Physical Review Letters

Tanaka, Yoshikazu ; Terada, Noriki ; Nakajima, Taro ; Taguchi, Munetaka ; Kojima, Taro ; Takata, Yasutaka ; Mitsuda, Setsuo ; Oura, Masaki ; Senba, Yasunori ; Ohashi, Haruhiko ; Shin, Shik

doi: 10.1103/PhysRevLett.109.127205pmid: 23005982

CuFeO 2 is one of the multiferroic materials and is the first case that the electric polarization is not explained by the magnetostriction model or the spin-current model. We have studied this material using soft x-ray resonant diffraction and found that superlattice reflection 0 1 - 2 q 0 appears in the ferroelectric and incommensurate magnetic ordered phase at the Fe L 2 , 3 absorption edges and moreover that the rotation of the x-ray polarization such as from σ to π or from π to σ is allowed at this reflection. These findings definitely provide direct evidence that the 3 d t 2 g ↓ orbital state of Fe ions has a long-range order in the ferroelectric state. The spin-orbit interaction in Fe ions plays a crucial role to the ferroelectricity in CuFeO 2 , coupling two nontrivial spin and orbital orders, both of which break the crystal symmetry.

Escande, D. F; Sattin, F. F

doi: 10.1103/PhysRevLett.109.129901pmid: N/A

Nakamura, Shin

doi: 10.1103/PhysRevLett.109.120602pmid: 23005930

We find novel phase transitions and critical phenomena that occur only outside the linear-response regime of current-driven nonequilibrium states. We consider the strongly interacting ( 3 + 1 ) -dimensional N = 4 large- N c S U ( N c ) supersymmetric Yang-Mills theory with a single flavor of fundamental N = 2 hypermultiplet as a microscopic theory. We compute its nonlinear nonballistic quark-charge conductivity by using the AdS/CFT correspondence. We find that the system exhibits a novel nonequilibrium first-order phase transition where the conductivity jumps and the sign of the differential conductivity flips at finite current density. A nonequilibrium critical point is discovered at the end point of the first-order regime. We propose a nonequilibrium steady-state analogue of thermodynamic potential in terms of the gravity-dual theory in order to define the transition point. Nonequilibrium analogues of critical exponents are proposed as well. The critical behavior of the conductivity is numerically confirmed on the basis of these proposals. The present work provides a new example of nonequilibrium phase transitions and nonequilibrium critical points.

Oja, R; Tyunina, M; Yao, L; Pinomaa, T; Kocourek, T; Dejneka, A; Stupakov, O; Jelinek, M; Trepakov, V; van Dijken, S; Nieminen, R. M

doi: 10.1103/PhysRevLett.109.127207pmid: 23005984

We use computational and experimental methods to study d 0 ferromagnetism at a charge-imbalanced interface between two perovskites. In SrTiO 3 / KTaO 3 superlattice calculations, the charge imbalance introduces holes in the SrTiO 3 layer, inducing a d 0 ferromagnetic half-metallic 2D hole gas at the interface oxygen 2 p orbitals. The charge imbalance overrides doping by vacancies at realistic concentrations. Varying the constituent materials shows ferromagnetism to be a general property of hole-type <?format ?> d 0 perovskite interfaces. Atomically sharp epitaxial d 0 SrTiO 3 / KTaO 3 , SrTiO 3 / KNbO 3 , and SrTiO 3 / NaNbO 3 interfaces are found to exhibit ferromagnetic hysteresis at room temperature. We suggest that the behavior is due to the high density of states and exchange coupling at the oxygen t 1 g band in comparison with the more studied d band t 2 g symmetry electron gas.

Vilfan, Andrej

doi: 10.1103/PhysRevLett.109.128105pmid: 23005993

We study the efficiency of self-propelled swimmers at low Reynolds numbers, assuming that the local energetic cost of maintaining a propulsive surface slip velocity is proportional to the square of that velocity. We determine numerically the optimal shape of a swimmer such that the total power is minimal while maintaining the volume and the swimming speed. The resulting shape depends strongly on the allowed maximum curvature. When sufficient curvature is allowed the optimal swimmer exhibits two protrusions along the symmetry axis. The results show that prolate swimmers such as Paramecium have an efficiency that is ∼ 20 % higher than that of a spherical body, whereas some microorganisms have shapes that allow even higher efficiency.

Lang, Thomas C; Meng, Zi Yang; Scherer, Michael M; Uebelacker, Stefan M; Assaad, Fakher F; Muramatsu, Alejandro F; Honerkamp, Carsten F; Wessel, Stefan F

doi: 10.1103/PhysRevLett.109.126402pmid: 23005964

Using a combination of quantum Monte Carlo simulations, functional renormalization group calculations and mean-field theory, we study the Hubbard model on the Bernal-stacked honeycomb bilayer at half-filling as a model system for bilayer graphene. The free bands consisting of two Fermi points with quadratic dispersions lead to a finite density of states at the Fermi level, which triggers an antiferromagnetic instability that spontaneously breaks sublattice and spin rotational symmetry once local Coulomb repulsions are introduced. Our results reveal an inhomogeneous participation of the spin moments in the ordered ground state, with enhanced moments at the threefold coordinated sites. Furthermore, we find the antiferromagnetic ground state to be robust with respect to enhanced interlayer couplings and extended Coulomb interactions.

Lee, Shu-Ping ; Alicea, Jason ; Refael, Gil

doi: 10.1103/PhysRevLett.109.126403pmid: 23005965

We show that a topological phase supporting Majorana fermions can form in a two-dimensional electron gas (2DEG) adjacent to an interdigitated superconductor-ferromagnet structure. An advantage of this setup is that the 2DEG can induce the required Zeeman splitting and superconductivity from a single interface, allowing one to utilize a wide class of 2DEGs including the surface states of bulk InAs. We demonstrate that the interdigitated device supports a robust topological phase when the finger spacing λ is smaller than half of the Fermi wavelength λ F . In this regime, the electrons effectively see a “smeared” Zeeman splitting and pairing field despite the interdigitation. The topological phase survives even in the opposite limit λ > λ F / 2 , although with a reduced bulk gap. We describe how to electrically generate a vortex in this setup to trap a Majorana mode, and predict an anomalous Fraunhofer pattern that provides a sharp signature of chiral Majorana edge states.

Yan, Wei ; Liu, Mengxi ; Dou, Rui-Fen ; Meng, Lan ; Feng, Lei ; Chu, Zhao-Dong ; Zhang, Yanfeng ; Liu, Zhongfan ; Nie, Jia-Cai ; He, Lin

doi: 10.1103/PhysRevLett.109.126801pmid: 23005971

Recent studies show that two low-energy van Hove singularities (VHSs) seen as two pronounced peaks in the density of states could be induced in a twisted graphene bilayer. Here, we report angle-dependent VHSs of a slightly twisted graphene bilayer studied by scanning tunneling microscopy and spectroscopy. We show that energy difference of the two VHSs follows Δ E vhs ∼ ℏ ν F Δ K between 1.0° and 3.0° (here ν F ∼ 1.1 × 10 6 m / s is the Fermi velocity of monolayer graphene, and Δ K = 2 K sin ⁡ ( θ / 2 ) is the shift between the corresponding Dirac points of the twisted graphene bilayer). This result indicates that the rotation angle between graphene sheets does not result in a significant reduction of the Fermi velocity, which quite differs from that predicted by band structure calculations. However, around a twisted angle θ ∼ 1.3 ° , the observed Δ E vhs ∼ 0.11 eV is much smaller than the expected value ℏ ν F Δ K ∼ 0.28 eV at 1.3°. The origin of the reduction of Δ E vhs at 1.3° is discussed.

Morris, R. G; Barthelemy, M. G

doi: 10.1103/PhysRevLett.109.128703pmid: 23006001

Transport processes on spatial networks are representative of a broad class of real world systems which, rather than being independent, are typically interdependent. We propose a measure of utility to capture key features that arise when such systems are coupled together. The coupling is defined in a way that is not solely topological, relying on both the distribution of sources and sinks, and the method of route assignment. Using a toy model, we explore relevant cases by simulation. For certain parameter values, a picture emerges of two regimes. The first occurs when the flows go from many sources to a small number of sinks. In this case, network utility is largest when the coupling is at its maximum and the average shortest path is minimized. The second regime arises when many sources correspond to many sinks. Here, the optimal coupling no longer corresponds to the minimum average shortest path, as the congestion of traffic must also be taken into account. More generally, results indicate that coupled spatial systems can give rise to behavior that relies subtly on the interplay between the coupling and randomness in the source-sink distribution.

Manjare, Manoj ; Yang, Bo ; Zhao, Y.-P

doi: 10.1103/PhysRevLett.109.128305pmid: 23005998

A new quasioscillatory translational motion has been observed for big Janus catalytic micromotors with a fast CCD camera. Such motional behavior is found to coincide with both the bubble growth and burst processes resulting from the catalytic reaction, and the competition of the two processes generates a net forward motion. Detailed physical models have been proposed to describe the above processes. It is suggested that the bubble growth process imposes a growth force moving the micromotor forward, while the burst process induces an instantaneous local pressure depression pulling the micromotor backward. The theoretic predictions are consistent with the experimental data.