Suppression of supercollision carrier cooling in high mobility graphene on SiC(0001¯)

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)


Thermoelectric transport in disordered metals without quasiparticles: The Sachdev-Ye-Kitaev models and holography

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)


A Crystal Ball for 2D Materials

April 18, 2017

Researchers predict new two-dimensional materials whose structures differ from their three-dimensional counterparts.

Synopsis on:
Arunima K. Singh et al.
Phys. Rev. B 95, 155426 (2017)


Stability and instability towards delocalization in many-body localization systems

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)


Chiral anomaly factory: Creating Weyl fermions with a magnetic field

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 Td 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)


Dynamically enriched topological orders in driven two-dimensional systems

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)


Self-focusing skyrmion racetracks in ferrimagnets

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)


Single-ion properties of the Seff = 12 XY antiferromagnetic pyrochlores NaACo2F7 (A=Ca2+, Sr2+)

Quantum effects in magnetic materials are prevalent when the magnetic ions support spin-½ moments, as in Cu2+-based materials. Strong quantum effects can also be achieved when the large angular momentum states are reduced to an effective spin-½ subspace (Seff=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 Seff=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 g tensors. Here, it is shown that a similar quantum model should apply to the recently discovered “high temperature” Co2+ pyrochlores, NaCaCo2F7 and NaSrCo2F7, 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 Seff=½ moments in these materials is also estimated.

K. A. Ross et al.
Phys. Rev. B 95, 144414 (2017)


Phase space manipulation of free-electron pulses from metal nanotips using combined terahertz near fields and external biasing

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)


Counter-rotating standing spin waves: A magneto-optical illusion

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)


Bosonic integer quantum Hall effect as topological pumping

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)


Heating up of Superconductors

January 27, 2017

This collection marks the 30th anniversary of the discovery of high-temperature superconductors. The papers selected highlight some of the advances that have been made to date, both in understanding why these compounds behave in the way they do, and in utilizing them in applications. The papers included in the collection have been made free to read.


The Rapid Communications section of Physical Review B is devoted to the accelerated publication of especially important new results. A Rapid Communication presents work that is important, interesting, or timely to those in a particular subfield.

More about Rapids

Current Issues

Vol. 95, Iss. 13-16 — April 2017

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Physics Next Workshops
March 20, 2017

The American Physical Society is initiating a new series of international workshops. These Physics Next workshops will be aimed at fostering new and emerging areas of physics research, focusing on topics that straddle traditional subject boundaries and are starting to “emerge from the noise.”

The first workshop is titled “Physics Next: Materials Design and Discovery,” and will take place on May 15 -17, 2017. More information.


Physical Review Materials Open for Submissions
April 4, 2017

APS is now accepting submissions for Physical Review Materials, the newest member of the Physical Review family of journals. PRMaterials expands the scope of the existing APS journals beyond their current emphasis on the physics of materials and will begin publishing mid-2017.

APS Endorses March for Science
March 10, 2017

After a thoughtful, deliberative process involving an examination of the alignment of the values of APS with the goals of the March for Science on April 22 in Washington, D.C., the APS Council Steering Committee, on behalf of the Council of Representatives, unanimously voted to endorse the march.

More Announcements

Trending in PRB

Topological tight-binding models from nontrivial square roots
J. Arkinstall et al.
Phys. Rev. B 95, 165109 (2017)

Reentrant superspin glass state and magnetization steps in the oxyborate Co2AlBO5
Jitender Kumar et al.
Phys. Rev. B 95, 144409 (2017)

Magnetic properties of ultrathin 3d transition-metal binary alloys. I. Spin and orbital moments, anisotropy, and confirmation of Slater-Pauling behavior
Martin A.W. Schoen et al.
Phys. Rev. B 95, 134410 (2017)

Single-ion properties of the Seff = 1/2 XY antiferromagnetic pyrochlores NaA′Co2F7 (A′=Ca2+, Sr2+)
K.A. Ross et al.
Phys. Rev. B 95, 144414 (2017)

Spin superfluid Josephson quantum devices
So Takei, Yaroslav Tserkovnyak, and Masoud Mohseni
Phys. Rev. B 95, 144402 (2017)

Locking of electron spin coherence above 20 ms in natural silicon carbide
D. Simin et al.
Phys. Rev. B 95, 161201 (2017)

Bosonic integer quantum Hall effect as topological pumping
Masaya Nakagawa and Shunsuke Furukawa
Phys. Rev. B 95, 165116 (2017)

Counter-rotating standing spin waves: A magneto-optical illusion
S. Shihab, L. Thevenard, A. Lemaitre, and C. Gourdon
Phys. Rev. B 95, 144411 (2017)

Topological states at the (001) surface of SrTiO3
Manali Vivek, Mark O. Goerbig, and Marc Gabay
Phys. Rev. B 95, 165117 (2017)

Self-focusing skyrmion racetracks in ferrimagnets
Se Kwon Kim, Kyung-Jin Lee, and Yaroslav Tserkovnyak
Phys. Rev. B 95, 140404 (2017)

Frustrated honeycomb-bilayer Heisenberg antiferromagnet: The spin-1/2 J1-J2-J1 model
R.F. Bishop and P.H.Y. Li
Phys. Rev. B 95, 134414 (2017)

Vortex-antivortex proliferation from an obstacle in thin film ferromagnets
Ezio Iacocca and Mark A. Hoefer
Phys. Rev. B 95, 134409 (2017)

High-pressure studies on the properties of FeGa3: Role of on-site Coulomb correlation
Debashis Mondal et al.
Phys. Rev. B 95, 134105 (2017)

Phonon-induced topological transition to a type-II Weyl semimetal
Lin-Lin Wang et al.
Phys. Rev. B 95, 165114 (2017)

Magnon planar Hall effect and anisotropic magnetoresistance in a magnetic insulator
J. Liu et al.
Phys. Rev. B 95, 140402 (2017)


Highlighting Impact and the Impact of Highlighting: PRB Editors’ Suggestions
January 4, 2016

Associate Editor Manolis Antonoyiannakis discusses the highlighting, as Editors’ Suggestions, of a small percentage of the papers published each week. We highlight papers primarily for their importance and impact in their respective fields, or because we find them particularly interesting or elegant. It turns out that the additional layer of scrutiny involved in the selection of papers as Editors’ Suggestions is associated with a significantly elevated and sustained citation impact.

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