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ABSTRACT: Simultaneous low-temperature electrical resistivity and Hall effect
measurements were performed on single-crystalline Bi2Se3 under applied
pressures up to 50 GPa. As a function of pressure, superconductivity is
observed to onset above 11 GPa with a transition temperature Tc and upper
critical field Hc2 that both increase with pressure up to 30 GPa, where they
reach maximum values of 7 K and 4 T, respectively. Upon further pressure
increase, Tc remains anomalously constant up to the highest achieved pressure.
Conversely, the carrier concentration increases continuously with pressure,
including a tenfold increase over the pressure range where Tc remains constant.
Together with a quasi-linear temperature dependence of Hc2 that exceeds the
orbital and Pauli limits, the anomalously stagnant pressure dependence of Tc
points to an unconventional pressure-induced pairing state in Bi2Se3 that is
unique among the superconducting topological insulators.
02/2013;
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ABSTRACT: We report a Fe K\beta x-ray emission spectroscopy study of local magnetic
moments in the rare-earth doped iron pnictide Ca_{1-x}RE_xFe_2As_2 (RE=La, Pr,
and Nd). In all samples studied the size of the Fe local moment is found to
decrease significantly with temperature and goes from ~0.9 \mu_B at T = 300 K
to ~0.45 \mu_B at T = 70 K. In the collapsed tetragonal (cT) phase of Nd- and
Pr-doped samples (T<70K) the local moment is quenched, while the moment remains
unchanged for the La-doped sample, which does not show lattice collapse. Our
results show that Ca_{1-x}RE_xFe_2As_2 (RE= Pr and Nd) exhibits a spin-state
transition and provide direct evidence for a non-magnetic Fe^{2+} ion in the
cT-phase, as predicted by Yildirim. We argue that the gradual change of the the
spin-state over a wide temperature range reveals the importance of multiorbital
physics, in particular the competition between the crystal field split Fe 3d
orbitals and the Hund's rule coupling.
12/2012;
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ABSTRACT: We study Bi2Se3 by polarization-dependent angle-resolved photoemission
spectroscopy (ARPES) and density-functional theory slab calculations. We find
that the surface state Dirac fermions are characterized by a layer-dependent
entangled spin-orbital texture, which becomes apparent through quantum
interference effects. This explains the discrepancy between the spin
polarization from spin-resovled ARPES - ranging from 20 to 85% - and the 100%
value assumed in phenomenological models. It also suggests a way to probe the
intrinsic spin texture of topological insulators, and to continuously
manipulate the spin polarization of photoelectrons and photocurrents all the
way from 0 to +/-100% by an appropriate choice of photon energy, linear
polarization, and angle of incidence.
12/2012;
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ABSTRACT: We present a study of entropy transport in Bi2Se3 at low temperatures and
high magnetic fields. In the zero-temperature limit, the magnitude of the
Seebeck coefficient quantitatively tracks the Fermi temperature of the 3D Fermi
surface at \Gamma-point as the carrier concentration changes by two orders of
magnitude (10$^{17}$ to 10$^{19}$cm$^{-3}$). In high magnetic fields, the
Nernst response displays giant quantum oscillations indicating that this
feature is not exclusive to compensated semi-metals. The analysis of quantum
oscillations quantifies the magnitude of the Zeeman energy and points to a
significant field-dependence of the Fermi energy across the quantum limit.
09/2012;
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ABSTRACT: A scheme is proposed to electrically measure the spin-momentum coupling in
the topological insulator surface state by injection of spin polarized
electrons from silicon. As a first approach, devices were fabricated consisting
of thin (<100nm) exfoliated crystals of Bi2Se3 on n-type silicon with
independent electrical contacts to silicon and Bi2Se3. Analysis of the
temperature dependence of thermionic emission in reverse bias indicates a
barrier height of 0.34 eV at the Si-Bi2Se3 interface. This robust Schottky
barrier opens the possibility of novel device designs based on sub-band gap
internal photoemission from Bi2Se3 into Si.
05/2012;
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ABSTRACT: We report $^{75}$As NMR studies on single crystals of rare-earth doped iron
pnictides superconductor Ca$_{1-x}$Pr$_{x}$Fe$_{2}$As$_{2}$ ($x$=0.075 and
0.15). The $^{75}$As spectra show a chemical pressure effect with doping and a
first order structure transition to the collapsed tetragonal phase upon
cooling. A sharp drop of the Knight shift is seen below the structural
transition, whereas $1/T_1$ is strongly enhanced at low-temperatures. These
evidences indicate quenching of Fe local magnetism and short-range ordering of
Pr$^{3+}$ moment in the collapsed tetragonal phase. The quenched Fe moment
through structure collapse suggests a strong interplay of structure and
magnetism, which is important for understanding the nature of the collapsed
tetragonal phase.
05/2012;
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ABSTRACT: Structural and electronic characterization of (Ca0.67Sr0.33)Fe2As2 has been
performed as a func- tion of pressure up to 12 GPa using conventional and
designer diamond anvil cells. The compound (Ca0.67Sr0.33)Fe2As2 behaves
intermediate between its end members-CaFe2As2 and SrFe2As2- displaying a
suppression of magnetism and the onset of superconductivity. Like other members
of the AEFe2As2 family, (Ca0.67Sr0.33)Fe2As2 undergoes a pressure-induced
isostructural volume collapse, which we associate with the development of As-As
bonding across the mirror plane of the structure. This collapsed tetragonal
phase abruptly cuts off the magnetic state, giving rise to superconductivity
with a maximum Tc=22.2 K. The maximum Tc of the superconducting phase is not
strongly correlated with any structural parameter, but its proximity to the
abrupt suppression of magnetism as well as the volume collapse transition
suggests that magnetic interactions and structural inhomogeneity may play a
role in its development. The pressure-dependent evolution of the ordered states
and crystal structures in (Ca,Sr)Fe2As2 provides an avenue to understand the
generic behavior of the other members of the AEFe2As2 family.
02/2012;
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ABSTRACT: Resistivity, magnetic susceptibility, neutron scattering, and x-ray crystallography measurements were used
to study the evolution of magnetic order and crystallographic structure in single-crystal samples of the
Ba1−xSrxFe2As2 and Sr1−yCayFe2As2 series. A nonmonotonic dependence of the magnetic ordering temperature T0 on chemical pressure is compared to the progression of the antiferromagnetic staggered moment, characteristics of the ordering transition, and structural parameters to reveal a distinct relationship between the magnetic energy scale and the tetrahedral bond angle, even far above T0. In Sr1−yCayFe2As2, an abrupt drop in T0 precisely at the Ca concentration where the tetrahedral structure approaches the ideal geometry indicates a strong coupling between the orbital bonding structure and the stabilization of magnetic order, providing strong constraints on the nature of magnetism in the iron-arsenide superconducting parent compounds.
Physical Review B 01/2012; 86:060504/1-5. · 3.69 Impact Factor
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ABSTRACT: Aliovalent rare-earth substitution into the alkaline-earth site of CaFe2As2 single crystals is used to fine tune structural, magnetic, and electronic properties of this iron-based superconducting system. Neutron and single-crystal x-ray scattering experiments indicate that an isostructural collapse of the tetragonal unit cell can be controllably induced at ambient pressures by the choice of substituent ion size. This instability is driven by the interlayer As-As anion separation, resulting in an unprecedented thermal expansion coefficient of 180×10−6 K−1. Electrical transport and magnetic susceptibility measurements reveal abrupt changes in the physical properties through the collapse as a function of temperature, including a reconstruction of the electronic structure. Superconductivity with onset transition temperatures as high as 47 K is stabilized by the suppression of antiferromagnetic order via chemical pressure, electron doping, or a combination of both. Extensive investigations are performed to understand the observations of partial volume-fraction diamagnetic screening, ruling out extrinsic sources such as strain mechanisms, surface states, or foreign phases as the cause of this superconducting phase that appears to be stable in both collapsed and uncollapsed structures.
Physical Review B 01/2012; 85(2):024525/1-14. · 3.69 Impact Factor
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ABSTRACT: We report x-ray diffraction, electrical resistivity, and magnetoresistance measurements on Bi2Se3 under high pressure and low temperature conditions. Pressure induces profound changes in both the room temperature value of the electrical resistivity as well as the temperature dependence of the resistivity. Initially, pressure drives Bi2Se3 toward increasingly insulating behavior and then, at higher pressures, the sample appears to enter a fully metallic state coincident with a change in the crystal structure. Within the low pressure phase, Bi2Se3 exhibits an unusual field dependence of the transverse magnetoresistance Δρ(xx) that is positive at low fields and becomes negative at higher fields. Our results demonstrate that pressures below 8 GPa provide a non-chemical means to controllably reduce the bulk conductivity of Bi2Se3.
Journal of Physics Condensed Matter 12/2011; 24(3):035602. · 2.55 Impact Factor
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Z-H Zhu,
G Levy,
B Ludbrook,
C N Veenstra,
J A Rosen,
R Comin,
D Wong,
P Dosanjh,
A Ubaldini,
P Syers,
N P Butch, J Paglione,
I S Elfimov,
A Damascelli
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ABSTRACT: The electronic structure of Bi(2)Se(3) is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultrahigh-vacuum conditions, can be overcome via in situ potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. Ab initio slab calculations reveal that these Rashba states are derived from 5-quintuple-layer quantum-well states. While the K-induced potential gradient enhances the spin splitting, this may be present on pristine surfaces due to the symmetry breaking of the vacuum-solid interface.
Physical Review Letters 10/2011; 107(18):186405. · 7.37 Impact Factor
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ABSTRACT: Although it is generally accepted that superconductivity is unconventional in the high-transition-temperature copper oxides, the relative importance of phenomena such as spin and charge (stripe) order, superconductivity fluctuations, proximity to a Mott insulator, a pseudogap phase and quantum criticality are still a matter of debate. In electron-doped copper oxides, the absence of an anomalous pseudogap phase in the underdoped region of the phase diagram and weaker electron correlations suggest that Mott physics and other unidentified competing orders are less relevant and that antiferromagnetic spin fluctuations are the dominant feature. Here we report a study of magnetotransport in thin films of the electron-doped copper oxide La(2 - x)Ce(x)CuO(4). We show that a scattering rate that is linearly dependent on temperature--a key feature of the anomalous normal state properties of the copper oxides--is correlated with the electron pairing. We also show that an envelope of such scattering surrounds the superconducting phase, surviving to zero temperature when superconductivity is suppressed by magnetic fields. Comparison with similar behaviour found in organic superconductors strongly suggests that the linear dependence on temperature of the resistivity in the electron-doped copper oxides is caused by spin-fluctuation scattering.
Nature 08/2011; 476(7358):73-5. · 36.28 Impact Factor
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ABSTRACT: Although it is generally accepted that superconductivity (SC) is
unconventional in the high- transition temperature copper oxides (high-Tc
cuprates), the relative importance of phenomena such as spin and charge
(stripe) order, SC fluctuations, proximity to a Mott insulator, a pseudogap
phase, and quantum criticality are still a matter of great debate1. In
electron-doped cuprates, the absence of an anomalous pseudogap phase in the
underdoped region of the phase diagram2 and weaker electron correlations3,4,
suggest that Mott physics and other unidentified competing orders are less
relevant and that antiferromagnetic (AFM) spin fluctuations are the dominant
feature. Here we demonstrate that a linear-temperature (T-linear) scattering
rate - a key feature of the anomalous normal state properties of the cuprates -
is correlated with the Cooper pairing (SC). Through a study of magnetotransport
in thin films of the electron-doped cuprate La2 xCexCuO4 (LCCO), we show that
an envelope of T-linear scattering surrounds the SC phase, and survives to zero
temperature when superconductivity is suppressed by magnetic fields. Comparison
with similar behavior found in organic superconductors5 strongly suggests that
the T-linear resistivity is caused by spin-fluctuation scattering. Our results
establish a fundamental connection between AFM spin fluctuations and the
pairing mechanism of high temperature superconductivity in the cuprates.
08/2011;
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ABSTRACT: We report measurements of the pressure dependence of the superconducting
transition temperature T_c in single crystal samples of the rare-earth doped
superconductor Ca$_{0.73}$La$_{0.27}$Fe$_2$As$_2$. We track T_c with two
techniques, via in-plane resistivity measurements and with a resonant tunnel
diode oscillator circuit which is sensitive to the skin depth. We show that
initially T_c rises steeply with pressure, forming a superconducting dome with
a maximum T_c of ~44 K at 20 kbar. We discuss this observation in the context
of other electron-doped iron pnictide superconductors, and conclude that the
application of pressure offers an independent way to tune T_c in this system.
07/2011;
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L Shu,
R E Baumbach,
M Janoschek,
E Gonzales,
K Huang,
T A Sayles, J Paglione,
J O'Brien,
J J Hamlin,
D A Zocco,
P-C Ho,
C A McElroy,
M B Maple
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ABSTRACT: X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on Ce(1-x)Yb(x)CoIn5 (0≤x≤1) reveal that many of the characteristic features of the x=0 correlated electron state are stable for x≤0.775 and that phase separation occurs for x>0.775. The stability of the correlated electron state is apparently due to cooperative behavior of the Ce and Yb ions, involving their unstable valences. Low-temperature non-Fermi liquid behavior is observed and varies with x, even though there is no readily identifiable quantum critical point. The superconducting critical temperature T(c) decreases linearly with x towards 0 K as x→1, in contrast with other HF superconductors where T(c) scales with T(coh).
Physical Review Letters 04/2011; 106(15):156403. · 7.37 Impact Factor
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ABSTRACT: Carrier and phonon dynamics in Bi2Se3 crystals are studied by a spatially resolved ultrafast pump-probe technique. Pronounced oscillations in differential reflection are observed with two distinct frequencies and are attributed to coherent optical and acoustic phonons, respectively. The rising time of the signal indicates that the thermalization and energy relaxation of hot carriers are both sub-ps in this material. We found that the thermalization and relaxation time decreases with the carrier density. The expansion of the differential reflection profile allows us to estimate an ambipolar carrier diffusion coefficient on the order of 500 cm2/s. A long-term slow expansion of the profile shows a thermal diffusion coefficient of 1.2 cm2/s.
Phys. Rev. B. 04/2011; 83(23).
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ABSTRACT: The effect of alkaline earth substitution on structural parameters was studied in high-quality single crystals of Ba1−xSrxFe2As2 and Sr1−xCaxFe2As2 grown by the self-flux method. The results of single-crystal and powder x-ray diffraction measurements suggest a continuous monotonic decrease of both a- and c-axis lattice parameters, the c/a tetragonal ratio, and the unit cell volume with decreasing alkaline earth atomic radius as expected by Vegard's law. As a result, the system experiences a continuously increasing chemical pressure effect in traversing the phase diagram from x = 0 in Ba1−xSrxFe2As2 to x = 1 in Sr1−xCaxFe2As2.
Journal of Physics Conference Series 02/2011; 273(1):012104.
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ABSTRACT: The effect of alkaline earth substitution on structural parameters was studied in high-quality single crystals of Ba1−xSrxFe2As2 and Sr1−xCaxFe2As2 grown by the self-flux method. The results of single-crystal and powder x-ray diffraction measurements suggest a continuous monotonic decrease of both a- and c-axis lattice parameters, the c/a tetragonal ratio, and the unit cell volume with decreasing alkaline earth atomic radius as expected by Vegard's law. As a result, the system experiences a continuously increasing chemical pressure effect in traversing the phase diagram from x = 0 in Ba1−xSrxFe2As2 to x = 1 in Sr1−xCaxFe2As2.
Journal of Physics Conference Series 01/2011; 273:12104.1-4.
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Physical Review B 01/2011; 83(13). · 3.69 Impact Factor
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ABSTRACT: We present the magnetic structure of the itinerant monoarsenide, FeAs, with
the B31 structure. Powder neutron diffraction confirms incommensurate modulated
magnetism with wavevector $\mathbf{q} = (0.395\pm0.001)\mathbf{c}^*$ at 4 K,
but cannot distinguish between a simple spiral and a collinear spin-density
wave structure. Polarized single crystal diffraction confirms that the
structure is best described as a non-collinear spin-density wave arising from a
combination of itinerant and localized behavior with spin amplitude along the
b-axis direction being (15 $\pm$ 5)% larger than in the a-direction.
Furthermore, the propagation vector is temperature dependence, and the
magnetization near the critical point indicates a two-dimensional Heisenberg
system. The nature of the magnetism in the simplest iron arsenide is of
fundamental importance in understanding the interplay between localized and
itinerant magnetism and superconductivity.
12/2010;
