Graphene, a two-dimensional monatomic layer of crystal carbon, has recently emerged as a potential material for next-generation optoelectronic devices owing to unique properties arising from its massless Dirac fermions. However, the intrinsic carrier dynamics of graphene has remained a mystery even for the simple case of carrier cooling after the photoexcitation. This is because the observed temporal variations of the nonequilibrium carriers in graphene have been thoroughly described in terms of a defect-induced extrinsic effect known as “supercollision” (SC). The SC process is based on defect-mediated electron-acoustic phonon scattering and theoretically has been predicted to reduce with the increase of mobility of a material. Here, the authors have prepared extremely high mobility graphene and traced the dynamics of photoexcited carriers in the Dirac bands directly by time- and angle-resolved photoemission spectroscopy. They successfully observed suppression of SC and extracted the intrinsic dynamical properties of graphene, such as anharmonic decay of the optical phonons and the bottleneck relaxation at the Dirac point. Breaking the limit of SC, their research also has technological significance in developing graphene-based optoelectronic devices.
Takashi Someya et al.
Phys. Rev. B 95, 165303 (2017)
Essentially all high temperature superconductors display a “strange metal” regime above their critical temperatures, where electrical and thermal transport cannot be explained within the traditional Fermi liquid paradigm of quasiparticle excitations. This paper focuses on the first, and newly discovered, solvable theory of a disordered metal without quasiparticle excitations, obtained by extending the Sachdev-Ye-Kitaev (SYK) models to finite spatial dimensions. The complete thermoelectric conductivity matrix is computed, and a surprising exact relation is found between the Seebeck coefficient and the derivative of the thermodynamic entropy with respect to the density. These computations are then compared with holographic theories which map strange metals onto black holes in theories of quantum gravity with one extra spatial dimension.
Richard A. Davison et al.
Phys. Rev. B 95, 155131 (2017)
Many-body localization plays an increasing role in condensed matter theory, both because it challenges the fundaments of statistical physics, and because it allows us to engineer several new, exotic, stable phases of matter. In this paper, the authors address the issue of the stability of a many-body localized material in contact with an ergodic grain, i.e., an imperfect bath made of a few interacting degrees of freedom. Thanks to detailed microscopic analysis and numerics, they conclude that such an ergodic grain eventually destabilizes the localized phase in the following cases: if the spatial dimension is higher than one, or if the spatial dimension is one but the localization length of the localized material is larger than a fixed threshold value. In realistic materials, these ergodic grains are always present as Griffiths regions where the disorder is anomalously small, and hence, the authors conclude that the localized phase in such materials is unstable, strictly speaking. Transport and thermalization are however exponentially suppressed in the distance between ergodic grains.
Wojciech De Roeck and François Huveneers
Phys. Rev. B 95, 155129 (2017)
The Weyl fermion – originally proposed to describe neutrinos – arises in a topological semimetallic phase in condensed matter systems, exhibiting such novel properties as surface Fermi arcs and a chiral anomaly. Although Weyl fermions have been observed, it is challenging to find materials that exhibit them near the Fermi level. The authors prove that Weyl fermions can be created in band-inverted materials in a large class of crystal systems by applying a magnetic field along various symmetry axes of the crystal. As the field direction is changed, the Weyl points move in momentum space, during which time pairs are created and annihilated. However, in the highly symmetric point group, the Weyl points cannot completely disappear: at least one pair must remain for any direction of the magnetic field. Furthermore, a semiclassical analysis shows that the magnetoresistance scales differently for Weyl fermions created by a magnetic field compared to intrinsic Weyl points. The ability to create Weyl fermions will lead to new material candidates in these crystal systems; controlling their positions opens the possibility to track and manipulate Fermi arcs.
Jennifer Cano et al.
Phys. Rev. B 95, 161306(R) (2017)
Time-periodic driving enables new nonequilibrium quantum phases of matter with topologically protected and quantum coherent motion that would be forbidden in thermal equilibrium. This work uncovers two new classes of driven topological phases in driven two-dimensional spin systems: i) Floquet symmetry protected topological phases, which are driven and interacting analogs of topological insulators whose edge states are protected against disorder and localization; and ii) Floquet enriched topological orders, in which time-dependent driving “breaks” the spins into anyonic particles with fractional statistics and oscillating topological charge. The physics of these phases is explored by constructing toy models that can be explicitly solved despite the presence of strong interactions.
Andrew C. Potter and Takahiro Morimoto
Phys. Rev. B 95, 155126 (2017)
Skyrmions, swirling magnetic textures with a topological character, have been gaining much attention in spintronics due to the fundamental interest as well as their touted utility as information carriers in ultradense and low-power memory devices. Generally, a skyrmion behaves as a massive particle moving in a viscous medium and experiencing a Magnus force, which is proportional to its winding number and the spin polarization of the magnet. There is a class of tunable ferrimagnets, such as rare-earth transition-metal alloys, exhibiting the angular momentum compensation point at which the spin density changes sign. This raises the possibility to engineer both the magnitude and the sign of the Magnus force acting on the skyrmions in the ferrimagnets. The authors exploit this to suggest that ferrimagnetic skyrmions can exhibit snake trajectories along the line of the vanishing spin density, analogous to the snake orbits of electrons in a nonuniform magnetic field. This can be utilized as dynamically self-focusing racetracks for skyrmions, paving the way for skyrmion-based memory devices.
Se Kwon Kim, Kyung-Jin Lee, and Yaroslav Tserkovnyak
Phys. Rev. B 95, 140404(R) (2017)
Quantum effects in magnetic materials are prevalent when the magnetic ions support spin-½ moments, as in Cu-based materials. Strong quantum effects can also be achieved when the large angular momentum states are reduced to an effective spin-½ subspace (=½) by the combined effect of spin-orbit coupling and the crystal electric field. A consequence of this is the introduction of anisotropy to the g tensor and the effective exchange interactions. Such anisotropic =½ models have recently been applied successfully to rare-earth based frustrated pyrochlore materials, where they are found to lead to particularly rich phenomenology for XY-like tensors. Here, it is shown that a similar quantum model should apply to the recently discovered “high temperature” Co pyrochlores, NaCaCoF and NaSrCoF, based on a fit to their single ion levels as measured via inelastic neutron scattering. The effect of the intrinsic crystalline disorder on the anisotropy of the =½ moments in these materials is also estimated.
K. A. Ross et al.
Phys. Rev. B 95, 144414 (2017)
This work studies the manipulation of photoelectron emission from metallic nanostructures by intense single-cycle terahertz transients. Specifically, the authors employ streaking spectroscopy to study the kinetic energy of photoelectrons emitted from metal nanotips exposed to tailored static and terahertz electrical fields. Supported by detailed numerical simulations, the measurements provide quantitative information on the temporal and spatial properties of terahertz near fields at metal nanotips. The study illustrates far-reaching control over the trajectories and phase-space density evolution of photoelectron wavepackets acted upon by strong static and dynamic near fields. The results are relevant for applications of nanoscopic photoelectron sources employed in ultrafast electron diffraction and microscopy.
Lara Wimmer, Oliver Karnbach, Georg Herink, and Claus Ropers
Phys. Rev. B 95, 165416 (2017)
In ferromagnetic layers, spin-wave modes can be excited by a laser pulse launching the magnetization out of equilibrium and into precession. The trajectory of the magnetization vector can be fully reconstructed using different magneto-optical effects, generally admitted to give a faithful picture of the dynamics. The authors show that this is not always the case and that magneto-optical effects can indeed be deceiving. For this, they study the case of a weakly absorbing ferromagnet, the semiconductor GaMnAs, in which perpendicular standing spin waves are excited by a laser pulse. Quite counterintuitively, the optical detection shows the first two excited modes to be of opposite chirality, whereas theory tells us the spin waves actually rotate in the same direction. The paper demonstrates that this unexpected and surprising effect is a pure optical illusion. It can perfectly be explained by taking into account absorption and optical phase shift inside the layer, the latter being particularly strong in weakly absorbing layers. These results provide a correct identification of spin-wave modes, enabling a trustworthy estimation of their respective weight as well as an unambiguous determination of the spin stiffness parameter.
S. Shihab, L. Thevenard, A. Lemaître, and C. Gourdon
Phys. Rev. B 95, 144411 (2017)
Topological pumping, which was originally proposed by Thouless, is a beautiful manifestation of quantum effects in transport phenomena. Thouless’s topological pumping is characterized by the topology of Bloch states in the space of momentum and time, and can be viewed as a dynamical analog of the integer quantum Hall effect of noninteracting fermions. Here, the authors propose a systematic procedure to construct nontrivial classes of topological pumping from strongly correlated quantum Hall states on a thin torus. In particular, this procedure is applied to the bosonic integer quantum Hall (BIQH) state formed by two species of bosons. The BIQH state is an example of a symmetry-protected topological (SPT) state of interacting bosons, and is characterized by nontrivial Hall responses. The authors find that the thin-torus counterpart of the BIQH state is the Haldane state of emergent spin-1 degrees of freedom, which is also a SPT state. The authors further show that an adiabatic change between the Haldane phase and trivial Mott insulators constitute an “off-diagonal” topological pumping, in which the translation of the lattice potential for one component induces a current in the other.
Masaya Nakagawa and Shunsuke Furukawa
Phys. Rev. B 95, 165116 (2017)