129 research outputs found
Magnetic, thermodynamic, and electrical transport properties of the noncentrosymmetric B20 germanides MnGe and CoGe
We present magnetization, specific heat, resistivity, and Hall effect
measurements on the cubic B20 phase of MnGe and CoGe and compare to
measurements of isostructural FeGe and electronic structure calculations. In
MnGe, we observe a transition to a magnetic state at K as identified
by a sharp peak in the ac magnetic susceptibility, as well as second phase
transition at lower temperature that becomes apparent only at finite magnetic
field. We discover two phase transitions in the specific heat at temperatures
much below the Curie temperature one of which we associate with changes to the
magnetic structure. A magnetic field reduces the temperature of this transition
which corresponds closely to the sharp peak observed in the ac susceptibility
at fields above 5 kOe. The second of these transitions is not affected by the
application of field and has no signature in the magnetic properties or our
crystal structure parameters. Transport measurements indicate that MnGe is
metal with a negative magnetoresistance similar to that seen in isostructural
FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about
0.5 carriers per formula unit also similar to that found in FeGe and MnSi. CoGe
is shown to be a low carrier density metal with a very small, nearly
temperature independent diamagnetic susceptibility.Comment: 16 pages 23 figure
Fermi surface evolution through a heavy fermion superconductor-to-antiferromagnet transition: de Haas-van Alphen effect in Cd-substituted CeCoIn
We report the results of de-Haas-van-Alphen (dHvA) measurements in Cd doped
CeCoIn and LaCoIn. Cd doping is known to induce an antiferromagnetic
order in the heavy fermion superconductor CeCoIn, whose effect can be
reversed with applied pressure. We find a slight but systematic change of the
dHvA frequencies with Cd doping in both compounds, reflecting the chemical
potential shift due to the addition of holes. The frequencies and effective
masses are close to those found in the nominally pure compounds with similar
changes apparent in the Ce and La compounds with Cd substitution. We observe no
abrupt changes to the Fermi surface in the high field paramagnetic state for corresponding to the onset of antiferromagnetic ordering at H=0 in
CeCo(InCd). Our results rule out electron localization as
the mechanism for the tuning of the ground state in CeCoIn with Cd doping
Local structure and site occupancy of Cd and Hg substitutions in CeTIn5 (T=Co, Rh, Ir)
The CeTIn5 superconductors (T=Co, Rh, or Ir) have generated great interest
due to their relatively Tc's, NFL behavior, and their proximity to AF order and
quantum critical points. In contrast to small changes with the T-species,
electron doping in CeT(In{1-x}Mx)5 with M=Sn and hole doping with Cd or Hg have
a dramatic effect on the electronic properties at very low concentrations. The
present work reports EXAFS measurements that address the substituent atom
distribution as a function of T, M, and x, near the superconducting phase.
Together with previous measurements for M=Sn, the proportion of the M atom
residing on the In(1) site, f{In(1)}, increases in the order M=Cd, Sn, and Hg,
ranging from about 40% to 70%, showing a strong preference for these
substituents to occupy the In(1) site (random=20%). In addition, f{In(1)}
ranges from 70% to 100% for M=Hg in the order T=Co, Rh, and Ir. These fractions
track the changes in the atomic radii of the various species, and help explain
the sharp dependence of Tc on substituting into the In site. However, it is
difficult to reconcile the small concentrations of M with the dramatic changes
in the ground state in the hole-doped materials with only an impurity
scattering model. These results therefore indicate that while such
substitutions have interesting local atomic structures with important
electronic and magnetic consequences, other local changes in the electronic and
magnetic structure are equally important in determining the bulk properties of
these materials.Comment: 10 pages, 7 figures, to appear in PR
Superconducting fluctuations and the Nernst effect: A diagrammatic approach
We calculate the contribution of superconducting fluctuations above the
critical temperature to the transverse thermoelectric response
, the quantity central to the analysis of the Nernst effect. The
calculation is carried out within the microscopic picture of BCS, and to linear
order in magnetic field. We find that as , the dominant contribution
to arises from the Aslamazov-Larkin diagrams, and is equal to the
result previously obtained from a stochastic time-dependent Ginzburg-Landau
equation [Ussishkin, Sondhi, and Huse, arXiv:cond-mat/0204484]. We present an
argument which establishes this correspondence for the heat current. Other
microscopic contributions, which generalize the Maki-Thompson and density of
states terms for the conductivity, are less divergent as .Comment: 11 pages, 5 figure
Observation of the Nernst signal generated by fluctuating Cooper pairs
Long-range order is destroyed in a superconductor warmed above its critical
temperature (Tc). However, amplitude fluctuations of the superconducting order
parameter survive and lead to a number of well established phenomena such as
paraconductivity : an excess of charge conductivity due to the presence of
short-lived Cooper pairs in the normal state. According to an untested theory,
these pairs generate a transverse thermoelectric (Nernst) signal. In amorphous
superconducting films, the lifetime of Cooper pairs exceeds the elastic
lifetime of quasi-particles in a wide temperature range above Tc; consequently,
the Cooper pairs Nernst signal dominate the response of the normal electrons
well above Tc. In two dimensions, the magnitude of the expected signal depends
only on universal constants and the superconducting coherence length, so the
theory can be unambiguously tested. Here, we report on the observation of a
Nernst signal in such a superconductor traced deep into the normal state. Since
the amplitude of this signal is in excellent agreement with the theoretical
prediction, the result provides the first unambiguous case for a Nernst effect
produced by short-lived Cooper pairs
SC-to-AFM transition in CeCo(In1-xCdx)5: De Haas-van Alphen Measurements
The results of de-Haas-van-Alphen (dHvA) measurements on Cd doped CeCoIn5 in the high magnetic field paramagnetic phase are reported. The replacement of trivalent In by divalent Cd is known to induce an antiferromagnetic order coexisting with superconductivity in this heavy fermion superconductor. We find a small but systematic change in the dHvA frequencies with Cd doping, reflecting the chemical potential shift due to the removal of conduction electrons. The frequencies and effective masses are close to those found in the undoped compound. We observe no abrupt change in the electron Fermi surface (FS) volume in the high field paramagnetic phase for x \u3e x c corresponding to the onset of antiferromagnetic ordering at zero magnetic field in CeCo(In1xCdx)5. Our results show that no significant change of the Fermi surface volume occurs to cause the AFM onset inside the SC state in CeCoIn5 with Cd doping. Only slight changes causing the Band 15 electron FS to be more cylindrical are observed making nesting more likely. © Published under licence by IOP Publishing Ltd
Antiferromagnetic Phases in the Fulde-Ferrell-Larkin-Ovchinnikov State of CeCoIn_5
The antiferromagnetic (AFM) order in the Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) superconducting state is analyzed on the basis of a Ginzburg-Landau
theory. To examine the possible AFM-FFLO state in CeCoIn_5, we focus on the
incommensurate AFM order characterized by the wave vector Q = Q_{0} \pm q_inc
with Q_0 =(\pi,\pi,\pi) and q_inc \parallel [110] or [1-10] in the tetragonal
crystal structure. We formulate the two component Ginzburg-Landau theory and
investigate the two degenerate incommensurate AFM order. We show that the
pinning of AFM moment due to the FFLO nodal planes leads to multiple phases in
magnetic fields along [100] or [010]. The phase diagrams for various coupling
constants between the two order parameters are shown for the comparison with
CeCoIn_5. Experimental results of the NMR and neutron scattering measurements
are discussed.Comment: 6pages, Proceedings of ICHE2010, To appear in J. Phys. Soc. Jpn.
Supp
Anomalous dissipation in the mixed state of underdoped cuprates close to the superconductor-insulator boundary
We present a comparative study of Nernst effect and resistivity in underdoped
samples of BiSrCuO and LaSrCuO. The
Nernst effect presents a peak in a region of the H-T diagram where resistivity
shows a non-metallic temperature dependence. Our results illustrate that the
mechanism of dissipation in the mixed state of underdoped cuprates is poorly
understood. Large quantum superconducting fluctuations and vanishing vortex
viscosity are among suggested explanations for an enhanced Nernst signal close
to the superconductor-insulator boundary.Comment: 5 pages including 3 figure
Ginzburg-Landau Analysis for the Antiferromagnetic Order in the Fulde-Ferrell-Larkin-Ovchinnikov Superconductor
Incommensurate antiferromangetic (AFM) order in the
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor is investigated on the
basis of the Ginzburg-Landau theory. We formulate the two component
Ginzburg-Landau model to discuss two degenerate incommensurate AFM states in
the tetragonal crystal structure. Owing to the broken translation symmetry in
the FFLO state, a multiple phase diagram of single-q phase and double-q phase
is obtained under the magnetic field along [100] or [010] direction. Magnetic
properties in each phase are investigated and compared with the neutron
scattering and NMR measurements for a heavy fermion superconductor CeCoIn_5. An
ultrasonic measurement is proposed for a future experimental study to identify
the AFM-FFLO state. The field orientation dependence of the AFM order in
CeCoIn_5 is discussed.Comment: 8 page
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