We report the emergence of electronic edge states in time-periodically driven strained armchair terminated graphene nanoribbons. This is done by considering a short-pulse spatial-periodic strain field. Then, the tight-binding Hamiltonian of the system is mapped into a one-dimensional ladder. The time periodicity is considered within the Floquet formalism. Thus the quasienergy spectrum is found numerically by diagonalizing the evolution operator. For some particular cases, the quasienergy spectrum is found analytically. We found that the system is able to support gapless and gapped phases. Very different edge states emerge for both the gapless and the gapful phases. In the case of the gapped phase, edge states emerge at the gap centered at zero quasienergy, although the Chern number is zero due to the chiral symmetry of the system. For the gapless phase, besides edge states at zero quasienergy, cosinelike edge states which merge and coexist with the bulk band are observed. To confirm the topological nature of these edge states, we analytically obtained the effective Hamiltonian and its spectrum for a particular case, finding that the edge states are topologically weak. Finally, we found analytically the evolution of band edges and their crossings as a function of the driven period. Topological modes arise at such crossings.

Resonant microwave absorption of a two-dimensional electron system in an AlGaAs/GaAs heterostructure excited by a near-field technique was investigated. Along with collective magnetoplasmon modes, we observed resonance that precisely follows the cyclotron resonance (CR) position and revealed no signs of a collective plasma depolarization shift. We show that the discovered CR mode is absent in the Faraday geometry, and it is localized at the edge of the exciting metal electrode. Such behavior points in favor of the single-particle Azbel'-Kaner nature of the discovered resonance.

A topological Weyl semimetal (TWS) is a metal where low-energy excitations behave like Weyl fermions of high-energy physics. Recently, it was shown that, due to the lower symmetry of condensed-matter systems, they can realize two distinct types of Weyl fermions. The type-I Weyl fermion in a metal is formed by a linear crossing of two bands at a point in the crystalline momentum space—Brillouin zone. The second type of TWSs host type-II Weyl points appearing at the touching points of electron and hole pockets, which is a result of tilted linear dispersion. The type-II TWS was predicted to exist in several compounds, including ${\mathrm{WTe}}_2$. Several angle-resolved photoemission spectroscopy studies of ${\mathrm{WTe}}_2$ were reported so far, having contradictory conclusions on the topological nature of observed Fermi arcs. In this paper, we report the results of spectroscopic imaging with a scanning tunneling microscope and first-principles calculations, establishing clear quasiparticle interference features of the surface states of ${\mathrm{WTe}}_2$. Our paper provides strong evidence for surface-state scattering. Although the surface Fermi arcs clearly are observed, it is still difficult to prove the existence of predicted type-II Weyl points in the bulk.

A new kind of spin-one chain Hamiltonian consisting of competing dimer and trimer projection operators is proposed. As the relative strengths and signs of the interactions are varied, the model exhibits a number of different phases including the gapped dimer phase and the gapless trimer phase with critical correlations described by a conformal field theory with central charge $c=2$. A symmetry-protected topological phase also exists in this model, even though the microscopic interactions are not the simple adiabatic extensions of the well-known Heisenberg and Affleck-Kennedy-Lieb-Tasaki models and contain both two- and three-particle permutations. A fourth phase is characterized by macroscopically degenerate ground states. While bearing almost a one-to-one resemblance to the phase diagram of the bilinear-biquadratic spin-one chain Hamiltonian, our model is rooted in a very different physical origin, namely, the two competing tendencies of spin-one particles to form singlets through either dimer or trimer formation.

High quality single crystals of ${\mathrm{BaBiO}}_3$ were grown by congruent melting technique and characterized with x-ray diffraction, x-ray photoemission, and transport property studies. The perovskite oxide ${\mathrm{BaBiO}}_3$ is a negative charge transfer gap high $T_c$ oxide parent superconducting compound exhibiting self-doping of holes into the oxygen $2p$ band. We study the low energy scale valence and conduction bands in detail from both a theoretical perspective as well as through x ray, absorption/emission, and photoelectron spectroscopies. X-ray spectroscopy verifies the results of density functional theory (DFT) regarding the overall band structure featuring strong O $2p$ character of the empty antibonding combination of the hybridized Bi $6s$ and O $2p$ states. From the analysis of the core level line shapes we conclude that the dominant O $2p$-Bi $6s$ hybridization energy scale determines the low energy scale electronic structure. This analysis provides further insight into the importance of self-doped oxygen $2p$ states in this high $T_c$ family of oxides.

The anomalous Hall effect is classified into two effects based on the mechanism. The first one is the intrinsic Hall effect due to the Berry curvature in momentum space. This is a Hall effect that solely arises from the band structure of solids. On the other hand, another contribution to the Hall effect, the so-called extrinsic mechanism, comes from impurity scatterings such as skew scattering and side jump. These two mechanisms are often discussed separately; the intrinsic Hall effect is related to the Berry curvature of the band while the skew scattering is studied using the scattering theory approaches. However, we here show that, in an electronic system with finite Berry curvature, the skew scattering by nonmagnetic impurities is described by the noncommutative nature of the real-space coordinates due to the Berry curvature of the Block wave functions. The anomalous Hall effect due to this skew scattering is estimated and compared with the intrinsic contribution.

The thermal response of graphene is expected to be extremely fast due to its low heat capacity and high thermal conductivity. In this work, the thermal response of suspended single-layer graphene membranes is investigated by characterization of their mechanical motion in response to a high-frequency modulated laser. A characteristic delay time $\tau$ between the optical intensity and mechanical motion is observed, which is attributed to the time required to raise the temperature of the membrane. We find, however, that the measured time constants are significantly larger than the predicted ones based on values of the specific heat and thermal conductivity. In order to explain the discrepancy between measured and modeled $\tau$, a model is proposed that takes a thermal boundary resistance at the edge of the graphene drum into account. The measurements provide a noninvasive way to characterize thermal properties of suspended atomically thin membranes, providing information that can be hard to obtain by other means.

Employing a Keldysh-Eilenberger technique, we theoretically study the generation of a spontaneous supercurrent and the appearance of the ${\phi }_0$ phase shift parallel to uniformly in-plane magnetized superconducting interfaces made of the surface states of a three-dimensional topological insulator. We consider two weakly coupled uniformly magnetized superconducting surfaces where a macroscopic phase difference between the $s$-wave superconductors can be controlled externally. We find that, depending on the magnetization strength and orientation on each side, a spontaneous supercurrent due to the ${\phi }_0$ states flows parallel to the interface at the nanojunction location. Our calculations demonstrate that nonsinusoidal phase relations of current components with opposite directions result in maximal spontaneous supercurrent at phase differences close to $\pi$. We also study the Andreev subgap channels at the interface and show that the spin-momentum locking phenomenon in the surface states can be uncovered through density of states studies. We finally discuss realistic experimental implications of our findings.

The state of a single particle injected onto the surface of the Fermi sea is a pure state if the temperature is zero and is a mixed state if the temperature is finite. Moreover, the state of an injected particle is orthogonal to the state of the Fermi sea at zero temperature, while it is not orthogonal at nonzero temperature. These changes in the quantum state of the injected particles can be detected using the temperature dependence of the shot noise that is generated when the particles one by one pass through a semitransparent quantum point contact. Namely, the shot noise produced by the mixed state is suppressed in comparison with the noise of the pure state. In addition, the correlations between the injected particles and the underlying Fermi sea, present at nonzero temperature, do enhance the shot noise. Furthermore, antibunching of injected particles with possible thermal excitations coming from another input channel of a quantum point contact does suppress shot noise. Here I analyze in detail these three effects, which are responsible for the temperature dependence of the shot noise, and discuss how to distinguish them experimentally.

Graphene on a substrate with a topological line defect possesses chiral edge states that exhibit linear dispersion and have opposite Fermi velocities for two valleys. The chiral edge states are localized at the line defect. With the presence of Rashba spin-orbital coupling, the dispersion of the chiral edge states splits into two. The optical excitation is modeled by the generalized semiconductor Bloch equation based on tight-binding theory. Charge, valley, and spin currents generated by normally incident plane waves through the photogalvanic effect as well as those generated by oblique light through the surface-plasmon drag effect are studied. Conditions for optical generation of purely localized valley or spin currents, which are solely originated from the chiral edge states, are discussed.

This study observes microwave beam splitting dependent on circular polarizations through nonuniform chiral metamaterials. Nonuniform chiral metamaterials with a gradation in refractive index are constructed using chiral meta-atoms that exhibit optical activities at microwave frequencies. Microwave scattering far-field patterns by the nonuniform chiral metamaterials demonstrate a deflection of microwaves, which transmit in a direction perpendicular to the refractive index gradient. Furthermore, circularly polarized microwaves with opposite “spins of light” go their separate ways in the nonuniform chiral metamaterials. This phenomenon is an optical analog of the Stern-Gerlach effects for electrons.

We show that frustrated exchange interactions could stabilize chiral states in a chiral ferromagnet by considering the competition between ferromagnetic (FM) nearest-neighbor (NN) interaction and antiferromagnetic (AFM) next-nearest-neighbor (NNN) interaction. In the low-field regime, a multisublattice helical state is found, which is different from the normal one-sublattice helical states in a FM and the two-sublattice helical states in a two-sublattice AFM. As the field increases further, the skyrmion lattice is even stable with a much larger energy-preferable window than a system without frustration. We argue that the enlargement of the stability window of skyrmions is a consequence of the reduced effective exchange interaction caused by the frustration. As a byproduct, the hysteresis loop of the frustrated chiral system shrinks as the magnetization goes to zero and then opens up again, which is known as the wasp-waist hysteresis loop. The critical field that separates the narrow and wide part of the wasp-waist loop depends exponentially on the strength of NNN coupling. Our results provide physical insight into the chiral states in the magnetic systems with the coexistence of frustrated exchange and chiral interaction and might even prove important for novel devices, where the stability window of skyrmions is a key asset.

Exchange magnetic coupling between Fe electrodes through a thin MgO interlayer in epitaxial junctions has been investigated as a function of temperature, MgO thickness, and interface quality. Depending on the MgO thickness, which has been varied from 1.5 to 4 monolayers, two opposite temperature dependences are clearly disentangled. For a thin MgO spacer, the main component decreases with temperature following a metalliclike behavior. On the contrary, for the thickest MgO layers, the main component increases with temperature, following an Arrhenius law. Moreover, the insertion of a monoatomic roughness at the bottom MgO interface, induced by the addition of a fraction of a Fe monolayer, exacerbates the metallic features as an oscillatory behavior from antiferromagnetic to ferromagnetic is observed. These results allow questioning the simple tunneling mechanism usually invoked for MgO coupling, and suggest a crossover behavior of the thin MgO spacer from metallic to insulating with a progressive opening of the gap.

Recent transport experiments in the cuprate superconductors linked the opening of the pseudogap to a change in electronic dispersion [S. Badoux et al., Nature (London) 531, 210 (2015)]. Transport measurements showed that the carrier density sharply changes from $x$ to $1+x$ at the pseudogap critical doping, in accordance with the change from Fermi arcs at low doping to a large hole Fermi surface at high doping. The SU(2) theory of cuprates shows that short-range antiferromagnetic correlations cause the arising of both charge and superconducting orders, which are related by an SU(2) symmetry. The fluctuations associated with this symmetry form a pseudogap phase. Here we derive the renormalized electronic propagator under the SU(2) dome, and calculate the spectral functions and transport quantities of the renormalized bands. We show that their evolution with doping matches both spectral and transport measurements.

${\mathrm{Bi}}_3{\mathrm{Mn}}_4{\mathrm{O}}_{12}\left({\mathrm{NO}}_3\right)$ (BMNO) is a honeycomb bilayers antiferromagnet, not showing any ordering down to very low temperatures despite having a relatively large Curie-Weiss temperature. Using ab initio density functional theory, we extract an effective spin Hamiltonian for this compound. The proposed spin Hamiltonian consists of antiferrimagnetic Heisenberg terms with coupling constants ranging up to third intralayer and fourth interlayer neighbors. Performing Monte Carlo simulation, we obtain the temperature dependence of magnetic susceptibility and so the Curie-Weiss temperature and find the coupling constants which best match with the experimental value. We discover that depending on the strength of the interlayer exchange couplings, two collinear spin configurations compete with each other in this system. Both states have in plane Néel character, however, at small interlayer coupling spin directions in the two layers are antiparallel (${\mathrm{N}}_1$ state) and discontinuously transform to parallel (${\mathrm{N}}_2$ state) by enlarging the interlayer couplings at a first order transition point. Classical Monte Carlo simulation and density matrix renormalization group calculations confirm that exchange couplings obtained for BMNO are in such a way that put this material at the phase boundary of a first order phase transition, where the trading between these two collinear spin states prevents it from setting in a magnetically ordered state.

We report on the determination of the full magnetic structure of ${\mathrm{Tb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ including the Ir sublattice by means of powder neutron diffraction. Below $T_N\sim 125\mathrm{K}$, both the Tb and the Ir sublattices order in a so-called all-in/all-out magnetic structure. However, additional magnetic intensity appears below 10 K on the (111) reflection indicating a change in the magnetic structure. We were able to describe the low-temperature magnetic structure with an additional $XY$ component on the Tb sublattices. The moment sizes at 1.5 K amount to $0.55\left(3\right){\mu }_B/\mathrm{Ir}$ and $5.22\left(3\right){\mu }_B/\mathrm{Tb}$. The ${\mathrm{Tb}}^{3+}$ moments are titled about $6^{\circ }$ off from the [111] direction. Moreover, it was found that the Tb moments couple antiferromagnetically with their six nearest-neighboring Ir moments below $T_N$. This is in contrast to the case of ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. The ${\mathrm{Ir}}^{4+}$ moment sizes of ${\mathrm{Tb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ are larger than that of ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ indicating that hybridization effects between the Ir $5d$ and the O $2p$ orbitals are smaller in ${\mathrm{Tb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$.

Materials showing anisotropic magnetocaloric effects could open the way for new designs of magnetic cooling. In the paper by Jin et al. [Phys. Rev. B 83, 184431 (2011)], a rotating magnetocaloric effect was pointed out in $\mathrm{TbMn}{\mathrm{O}}_3$ single crystals. The reported effect arises from the rotation of $\mathrm{TbMn}{\mathrm{O}}_3$ crystals between their $b$ and $a$ axes. However, in this Comment we demonstrate that $\mathrm{TbMn}{\mathrm{O}}_3$ crystals generate a rotating magnetocaloric effect that is at least two times larger than that reported by Jin et al. On the other hand, the newly reported rotating magnetocaloric effect is not directly resulted from the magnetocrystalline anisotropy, also being in contrast with Jin et al. data.

We comment on the recent work [N. Harrison et al., Phys. Rev. B 92, 224505 (2015)] which attempts to explain the sign reversal and quantum oscillations of the Hall coefficient observed in cuprates from a single-nodal diamond-shaped electron pocket with concave arc segments. Given the importance of this work, it calls for closer scrutiny. Their conclusion of sign reversal of the Hall coefficient depends on a nongeneric rounding of the sharp vertices. Moreover, their demonstration of quantum oscillation in the Hall coefficient from a single pocket is unconvincing. We maintain that at least two pockets with different scattering rates is necessary to explain the observed quantum oscillations of the Hall coefficient.

We provide arguments relating to those recently made in a comment by Chakravarty and Wang, who question the validity of our proposed charge-density wave Fermi surface reconstruction model and its relation to sign changes in the Hall effect. First, we show that the form of rounding of the vertices (i.e. sharp corners) of the reconstructed electron pocket, as used in our model calculations of the Hall coefficient, is consistent with Bragg reflection from the periodic potential of a charge-density wave, rather than being arbitrarily chosen. Second, we provide further justifications for why an oscillatory transport scattering time provides a useful means for modeling Shubnikov–de Haas oscillations in the Hall effect, in the situation where a Fermi surface pocket departs from the ideal circular form. Third, we discuss recent experimental evidence gathered from two different families of underdoped cuprates supporting the existence of a single electron pocket produced by biaxial charge-density wave order as a universal phenomena.

The evolution of the electronic properties of guest Na and Rb atoms in a disordered deformable lattice is investigated for a series of guest-atom densities $n$. The quasi-two-dimensional host $M_{7.8-\delta }{\mathrm{Al}}_{7.8}{\mathrm{Si}}_{8.2}{\mathrm{O}}_{32.0}$ ($M$=Na, Rb), known as zeolite P, is used. The Na system is a stubborn bipolaronic insulator to the maximum $n$ of 1.03. In contrast, the Rb system exhibits a crossover from a bipolaronic insulator to a conducting phase analogous to a disordered metal at $n$ = 0.89. A critical region undergoing polaronic melting appears in the vicinity of the crossover on the insulating side, evidenced by a reduction in the small bipolaron absorption band and a drop in the activation energy. Transition to the conducting phase coincides with the appearance of a midinfrared band and an increase in the charge-carrier decay length, suggesting the polaronic and extended nature of the carriers. These findings constitute rare examples of electron-lattice coupling opening (or closing) a mobility gap and scaling the continuity (or discontinuity) of a conducting transition in the face of disorder.