Laser-assisted field evaporation of materials is often interpreted as an equilibrium phenomenon that can be described in terms of a thermally assisted process due to the increase of the lattice temperature defined for equilibrium phonon distribution. Our simulations, taking into account nonequilibrium phonon distribution, produced by femtosecond laser radiation, and coupled carrier-phonon kinetics in the sample, show that the evaporation process is assisted by hot phonons. An out-of-thermal-equilibrium phenomenon on the picosecond time scale is thus demonstrated in the case of femtosecond-laser-assisted field evaporation of semiconductors. It is experimentally confirmed by atom-probe measurements using an original double-pulse correlation setup.

Nitrogenated holey graphene (NHG) is a recently synthesized two-dimensional material. In this paper the structural and electronic properties of heterostructures of graphene and NHG are investigated using first-principles and tight-binding calculations. Due to the lattice mismatch between NHG and graphene, the formation of a moiré pattern is preferred in the graphene/NHG heterostructure, instead of a lattice-coherent structure. In moiré-patterned graphene/NHG, the band gap opening at the $K$ point is negligible, and the linear band dispersion of graphene survives. Applying an electric field modifies the coupling strength between the two atomic layers. The Fermi velocity $v_F$ is reduced as compared to the one of pristine graphene, and its magnitude depends on the twist angle $\theta$ between graphene and NHG: For $\theta =0^{\circ }, v_F$ is 30% of that of graphene, and it increases rapidly to a value of 80% with increasing $\theta$. The heterostructure exhibits electron-hole asymmetry in $v_F$, which is large for small $\theta$. In NHG encapsulated between two graphene layers, a “Dirac ring” appears around the $K$ point. Its presence is robust with respect to the relative stacking of the two graphene layers. These findings can be useful for future applications of graphene/NHG heterostructures.

We investigate the pumped charge and spin at zero bias by a modulation of two control parameters using the full counting statistics with quantum master equation approach. First we study higher order effects of the pumping frequency in general Markov systems and show in this limit the equivalence between our approach and the real-time diagrammatic approach. An adiabatic modulation of the control parameters induces the Berry-Sinitsyn-Nemenman (BSN) phase. We show that the origin of the BSN phase is a nonadiabatic effect. The pumped charge (spin) is given by a summation of (i) a time integral of the instantaneous steady charge (spin) current and (ii) a geometric surface integral of the BSN curvature, which results from the BSN phase. In quantum dots (QDs) weakly coupled to two leads, we show that (i) is usually dominant if the thermodynamic parameters are modulated, although it is zero if the thermodynamic parameters are fixed to zero bias. To observe the spin effects, we consider collinear magnetic fields, which relate to spins through the Zeeman effect, with different amplitudes applying to the QDs and the leads. For interacting one level QD, we calculate analytically the pumped charge and spin by modulating the magnetic fields and the coupling strengths to the leads in the noninteracting and strong interacting limits. We show that the difference between these two limits appears through the instantaneous averages of the numbers of the electron with up and down spin in the QD. For the quantum pump by the modulation of the magnetic fields of the QD and one lead, the energy dependences of linewidth functions, which are usually neglected, are essential.

In Weyl materials the valence and conduction electron bands touch at an even number of isolated points in the Brillouin zone. In the vicinity of these points the electron dispersion is linear and may be described by the massless Dirac equation. This results in nontrivial topology of the Berry connection curvature. One of its consequences is the existence of peculiar surface electron states whose Fermi surfaces form arcs connecting projections of the Weyl points onto the surface plane. Band bending near the boundary of the crystal also produces surface states. We show that in Weyl materials band bending near the crystal surface gives rise to a spiral structure of energy surfaces of arc states. The corresponding Fermi surface has the shape of a spiral that winds about the projection of the Weyl point onto the surface plane. The direction of the winding is determined by the helicity of the Weyl point and the sign of the band-bending potential. For close valleys the arc state morphology may be understood in terms of the avoided crossing of oppositely winding spirals.

Using a combination of x-ray absorption spectroscopy (XAS) experiments and first-principles calculations, we demonstrate that insulating ${\mathrm{KCuO}}_2$ contains Cu in an unusually high formal 3+ valence state, and the ligand-to-metal (O-to-Cu) charge-transfer energy is intriguingly negative ($\mathrm{\Delta }\sim -1.5$ eV) and has a dominant ($\sim 60\%$) ligand-hole character in the ground state akin to the high $T_c$ cuprate Zhang-Rice state. Unlike most other formal ${\mathrm{Cu}}^{3+}$ compounds, the Cu $2p$ XAS spectra of ${\mathrm{KCuO}}_2$ exhibit pronounced $3d^8$ (${\mathrm{Cu}}^{3+}$) multiplet structures, which account for $\sim 40\%$ of its ground state wave function. Ab initio calculations elucidate the origin of the band gap in ${\mathrm{KCuO}}_2$ as arising primarily from strong intracluster Cu $3d$-O $2p$ hybridizations ($t_{\mathrm{pd}}$); the value of the band gap decreases with a reduced value of $t_{\mathrm{pd}}$. Further, unlike conventional negative-charge-transfer insulators, the band gap in ${\mathrm{KCuO}}_2$ persists even for vanishing values of Coulomb repulsion $U$, underscoring the importance of single-particle band-structure effects connected to the one-dimensional nature of the compound.

The circular polarization of direct gap emission of Ge is studied in optically excited tensile-strained Ge-on-Si heterostructures as a function of doping and temperature. Owing to the spin-dependent optical selection rules, the radiative recombinations involving strain-split light $\left(c\mathrm{\Gamma }\text{-LH}\right)$ and heavy hole $\left(c\mathrm{\Gamma }\text{-HH}\right)$ bands are unambiguously resolved. The fundamental $c\mathrm{\Gamma }\text{-LH}$ transition is found to have a low temperature circular polarization degree of about 85%, despite an off-resonance excitation of more than 300 meV. By photoluminescence (PL) measurements and tight-binding calculations we show that this exceptionally high value is due to the characteristic energy dependence of the optically induced electron spin population. Finally, our observation of a direct gap doublet clarifies that the light hole contribution, previously considered to be negligible, can dominate the room temperature PL even at low tensile strain values of $\approx 0.2\%$.

We have performed spin and charge transport measurements in dual gated high mobility bilayer graphene encapsulated in hexagonal boron nitride. Our results show spin relaxation lengths ${\lambda }_s$ up to $13\mu \mathrm{m}$ at room temperature with relaxation times ${\tau }_s$ of 2.5 ns. At 4 K, the diffusion coefficient rises up to $0.52{\mathrm{m}}^2/\mathrm{s}$, a value five times higher than the best achieved for graphene spin valves up to date. As a consequence, ${\lambda }_s$ rises up to $24\mu \mathrm{m}$ with ${\tau }_s$ as high as 2.9 ns. We characterized three different samples and observed that the spin relaxation times increase with the device length. We explain our results using a model that accounts for the spin relaxation induced by the nonencapsulated outer regions.

Motivated by recent experiments that reveal expansive fractional quantum Hall states in the $n=1$ graphene Landau level and suggest a nontrivial role of the spin degree of freedom [F. Amet, A. J. Bestwick, J. R. Williams, L. Balicas, K. Watanabe, T. Taniguchi, and D. Goldhaber-Gordon, Nat. Commun. 6, 5838 (2015)], we perform an accurate quantitative study of the competition between fractional quantum Hall states with different spin polarizations in the $n=1$ graphene Landau level. We find that the fractional quantum Hall effect is well described in terms of composite fermions, but the spin physics is qualitatively different from that in the $n=0$ Landau level. In particular, for the states at filling factors $\nu =s/(2s\pm 1), s$ positive integer, a combination of exact diagonalization and the composite fermion theory shows that the ground state is fully spin polarized and supports a robust spin-wave mode even in the limit of vanishing Zeeman coupling. Thus, even though composite fermions are formed, a mean-field description that treats them as weakly interacting particles breaks down, and the exchange interaction between them is strong enough to cause a qualitative change in the behavior by inducing full spin polarization. We also verify that the fully spin-polarized composite fermion Fermi sea has lower energy than the paired Pfaffian state at the relevant half fillings in the $n=1$ graphene Landau level, indicating an absence of composite fermion pairing at half filling in the $n=1$ graphene Landau level.

Trial wave functions that can be represented by summing over locally coupled degrees of freedom are called tensor network states (TNSs); they have seemed difficult to construct for two-dimensional topological phases that possess protected gapless edge excitations. We show it can be done for chiral states of free fermions, using a Gaussian Grassmann integral, yielding $p_x\pm i{p}_y$ and Chern insulator states, in the sense that the fermionic excitations live in a topologically nontrivial bundle of the required type. We prove that any strictly short-range quadratic parent Hamiltonian for these states is gapless; the proof holds for a class of systems in any dimension of space. The proof also shows, quite generally, that sets of compactly supported Wannier-type functions do not exist for band structures in this class. We construct further examples of TNSs that are analogs of fractional (including non-Abelian) quantum Hall phases; it is not known whether parent Hamiltonians for these are also gapless.

We study Kondo physics of a $\text{spin}-\frac{1}{2}$ impurity in electronic matter with strong spin-orbit interaction, which can be realized by depositing magnetic adatoms on the surface of a three-dimensional topological insulator. We show that magnetic properties of topological surface states and the very existence of Kondo screening strongly depend on details of the bulk material, and specifics of surface preparation encoded in time-reversal preserving boundary conditions for electronic wavefunctions. When this tunable Kondo effect occurs, the impurity spin is screened by purely orbital motion of surface electrons. This mechanism gives rise to a transverse magnetic response of the surface metal, and to spin textures that can be used to experimentally probe signatures of a Kondo resonance. Our predictions are particularly relevant for STM measurements in $\mathrm{Pb}\mathrm{Te}$-class crystalline topological insulators, but we also discuss implications for other classes of topological materials.

The proximity-induced pair potential in a topological insulator–superconductor hybrid features an interesting superposition of a conventional spin-singlet component from the superconductor and a spin-triplet one induced by the surface state of the topological insulator. This singlet-triplet superposition can be altered by the presence of a magnetic field. We study the interplay between topological order and superconducting correlations performing a symmetry analysis of the induced pair potential, using Green functions techniques to theoretically describe ballistic junctions between superconductors and topological insulators under magnetic fields. We relate a change in the conductance from a gapped profile into one with a zero-energy peak with the transition into a topologically nontrivial regime where the odd-frequency triplet pairing becomes the dominant component in the pair potential. The nontrivial regime, which provides a signature of odd-frequency triplet superconductivity, is reached for an out-of-plane effective magnetization with strength comparable to the chemical potential of the superconductor or for an in-plane one, parallel to the normal-superconductor interface, with strength of the order of the superconducting gap. Strikingly, in the latter case, a misalignment with the interface yields an asymmetry with the energy in the conductance unless the total contribution of the topological surface state is considered.

Motivated by the studies of the superconducting pairing states in the iron-based superconductors, we analyze the effects of Brillouin zone folding procedure from a space-group symmetry perspective for a general class of materials with the $P4/nmm$ space group. The Brillouin zone folding amounts to working with an effective 1-Fe unit cell, instead of the crystallographic 2-Fe unit cell. We show that the folding procedure can be justified by the validity of a glide reflection symmetry throughout the crystallographic Brillouin zone and by the existence of a minimal double degeneracy along the edges of the latter. We also demonstrate how the folding procedure fails when a local spin-orbit coupling is included although the latter does not break any of the space-group symmetries of the bare Hamiltonian. In light of these general symmetry considerations, we further discuss the implications of the glide reflection symmetry for the superconducting pairing in an effective multiorbital $t-J_1-J_2$ model. We find that, for spin-singlet pairing states, the P4/$nmm$ space-group symmetry allows only even parity under the glide reflection and zero total momentum.

In iron pnictides, high temperature superconductivity emerges after suppressing antiferromagnetism by doping. Here, we show that antiferromagnetism in ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{FeAs}}_2$ is robust against and even enhanced by doping. Using ${}^{75}\mathrm{As}$-nuclear magnetic resonance and nuclear quadrupole resonance techniques, we find that an antiferromagnetic order occurs below the Néel temperature $T_{\mathrm{N}}=62$ K at a high doping concentration $\left(x=0.15\right)$ where superconductivity sets in at the transition temperature $T_{\mathrm{c}}=35$ K. In the superconducting state coexisting with antiferromagnetism, the nuclear-spin-lattice relaxation rate $1/T_1$ becomes proportional to $T$, indicating gapless excitations. Unexpectedly, $T_{\mathrm{N}}$ is enhanced with increasing doping, rising up to $T_{\mathrm{N}}=70$ K at $x=0.24$. The obtained phase diagram of this system enriches the physics of iron-based high-$T_{\mathrm{c}}$ superconductors.

We have investigated the physical properties of a pyrochlore hafnate ${\mathrm{Nd}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$ using ac magnetic susceptibility ${\chi }_{\mathrm{ac}}\left(T\right)$, dc magnetic susceptibility $\chi \left(T\right)$, isothermal magnetization $M\left(H\right)$, and heat capacity $C_{\mathrm{p}}\left(T\right)$ measurements, and determined the magnetic ground state by neutron powder diffraction. An upturn is observed below 6 K in $C_{\mathrm{p}}\left(T\right)/T$, however both $C_{\mathrm{p}}\left(T\right)$ and $\chi \left(T\right)$ do not show any clear anomaly down to 2 K. The ${\chi }_{\mathrm{ac}}\left(T\right)$ shows a well-pronounced anomaly indicating an antiferromagnetic transition at $T_{\mathrm{N}}=0.55$ K. The long-range antiferromagnetic ordering is confirmed by neutron diffraction. The refinement of the neutron diffraction pattern reveals an all-in/all-out antiferromagnetic structure, where for successive tetrahedra the four ${\mathrm{Nd}}^{3+}$ magnetic moments point alternatively all-into or all-out-of the tetrahedron, with an ordering wave vector k = (0, 0, 0) and an ordered state magnetic moment of $m=0.62\left(1\right){\mu }_{\mathrm{B}}$/Nd at 0.1 K. The ordered moment is strongly reduced, reflecting strong quantum fluctuations in ordered state.

In noncentrosymmetric materials, the relativistic effect extensively modifies the energy band of magnons as well as that of electrons. With use of microfabricated microwave antennae, we have demonstrated that the propagation of magnons with large momentum is nonreciprocal in a noncentrosymmetric ferromagnet ${\mathrm{LiFe}}_5{\mathrm{O}}_8$. The nonreciprocity is clearly explained by the effect of the asymmetric magnon band originating from the relativistic Dzyaloshinskii-Moriya interaction. This result may pave a new path to designing a magnonic device based on the relativistic band engineering.

We follow the evolution of the ionization potential (IP) for the paradigmatic quasi-one-dimensional transacetylene family of conjugated molecules, from short to long oligomers and to the infinite polymer transpolyacetylene (TPA). Our results for short oligomers are very close to experimental available data. We find that the IP varies with oligomer length and converges to the given value for TPA with a smooth, coupled inverse-length-exponential behavior. Our prediction is based on an “internally consistent” scheme to adjust the exchange mixing parameter $\alpha$ of the PBEh hybrid density functional, so as to obtain a description of the electronic structure consistent with the quasiparticle approximation for the IP. This is achieved by demanding that the corresponding quasiparticle correction, in the $GW$@PBEh approximation, vanishes for the IP when evaluated at $\mathrm{PBEh}({\alpha }^{\text{ic}}$). We find that ${\alpha }^{\text{ic}}$ is also system-dependent and converges with increasing oligomer length, enabling the dependence of the IP and other electronic properties to be identified.