy will yield a complete picture of the dynamics within and between electron and phonon subsystems, helping to unravel complex phases in which the intertwined nature of these systems has a strong influence on emergent properties.

We present a systematic study of the filling constraints to realize a “trivial” insulator symmetric under magnetic space group $\mathcal{M}$. The filling $\nu$ must be an integer multiple of $m^{\mathcal{M}}$ to avoid spontaneous symmetry breaking or fractionalization in gapped phases. We improve the value of $m^{\mathcal{M}}$ in the literature and prove the tightness of the constraint for the majority of magnetic space groups. The result may shed light on the material search of exotic magnets with fractionalization.

We study signatures of magnetic quantum oscillations in three-dimensional nodal line semimetals at zero temperature. The extended nature of the degenerate bands can result in a Fermi surface geometry with topological genus one, as well as a Fermi surface of electron and hole pockets encapsulating the nodal line. Moreover, the underlying two-band model to describe a nodal line is not unique, in that there are two classes of Hamiltonian with distinct band topology giving rise to the same Fermi-surface geometry. After identifying the extremal cyclotron orbits in various magnetic field directions, we study their concomitant Landau levels and resulting quantum oscillation signatures. By Landau-fan-diagram analyses, we extract the nontrivial $\pi$ Berry phase signature for extremal orbits linking the nodal line.

Experimental evidence for Majorana bound states (MBSs) is so far mainly based on the robustness of a zero-bias conductance peak. However, similar features can also arise due to Andreev bound states (ABSs) localized at the end of an island. We show that these two scenarios can be distinguished by an interferometry experiment based on embedding a Coulomb-blockaded island into an Aharonov-Bohm ring. For two ABSs, when the ground state is nearly degenerate, cotunneling can change the state of the island, and interference is suppressed. By contrast, for two MBSs the ground state is nondegenerate, and cotunneling has to preserve the island state, which leads to $h/e$-periodic conductance oscillations with magnetic flux. Such interference setups can be realized with semiconducting nanowires or two-dimensional electron gases with proximity-induced superconductivity and may also be a useful spectroscopic tool for parity-flip mechanisms.

We measure electrically detected magnetic resonance on organic light-emitting diodes made of the polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] at room temperature and high magnetic fields where spectral broadening of the resonance due to spin-orbit coupling (SOC) exceeds that due to the local hyperfine fields. Density-functional-theory calculations on an open-shell model of the material reveal $g$-tensors of charge-carrier spins in the lowest unoccupied (electron) and highest occupied (hole) molecular orbitals. These tensors are used for simulations of magnetic resonance line shapes. Besides providing the first quantification and direct observation of SOC effects on charge-carrier states in these weakly SO-coupled hydrocarbons, this procedure demonstrates that spin-related phenomena in these materials are fundamentally monomolecular in nature.

We develop a first-principles approach based on many-body perturbation theory to investigate the effects of the interaction between electrons and carrier plasmons on the electronic properties of highly doped semiconductors and oxides. Through the evaluation of the electron self-energy, we account simultaneously for electron-plasmon and electron-phonon coupling in theoretical calculations of angle-resolved photoemission spectra, electron linewidths, and relaxation times. We apply this methodology to electron-doped anatase ${\mathrm{TiO}}_2$ as an illustrative example. The simulated spectra indicate that electron-plasmon coupling in ${\mathrm{TiO}}_2$ underpins the formation of satellites at energies comparable to those of polaronic spectral features. At variance with phonons, however, the energy of plasmons and their spectral fingerprints depends strongly on the carrier concentration, revealing a complex interplay between plasmon and phonon satellites. The electron-plasmon interaction accounts for approximately 40% of the total electron-boson interaction strength, and it is key to improve the agreement with measured quasiparticle spectra.

The compound ${\mathrm{EuCo}}_{2-y}{\mathrm{As}}_2$ with the tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ structure is known to contain ${\mathrm{Eu}}^{+2}$ ions with spin $S=\frac{7}{2}$ that order below a temperature $T_{\mathrm{N}}\approx 47$ K into an antiferromagnetic (AFM) proper helical structure with the ordered moments aligned in the tetragonal $ab$ plane, perpendicular to the helix axis along the $c$ axis, with no contribution from the Co atoms. Here we carry out a detailed investigation of the properties of single crystals. We consistently find about 5% vacancies on the Co site from energy-dispersive x-ray analysis and x-ray diffraction refinements. Enhanced ordered and effective moments of the Eu spins are found in most of our crystals. Electronic structure calculations indicate that the enhanced moments arise from polarization of the $d$ bands, as occurs in ferromagnetic Gd metal. Electrical resistivity measurements indicate metallic behavior. The low-field in-plane magnetic susceptibilities ${\chi }_{ab}(T<T_{\mathrm{N}})$ for several crystals are reported that are fitted well by unified molecular field theory (MFT), and the Eu–Eu exchange interactions $J_{ij}$ are extracted from the fits. High-field magnetization $M$ data for magnetic fields $H\parallel ab$ reveal what appears to be a first-order spin-flop transition followed at higher field by a second-order metamagnetic transition of unknown origin, and then by another second-order transition to the paramagnetic (PM) state. For $H\parallel c$, the magnetization shows only a second-order transition from the canted AFM to the PM state, as expected. The critical fields for the AFM to PM transition are in approximate agreement with the predictions of MFT. Heat capacity $C_{\mathrm{p}}$ measurements in zero and high $H$ are reported. Phase diagrams for $H\parallel c$ and $H\parallel ab$ versus $T$ are constructed from the high-field $M(H,T)$ and $C_{\mathrm{p}}(H,T)$ measurements. The magnetic part $C_{\mathrm{mag}}(T,H=0)$ of $C_{\mathrm{p}}(T,H=0)$ is extracted and is fitted rather well below $T_{\mathrm{N}}$ by MFT, although dynamic short-range AFM order is apparent in $C_{\mathrm{mag}}\left(T\right)$ up to about 70 K, where the molar entropy attains its high-$T$ limit of $Rln8$.

Standard plane-wave pseudopotential (PWPP) calculations for slabs such as graphene become extremely demanding, as soon as the exact exchange (EXX) of density functional theory is applied. Even if the Krieger-Li-Iafrate (KLI) approximation for the EXX potential is utilized, such EXX-PWPP calculations suffer from the fact that an accurate representation of the occupied states throughout the complete vacuum between the replicas of the slab is required. In this contribution, a robust and efficient extension scheme for the PWPP states is introduced, which ensures the correct exponential decay of the slab states in the vacuum for standard cutoff energies and therefore facilitates EXX-PWPP calculations for very wide vacua and rather thick slabs. Using this scheme, it is explicitly verified that the Slater component of the EXX/KLI potential decays as $-1/z$ over an extended region sufficiently far from the surface (assumed to be perpendicular to the $z$ direction) and from the middle of the vacuum, thus reproducing the asymptotic behavior of the exact EXX potential of a single slab. The calculations also reveal that the orbital-shift component of the EXX/KLI potential is quite sizable in the asymptotic region. In spite of the long-range exchange potential, the replicas of the slab decouple rather quickly with increasing width of the vacuum. Relying on the identity of the work function with the Fermi energy obtained with a suitably normalized total potential, the present EXX/KLI calculations predict work functions for both graphene and the Si(111) surface which are substantially larger than the corresponding experimental data. Together with the size of the orbital-shift potential in the asymptotic region, the very large EXX/KLI work functions indicate a failure of the KLI approximation for nonmetallic slabs.

It is of great interest to design and make materials in which ferroelectric polarization is coupled to other order parameters such as lattice, magnetic, and electronic instabilities. Such materials will be invaluable in next-generation data storage devices. Recently, remarkable progress has been made in understanding improper ferroelectric coupling mechanisms that arise from lattice and magnetic instabilities. However, although theoretically predicted, a compact lattice coupling between electronic and ferroelectric (polar) instabilities has yet to be realized. Here we report detailed crystallographic studies of a perovskite ${\mathrm{Hg}}^{\mathbf{A}}{\mathrm{Mn}}_3^{{\mathbf{A}}^{\prime }}{\mathrm{Mn}}_4^{\mathbf{B}}{\mathrm{O}}_{12}$ that is found to exhibit a polar ground state on account of such couplings that arise from charge and orbital ordering on both the ${\mathrm{A}}^{\prime }$- and B-sites, which are themselves driven by a highly unusual ${\mathrm{Mn}}^{{\mathrm{A}}^{\prime }}-{\mathrm{Mn}}^{\mathrm{B}}$ intersite charge transfer. The inherent coupling of polar, charge, orbital, and hence magnetic degrees of freedom make this a system of great fundamental interest, and demonstrating ferroelectric switching in this and a host of recently reported hybrid improper ferroelectrics remains a substantial challenge.

We demonstrate the triggering and detection of coherent electron-nuclear spin oscillations related to the hyperfine interaction in Ga deep paramagnetic centers in GaAsN by band-to-band photoluminescence without an external magnetic field. In contrast to other point defects such as ${\mathrm{Cr}}^{4+}$ in SiC, ${\mathrm{Ce}}^{3+}$ in yttrium aluminum garnet crystals, nitrogen-vacancy centers in diamond, and P atoms in silicon, the bound-electron spin in Ga centers is not directly coupled to the electromagnetic field via the spin-orbit interaction. However, this apparent drawback can be turned into an advantage by exploiting the spin-selective capture of conduction band electrons to the Ga centers. On the basis of a pump-probe photoluminescence experiment we measure directly in the temporal domain the hyperfine constant of an electron coupled to a gallium defect in GaAsN by tracing the dynamical behavior of the conduction electron spin-dependent recombination to the defect site. The hyperfine constants and the relative abundance of the nuclei isotopes involved can be determined without the need of an electron spin resonance technique and in the absence of any magnetic field. Information on the nuclear and electron spin relaxation damping parameters can also be estimated from the oscillation amplitude decay and the long-time-delay behavior.

Thermal ripples of graphene are well understood at room temperature, but their quantum counterparts at low temperatures are in need of a realistic quantitative description. Here we present atomistic path-integral Monte Carlo simulations of freestanding graphene, which show upon cooling a striking classical-quantum evolution of height and angular fluctuations. The crossover takes place at ever-decreasing temperatures for ever-increasing wavelengths so that a completely quantum regime is never attained. Zero-temperature quantum graphene is flatter and smoother than classical graphene at large scales yet rougher at short scales. The angular fluctuation distribution of the normals can be quantitatively described by coexistence of two Gaussians, one classical strongly $T$-dependent and one quantum about $2^{\circ }$ wide, of zero-point character. The quantum evolution of ripple-induced height and angular spread should be observable in electron diffraction in graphene and other two-dimensional materials, such as ${\mathrm{MoS}}_2$, bilayer graphene, boron nitride, etc.

We study the coherent evolution of an InAs self-assembled quantum dot (QD) ensemble in the ultrafast regime. The evolution of the entire frequency distribution is revealed by performing prepulse two-dimensional (2D) coherent spectroscopy. Charged and neutral QDs display distinct nonlinear responses arising from two-level trion and four-level exciton-biexciton systems, respectively, and each signal is clearly separated in 2D spectra. Whereas the signals for charged QDs are symmetric with respect to the detuning, those for neutral QDs are asymmetric due to the asymmetric four-level energy structure. Experimental results for charged and neutral QDs are well reproduced by solving the optical Bloch equations, including detuning and excitation-induced dephasing (EID) effects. The temperature dependence suggests that wetting-layer carriers play an important role in EID.

Fracton order is a new kind of quantum order characterized by topological excitations that exhibit remarkable mobility restrictions and a robust ground-state degeneracy (GSD) which can increase exponentially with system size. In this paper, we present a generic lattice construction (in three dimensions) for a generalized X-cube model of fracton order, where the mobility restrictions of the subdimensional particles inherit the geometry of the lattice. This helps explain a previous result that lattice curvature can produce a robust GSD, even on a manifold with trivial topology. We provide explicit examples to show that the (zero-temperature) phase of matter is sensitive to the lattice geometry. In one example, the lattice geometry confines the dimension-1 particles to small loops, which allows the fractons to be fully mobile charges, and the resulting phase is equivalent to (3+1)-dimensional toric code. However, the phase is sensitive to more than just lattice curvature; different lattices without curvature (e.g., cubic or stacked kagome lattices) also result in different phases of matter, which are separated by phase transitions. Unintuitively, however, according to a previous definition of phase [X. Chen et al., Phys. Rev. B 82, 155138 (2010)], even just a rotated or rescaled cubic results in different phases of matter, which motivates us to propose a coarser definition of phase for gapped ground states and fracton order. This equivalence relation between ground states is given by the composition of a local unitary transformation and a quasi-isometry (which can rotate and rescale the lattice); equivalently, ground states are in the same phase if they can be adiabatically connected by varying both the Hamiltonian and the positions of the degrees of freedom (via a quasi-isometry). In light of the importance of geometry, we further propose that fracton orders should be regarded as a geometric order.

We perform a microscopic analysis of how the constraints imposed by conservation laws affect $q=0$ Pomeranchuk instabilities in a Fermi liquid. The conventional view is that these instabilities are determined by the static interaction between low-energy quasiparticles near the Fermi surface, in the limit of vanishing momentum transfer $q$. The condition for a Pomeranchuk instability is set by $F_l^{c\left(s\right)}=-1$, where $F_l^{c\left(s\right)}$ (a Landau parameter) is a properly normalized partial component of the antisymmetrized static interaction $F(k,k+q;p,p-q)$ in a charge (c) or spin (s) subchannel with angular momentum $l$. However, it is known that conservation laws for total spin and charge prevent Pomeranchuk instabilities for $l=1$ spin- and charge-current order parameters. Our study aims to understand whether this holds only for these special forms of $l=1$ order parameters or is a more generic result. To this end we perform a diagrammatic analysis of spin and charge susceptibilities for charge and spin density order parameters, as well as perturbative calculations to second order in the Hubbard $U$. We argue that for $l=1$ spin-current and charge-current order parameters, certain vertex functions, which are determined by high-energy fermions, vanish at $F_{l=1}^{c\left(s\right)}=-1$, preventing a Pomeranchuk instability from taking place. For an order parameter with a generic $l=1$ form factor, the vertex function is not expressed in terms of $F_{l=1}^{c\left(s\right)}$, and a Pomeranchuk instability may occur when $F_1^{c\left(s\right)}=-1$. We argue that for other values of $l$, a Pomeranchuk instability may occur at $F_l^{c\left(s\right)}=-1$ for an order parameter with any form factor.

Recent experiments [Banerjee et al., arXiv:1710.00492] have measured thermal conductance of the $\nu =5/2$ edge in a GaAs electron gas and found it to be quantized as $K\approx 5/2$ (in appropriate dimensionless units). This result is unexpected, as prior numerical work predicts that the $\nu =5/2$ state should be the anti-Pfaffian phase of matter, which should have quantized $K=3/2$. The purpose of this Rapid Communication is to propose a possible solution to this conflict: If the Majorana edge mode of the anti-Pfaffian does not thermally equilibrate with the other edge modes, then $K=5/2$ is expected. I briefly discuss a possible reason for this nonequilibration and what should be examined further to determine if this is the case.

The combination of space inversion and time-reversal symmetries results in doubly degenerate Bloch states with opposite spin. Many lattices with these symmetries can be constructed by combining a noncentrosymmetric potential (lacking this degeneracy) with its inverted copy. Using simple models, we unravel the evolution of local spin splitting during this process of inversion symmetry restoration, in the presence of spin-orbit interaction and sublattice coupling. Importantly, through an analysis of quantum mechanical commutativity, we examine the difficulty of identifying states that are simultaneously spatially segregated and spin polarized. We also explain how surface-sensitive experimental probes (such as angle-resolved photoemission spectroscopy, or ARPES) of “hidden spin polarization” in layered materials are susceptible to unrelated spin splitting intrinsically induced by broken inversion symmetry at the surface.

In this paper theoretical and numerical studies of perfect/nearly perfect conversion of a plane wave into a surface wave are presented. The problem of determining the electromagnetic properties of an inhomogeneous lossless boundary which would fully transform an incident plane wave into a surface wave propagating along the boundary is considered. An approximate field solution which produces a slowly growing surface wave and satisfies the energy conservation law is discussed and numerically demonstrated. The results of the study are of great importance for the future development of such devices as perfect leaky-wave antennas and can potentially lead to many novel applications.

The magnetoconductance of a long channel InAs nanowire based field effect transistor in the quasiballistic regime under large magnetic field is investigated. The quasi-1D nanowire is fully characterized by a bias voltage spectroscopy and measurements under magnetic field up to 50 T applied either perpendicular or parallel to the nanowire axis lifting the spin and orbital degeneracies of the subbands. Under normal magnetic field, the conductance shows quantized steps due to the backscattering reduction and a decrease due to depopulation of the 1D modes. Under axial magnetic field, a quasioscillatory behavior is evidenced due to the coupling of the magnetic field with the angular momentum of the wave function. In addition the formation of cyclotron orbits is highlighted under high magnetic field. The experimental results are compared with theoretical calculation of the 1D band structure and related parameters.

The quantum phase transition to a ${\mathbb{Z}}_3$-ordered Kekulé valence bond solid in two-dimensional Dirac semimetals is governed by a fermion-induced quantum critical point, which renders the putatively discontinuous transition continuous. We study the resulting universal critical behavior in terms of a functional RG approach, which gives access to the scaling behavior on the symmetry-broken side of the phase transition, for general dimensions and number of Dirac fermions. In particular, we investigate the emergence of the fermion-induced quantum critical point for spacetime dimensions $2<D<4$. We determine the integrated RG flow from the Dirac semimetal to the symmetry-broken regime and analyze the underlying fixed-point structure. We show that the fermion-induced criticality leads to a scaling form with two divergent length scales, due to the breaking of the discrete ${\mathbb{Z}}_3$ symmetry. This provides another source of scaling corrections, besides the one stemming from being in the proximity to the first-order transition.

A common characteristic of quasi-one-dimensional (q1D) conductors $\beta \text{−}{A}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5(A$ = Li, Na, and Ag) is that the charge ordering (CO), the ground state (GS) at ambient pressure, and the superconducting (SC) phases, the GS under high pressure, are competing with each other. We have explored high-pressure properties of divalent $\beta$-vanadium bronzes, $\beta \text{−}{A}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5(A$ = Ca, Sr, and Pb), which are $A$-cation stoichiometry finely controlled single-crystal/powder samples, and found the absence of the SC phase. In these observations, however, we observed enormous and novel phase transitions, a kind of “devil's staircase”-type phase transitions in the charge ordering (CO) phases. The most surprising discovery in this devil's staircase, which was found mainly in $\beta \text{−}{\mathrm{Sr}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$, is that all the charge modulation vectors of many kinds of CO phases can be represented as a primitive lattice translation vector along the $b$ axis multiplied by several odd numbers. This discovery surely demonstrates interplay between the charge degree freedom and the crystallographic symmetry. We propose two possible mechanisms to explain this phenomenon: “self-charge transfer (carrier redistribution)” between the two subsystems in these compounds and “sequential symmetry reduction” that was discussed in Landau theory of phase transitions. In $\beta \text{−}{\mathrm{Ca}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ we also found a $P\text{−}T$ phase diagram similar in outlook but different in detail. The devil's staircase was also observed but it is an incomplete one. Furthermore, the charge modulation vectors in it are shorter than those in $\beta \text{−}{\mathrm{Sr}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$. In $\beta \text{−}{\mathrm{Pb}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$, which has no CO phase at ambient pressure, the pressure-induced antiferromagnetic ordering was observed at around 50 K above 0.5 GPa. Using these two kinds of mechanisms, we also explain the global high-pressure properties in all the stoichiometric divalent $\beta$-vanadium bronzes, which were observed as a wide variety of electromagnetic states. In addition, we also discuss a possible key for the presence/absence of the SC phase under pressure.