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M. G. Kim,
J. Soh,
J. Lang,
M. P. M. Dean,
A. Thaler,
S. L. Bud'ko, P. C. Canfield,
E. Bourret-Courchesne,
A. Kreyssig,
A. I. Goldman,
R. J. Birgeneau
[show abstract]
[hide abstract]
ABSTRACT: We have employed the x-ray resonant magnetic scattering (XRMS) technique at
the Ru $L_2$ edge of the Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ ($x = 0.205$)
superconductor. We show that pronounced resonance enhancements at the Ru $L_2$
edge are observed at the wave vector which is consistent with the
antiferromagnetic propagation vector of the Fe in the undoped BaFe$_2$As$_2$.
We also demonstrate that the XRMS signals at the Ru $L_2$ edge follow the
magnetic ordering of the Fe with a long correlation length, $\xi_{ab} >
2850\pm400$ \AA . Our experimental observation shows that the Ru is
spin-polarized in Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ compounds.
04/2013;
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M. G. Kim,
G. S. Tucker,
D. K. Pratt,
S. Ran,
A. Thaler,
A. D. Christianson,
K. Marty,
A. Podlesnyak,
S. L. Bud'ko, P. C. Canfield,
A. Kreyssig,
A. I. Goldman,
R. J. McQueeney
[show abstract]
[hide abstract]
ABSTRACT: Inelastic neutron scattering measurements on
Ba(Fe$_{0.963}$Ni$_{0.037}$)$_2$As$_2$ manifest a neutron spin resonance in the
superconducting state with anisotropic dispersion within the Fe layer. Whereas
the resonance is sharply peaked at Q$_{AFM}$ along the orthorhombic a axis, the
resonance disperses upwards away from Q$_{AFM}$ along the b axis. In contrast
to the downward dispersing resonance and hour-glass shape of the spin
excitations in superconducting cuprates, the resonance in electron-doped
BaFe$_2$As$_2$ compounds possesses a magnon-like upwards dispersion.
04/2013;
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[show abstract]
[hide abstract]
ABSTRACT: We present detailed temperature and field dependent data obtained from
magnetization, resistivity, heat capacity, Hall resistivity and thermoelectric
power measurements performed on single crystals of CeZn11. The compounds orders
antiferromagnetically at $\sim$ 2 K. The zero-field resistivity and TEP data
show features characteristic of a Ce-based intermetallic with crystal electric
field splitting and possible correlated, Kondo lattice effects. We constructed
the T-H phase diagram for the magnetic field applied along the easy, [110],
direction which shows that the magnetic field required to suppress T_N below
0.4 K is in the range of 45-47.5 kOe. A linear behavior of the rho(T) data,
H||[110], was observed only for H=45 kOe for 0.46 K<T<1.96 K followed by the
Landau-Fermi-liquid regime for a limited range of fields, 47.5 kOe< H<60 kOe.
From the analysis of our data, it appears that CeZn11 is a weakly to moderately
correlated local moment compound with rather small Kondo temperature. The
thermoelectric and transport properties of CeZn11 are mostly governed by the
CEF effects. Given the very high quality of our single crystals, quantum
oscillations are found for both CeZn11 and its non-magnetic analogue, LaZn11.
04/2013;
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[show abstract]
[hide abstract]
ABSTRACT: We report measurements of the electrical and thermal transport on the
hexagonal heavy-fermion metal YbAgGe for temperatures $T\geq$ 40 mK and in
magnetic fields $H\parallel ab$ up to 14 T. This distorted Kagome-lattice
system displays a series of magnetic states and a quantum critical point at
$H_c =4.5$ T. The Lorenz ratio $L(T)/L_0$ displays a marked reduction only
close to $H_c$. A $T$-linear contribution below 120 mK, present at all
different fields, allows to extrapolate the Lorenz ratio towards T=0. At the
critical field this yields $L/L_0=0.92\pm 0.03$, suggesting a violation of the
Wiedemann-Franz law due to strong inelastic scattering.
04/2013;
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[show abstract]
[hide abstract]
ABSTRACT: Electrical transport measurements of the heavy fermion compound YbFe2Zn20
were carried out under pressures up to 8.23 GPa and down to temperatures of
nearly 0.3 K. The pressure dependence of the low temperature Fermi-liquid state
was assessed by fitting rho(T) = rho_0 + AT^n with n = 2 for T < T_FL. Power
law analysis of the low temperature resistivities indicates n = 2 over a broad
temperature range for P < 5 GPa. However, at higher pressures, the quadratic
temperature dependence is only seen at the very lowest temperatures, and
instead shows a wider range of n < 2 power law behavior in the low temperature
resistivities. As pressure was increased, T_FL diminished from ~11 K at ambient
pressure to ~0.6 K at 8.23 GPa. Over the same pressure range, the A parameter
increased dramatically with a functional form of A proportional to (P-Pc)^-2
with Pc~9.8GPa being the critical pressure for a possible quantum critical
point.
04/2013;
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[show abstract]
[hide abstract]
ABSTRACT: By using nuclear magnetic resonance and ac-susceptibility, the characteristic
correlation times for the vortex dynamics, in an iron-based superconductor, have been derived.
Upon cooling, the vortex dynamics displays a crossover consistent with a vortex glass transition.
The correlation times, in the fast motions regime, merge onto a universal curve which is fit by
the Vogel-Fulcher law, rather than by an Arrhenius law. Moreover, the pinning barrier shows a
weak dependence on the magnetic field which can be heuristically justified within a fragile glass
scenario. In addition, the glass freezing temperatures obtained by the two techniques merge onto
the de Almeida-Thouless line. Finally the phase diagram for the mixed phase has been derived.
EPL (Europhysics Letters) 04/2013; 102(1):17005. · 2.17 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We report on the infrared studies of the interlayer charge dynamics of a prototypical pnictide superconductor Ba(Fe_{0.926}Co_{0.074})_{2}As_{2}. We succeeded in probing the intrinsic interlayer response by performing infrared experiments on the crystals with a cleaved ac surface. Our experiments identify the coexistence of the suppression of the electronic spectral weight and the development of a coherent Drude-like response in the normal state. The formation of the interlayer condensate is clearly observed in the superconducting state and appears to be linked to coherent contribution to the normal-state conductivity.
Physical Review Letters 03/2013; 110(9):097003. · 7.37 Impact Factor
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A. Kreyssig,
M. A. Green,
Y Lee,
G. D. Samolyuk,
P. Zajdel,
J. W. Lynn,
S. L. Bud'ko,
M. S. Torikachvili,
N. Ni,
S Nandi,
J. Leao,
S. J. Poulton,
D. N. Argyriou,
B. N. Harmon, P. C. Canfield,
R. J. McQueeney,
A. I. Goldman
[show abstract]
[hide abstract]
ABSTRACT: Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced transition to a non-magnetic, volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.
Physical review. B, Condensed matter 02/2013; 78:184517.
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[show abstract]
[hide abstract]
ABSTRACT: We have performed detailed studies of the temperature evolution of the electronic structure in Ba(Fe_{1-x}Ru_{x})_{2}As_{2} using angle resolved photoemission spectroscopy. Surprisingly, we find that the binding energy of both hole and electron bands changes significantly with temperature in both pure and Ru substituted samples. The hole and electron pockets are well nested at low temperature in unsubstituted (BaFe_{2}As_{2}) samples, which likely drives the spin density wave and resulting antiferromagnetic order. Upon warming, this nesting is degraded as the hole pocket shrinks and the electron pocket expands. Our results demonstrate that the temperature dependent nesting may play an important role in driving the antiferromagnetic-paramagnetic phase transition.
Physical Review Letters 02/2013; 110(6):067002. · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: In-plane resistivity measurements as a function of temperature, magnetic
field and its orientation with respect to the crystallographic $ab-$plane were
used to study the upper critical field, $H_{c2}$, of two overdoped compositions
of the iron-based superconductor Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$, $x=$0.054 and
$x$=0.072. Measurements were performed using precise alignment (with accuracy
less than 0.1$^o$) of magnetic field with respect to the Fe-As-plane. The
dependence of the $H_{c2}$ on angle $\theta$ between the field and the
$ab$-plane was measured in isothermal conditions in a broad temperature range.
We found that the shape of $H_{c2}(\theta)$, clearly deviates from
Ginzburg-Landau functional form.
01/2013;
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Physical Review B. 01/2013; 87(1):014506.
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E. D. Mun,
S. L. Bud'ko,
C. Martin,
H. Kim,
M. A. Tanatar,
J. -H. Park,
T. Murphy,
G. M. Schmiedeshoff,
N. Dilley,
R. Prozorov, P. C. Canfield
[show abstract]
[hide abstract]
ABSTRACT: In this paper, we present the systematic measurements of the temperature and
magnetic field dependences of the thermodynamic and transport properties of the
Yb-based heavy fermion YbPtBi for temperatures down to 0.02 K with magnetic
fields up to 140 kOe to address the possible existence of a field-tuned quantum
critical point. Measurements of magnetic field and temperature dependent
resistivity, specific heat, thermal expansion, Hall effect, and thermoelectric
power indicate that the AFM order can be suppressed by applied magnetic field
of $H_{c}$ $\sim$ 4 kOe. In the $H-T$ phase diagram of YbPtBi, three regimes of
its low temperature states emerges: (I) AFM state, characterized by spin
density wave (SDW) like feature, which can be suppressed to $T$ = 0 by the
relatively small magnetic field of $H_{c}$ $\sim$ 4\,kOe, (II) field induced
anomalous state in which the electrical resistivity follows $\Delta\rho(T)
\propto T^{1.5}$ between $H_{c}$ and $\sim$ 8 kOe, and (III) Fermi liquid (FL)
state in which $\Delta\rho(T) \propto T^{2}$ for $H \geq$ 8 kOe. Regions I and
II are separated at $T$ = 0 by what appears to be a quantum critical point.
Whereas region III appears to be a FL associated with the hybridized 4$f$
states of Yb, region II may be a manifestation of a spin liquid state.
11/2012;
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M G Kim,
J Lamsal,
T W Heitmann,
G S Tucker,
D K Pratt,
S N Khan,
Y B Lee,
A Alam,
A Thaler,
N Ni,
S Ran,
S L Bud'ko,
K J Marty,
M D Lumsden, P C Canfield,
B N Harmon,
D D Johnson,
A Kreyssig,
R J McQueeney,
A I Goldman
[show abstract]
[hide abstract]
ABSTRACT: The spin fluctuation spectra from nonsuperconducting Cu-substituted, and superconducting Co-substituted, BaFe_{2}As_{2} are compared quantitatively by inelastic neutron scattering measurements and are found to be indistinguishable. Whereas diffraction studies show the appearance of incommensurate spin-density wave order in Co and Ni substituted samples, the magnetic phase diagram for Cu substitution does not display incommensurate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitutional impurity effects in the Fe plane play a significant role in controlling magnetism and the appearance of superconductivity, with Cu distinguished by enhanced impurity scattering and split-band behavior.
Physical Review Letters 10/2012; 109(16):167003. · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: By using Nuclear Magnetic Resonance and ac-susceptibility, the characteristic
correlation times for the vortex dynamics, in an iron-based superconductor,
have been derived. Upon cooling, the vortex dynamics displays a crossover
conceivable with a vortex glass transition. The correlation times in the fast
motions regime merge into a universal curve which can be fit by the
Vogel-Fulcher law. Moreover, the glass freezing temperatures obtained by the
two techniques merge nicely into the de Almeida-Thouless line. The pinning
barrier shows a weak dependence on the magnetic field which can be
heuristically justified within a fragile glass scenario.
08/2012;
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S. J. Moon,
A. A. Schafgans,
S. Kasahara,
T. Shibauchi,
T. Terashima,
Y. Matsuda,
M. A. Tanatar,
R. Prozorov,
A. Thaler, P. C. Canfield,
A. S. Sefat,
D. Mandrus,
D. N. Basov
[show abstract]
[hide abstract]
ABSTRACT: We report on infrared studies of charge dynamics in a prototypical pnictide
system: the BaFe2As2 family. Our experiments have identified hallmarks of the
pseudogap state in the BaFe2As2 system that mirror the spectroscopic
manifestations of the pseudogap in the cuprates. The magnitude of the infrared
pseudogap is in accord with that of the spin-density-wave gap of the parent
compound. By monitoring the superconducting gap of both P- and Co-doped
compounds, we find that the infrared pseudogap is unrelated to
superconductivity. The appearance of the pseudogap is found to correlate with
the evolution of the antiferromagnetic fluctuations associated with the
spin-density-wave instability. The strong-coupling analysis of infrared data
further reveals the interdependence between the magnetism and the pseudogap in
the iron pnictides.
07/2012;
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S J Moon,
A A Schafgans,
S Kasahara,
T Shibauchi,
T Terashima,
Y Matsuda,
M A Tanatar,
R Prozorov,
A Thaler, P C Canfield,
A S Sefat,
D Mandrus,
D N Basov
[show abstract]
[hide abstract]
ABSTRACT: We report on infrared studies of charge dynamics in a prototypical pnictide system: the BaFe_{2}As_{2} family. Our experiments have identified hallmarks of the pseudogap state in the BaFe_{2}As_{2} system that mirror the spectroscopic manifestations of the pseudogap in the cuprates. The magnitude of the infrared pseudogap is in accord with that of the spin-density-wave gap of the parent compound. By monitoring the superconducting gap of both P- and Co-doped compounds, we find that the infrared pseudogap is unrelated to superconductivity. The appearance of the pseudogap is found to correlate with the evolution of the antiferromagnetic fluctuations associated with the spin-density-wave instability. The strong-coupling analysis of infrared data further reveals the interdependence between the magnetism and the pseudogap in the iron pnictides.
Physical Review Letters 07/2012; 109(2):027006. · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Single crystals of CaNi2 and CaNi3 are successfully grown out of excess Ca. Both compounds manifest a metallic ground state with enhanced, temperature-dependent magnetic susceptibility. The relatively high Stoner factors of Z=0.79 and 0.87 found for CaNi2 and CaNi3, respectively, reveal their close vicinity to ferromagnetic instabilities. The pronounced field dependence of the magnetic susceptibility of CaNi3 at low temperatures (T<25 K) suggests strong ferromagnetic fluctuations. A corresponding contribution to the specific heat with a temperature dependence of T3lnT is also observed.
Phys. Rev. B. 06/2012; 85(22).
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G. S. Tucker,
D. K. Pratt,
M. G. Kim,
S. Ran,
A. Thaler,
G. E. Granroth,
K. Marty,
W. Tian,
J. L. Zarestky,
M. D. Lumsden,
S. L. Bud'ko, P. C. Canfield,
A. Kreyssig,
A. I. Goldman,
R. J. McQueeney
[show abstract]
[hide abstract]
ABSTRACT: Inelastic neutron scattering measurements on Ba(Fe0.925Mn0.075)2As2 manifest
spin fluctuations at two different wavevectors in the Fe square lattice,
(1/2,0) and (1/2,1/2), corresponding to the expected stripe spin-density wave
order and checkerboard antiferromagnetic order, respectively. Below T_N=80 K,
long-range stripe magnetic ordering occurs and sharp spin wave excitations
appear at (1/2,0) while broad and diffusive spin fluctuations remain at
(1/2,1/2) at all temperatures. Low concentrations of Mn dopants nucleate local
moment spin fluctuations at (1/2,1/2) that compete with itinerant spin
fluctuations at (1/2,0) and may disrupt the development of superconductivity.
06/2012;
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[show abstract]
[hide abstract]
ABSTRACT: Temperature-dependent, in-plane, thermoelectric power data are presented for
single crystals of Ba(Fe1-xCox)2As2, 0\leqx\leq0.05, and Ba(Fe1-xRhx)2As2,
(0\leqx\leq0.171). Given that previous thermoelectric power and angle resolved
photoemission spectroscopy studies of Ba(Fe1-xCox)2As2 delineated a rather
large Co-concentration range for Lifshitz transitions to occur, and the
underdoped side of the phase diagram is poorly explored, new measurements of
thermoelectric power on tightly spaced concentrations of Co, 0\leqx\leq0.05,
were carried out. The data suggest evidence of a Lifshitz transition, but
instead of a discontinuous jump in thermoelectric power between 0\leqx\leq0.05,
a more gradual evolution in the S(T) plots as x is increased was observed. The
thermoelectric power data of Ba(Fe1-xRhx)2As2 show very similar behavior to
that of Co substituted BaFe2As2. The previously outlined T - x phase diagrams
for both systems are further confirmed by these thermoelectric power data.
06/2012;
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[show abstract]
[hide abstract]
ABSTRACT: We demonstrate that the thermopower (S) can be used to probe the spin
fluctuations (SF) in the proximity to the quantum critical point (QCP) in
Fe-based superconductors. The sensitivity of S to the entropy of charge
carriers allows us to observe an increase of S/T in Ba(Fe(1-x)Co(x))2As2 close
to the spin-density-wave (SDW) QCP. This behavior is due to the coupling of
low-energy conduction electrons to two-dimensional SF, similar to the
heavy-fermion systems. The low-temperature enhancement of S/T in the doping
range 0.02 < x < 0.1 is bordered by the two Lifshitz transitions, and it
corresponds to the superconducting doping region, where similarity between the
electron and nonreconstructed hole pockets exists. The maximal S/T is observed
in the proximity of commensurate-to-incommensurate SDW transition, for critical
x_c ~ 0.06, close to the highest superconducting T_c. This analysis indicates
that the low-T thermopower is influenced by the critical spin fluctuations
which are important for the superconducting mechanism.
06/2012;
