July 2011

www.epljournal.org

Highlights from the previous volumes

Shortcut to adiabaticity with ultracold atoms

Adiabatic transformations are widely used in physics, and,for instance, are at the heart of the manipulation of quan-tum states. In such transformations, the Hamiltonianshould typically vary slowly with time, such that the sys-tem always remains close to equilibrium. The drawback isthe long transition time, which in some situations becomesunpractical due to finite lifetime or coherence time of thestate under study. Alternatively, a “shortcut to adiabatic-ity” is a specifically designed temporal trajectory of theHamiltonian, which connects the initial to the final statein a shorter time. The system is out of equilibrium dur-ing the transition, but the final state is identical to thatobtained via an adiabatic transition (see the left panel ofthe figure).

We have applied this strategy for the first time, torapidly decompress an interacting Bose-Einstein conden-sate (BEC) held in a magnetic trap. Reducing the trapconfinement also shifts the cloud vertically by a largeamount, due to gravity. We implemented a 30ms longtrajectory designed for a 10-fold reduction of the trap fre-quency. Because of experimental imperfections, the finalstate we obtained is not an equilibrium one. However, wewere able to demonstrate a large reduction of BEC exci-tations (dipole and breathing modes) when comparing theshortcut to other standard decompression schemes (see theright panel of the figure). This trajectory was also shownto work for a thermal cloud with negligible interactions,hinting at the broad range of application of this technique.

Left: principle of shortcut to adiabaticity. The blue and red

curves represent the BEC wave function and trapping potential

respectively. Right: comparison of BEC excitations produced

by standard and shortcut decompressions.

Original article by Schaff J.-F. et al.EPL, 93 (2011) 23001

Spin-charge locking and tunnelinginto a helical metal

Spintronics aims at exploiting the electron spin for newdevice functionalities. In recent years a new spintronicparadigm, based on the spin-orbit interaction, has beenproposed aiming to gain spin control by electric fields. Inthis respect, topological insulators (TI) appear as a verypromising opportunity. At the surface of a TI gapless ex-citations occur with extraordinarily strong spin-orbit cou-pling: a given surface momentum is associated with a sin-gle spin direction, such that the states on the Fermi sur-face have a well-defined helicity. In this paper we present atheoretical study of the dynamics of the electrons movingon the surface of a three-dimensional TI, i.e. in a two-dimensional helical metal (HM). When the HM is broughtinto contact with a ferromagnet there arises an uncon-ventional magnetoresistance. The origin of the effect isthe spin-orbit coupling: since the electron momentum isconnected to a single spin state, a current flow createsa nonequilibrium spin polarization. This current-inducedspin polarization increases or decreases the spin-dependentvoltage difference between the helical metal and the ma-jority or minority carriers in the FM and thus modifies thetunneling current. By reversing the flow of the current inthe helical metal the two spin species exchange their role.In the ideal case the tunneling current between the FMand the HM can be switched on and off depending on therelative orientation of the magnetization of the FM withrespect to the direction of the current flow in the HM.

A ferromagnet (green) that is coupled to a topological insulator

(grey) such that electrons can tunnel between the two materi-

als. It is found that a current flow in the surface states of the

topological insulator strongly modifies the tunneling current.

Original article by Schwab P. et al.EPL, 93 (2011) 67004

Copyright c© EPLA, 2011

Highlights from the previous volumes

Optical evidence of nematicity in iron-basedsuperconductors

A nematic order recently arose as a robust electronic statedescribing the nature of the pseudogap phase in the high-temperature superconducting cuprates. In the field ofliquid crystals, a nematic state derives from a transitionbreaking the rotational symmetry of the high-temperaturephase but preserving the translational one. Besides thecuprates, the novel iron-based superconductors in theirparent and underdoped phase recently emerged as an al-ternative playground for studying an electron nematic-ity in a correlated system. The iron-arsenide supercon-ductors harbor indeed an antiferromagnetic ground state,which is either preceded or accompanied by a structuraltetragonal-orthorhombic phase transition at Ts. Thisstructural transition breaks the fourfold symmetry of thehigh-temperature lattice and leads to an anisotropic con-ducting state. This broken rotational symmetry has thus adirect impact in the optical properties. We investigate theoptical conductivity σ1(ω) with light polarized along thein-plane orthorhombic a- and b-axes of Ba(Fe1−xCox )2As2for x = 0 and 2.5% under uniaxial pressure across theirphase transitions. The charge dynamics on these de-twinned, single-domain samples reveals an in-plane opti-cal anisotropy (i.e., linear dichroism) which extends torelatively high frequencies and at T > Ts. This revealssubstantial nematic susceptibility as well as the electronicnature of the structural transition. Another key resultconsists in the opportunity to disentangle the distinct be-haviors of the Drude weights and scattering rates of theitinerant charge carriers, which are both enhanced alongthe a-axis with respect to the b-axis. Our findings allow usto clarify the long-standing striking anisotropy (ρb > ρa)of the dc resistivity.

0 100 200 3000.0

0.2

0.4

0.6

0

50

100

150–30

–20

–10

0

Temperature (K)

Δρ(T

)/ρ(T

)=2(ρ b−ρ

a)/(ρ

a+ρ b

)

Δσ1 (ω

1 ) (Ωcm

) −1

Δσ1 (ω

2 ) (Ωcm

) −1

Δρ/ρΔσ1(ω1)Δσ1(ω2)

Δρopt/ρ

x=0.025

Temperature dependence of the dichroism Δσ1(ω) = σ1(ω, E ‖a)−σ1(ω, E ‖ b) for x = 0.025 at selected frequencies compared

to the dc anisotropy ratio (Δρ/ρ). The anisotropy in the dc

limit of the optical conductivity (Δρopt/ρ) is reported, as well.

The vertical dashed and dash-dotted lines mark the magnetic

and structural phase transitions, respectively.

Original article by Dusza A. et al.EPL, 93 (2011) 37002

Design of a low band gap oxide ferroelectric:Bi6Ti4O17

The manipulation of band gaps in oxides while retainingfunction is a long-standing problem. Xu and co-workersdiscuss a strategy based on manipulation of the Coulombpotential by artificial layering and illustrate its use to pro-duce a titanate ferroelectric with a band gap below 2 eV.This is 1 eV below the lowest band gap of a titanate fer-roelectric compound and is in a range that is potentiallyuseful for solar and other optical applications. They startwith the known ferroelectric Bi4Ti3O12, which has a bandgap of approximately 3 eV. This compound may be re-garded as a layered system consisting of stacks of alternat-ing perovskite and fluorite structure blocks. Importantly,the ferroelectric polarization is substantial and close to theplane of the layers. The authors used a combination offirst-principles relaxations and electronic-structure calcu-lations to show that they could alter the Coulomb poten-tial enough to shift the relative positions of the cation de-rived conduction bands relative to the O 2p valence bandsby 1 eV, while nonetheless retaining a sizable ferroelectricpolarization. This manipulation of the Coulomb poten-tial is illustrated in the figure, which shows the calculatedshifts in the 1s core levels of the Bi and O atoms as afunction of position along the layer stacking direction inthe unit cell. The mechanism that the authors use is gen-eral and could be applied to other layered oxide systemsincluding other ferroelectrics. The ability to shift bandsin engineered oxide structure on the eV scale may also beimportant for other applications such as oxide electronicsand photo-catalysis.

1s core level variation for O and Bi along the long axis of the

unit cell as shown on the right. This represents the variation

in the Coulomb potential that is induced by the layer stacking

in this material.

Original article by Xu Bo et al.EPL, 94 (2011) 37006