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T. Shimojima,
T. Sonobe,
W. Malaeb,
K. Shinada,
A. Chainani,
S. Shin,
T. Yoshida,
S. Ideta,
A. Fujimori,
H. Kumigashira, [......],
C. H. Lee,
A. Iyo,
H. Eisaki,
K. Ohgushi,
S. Kasahara,
T. Terashima,
H. Ikeda,
T. Shibauchi, Y. Matsuda,
K. Ishizaka
[show abstract]
[hide abstract]
ABSTRACT: In the normal state above the superconducting transition temperature (Tc),
the momentum-resolved electronic structure of the high-Tc cuprate shows a
depletion dubbed the psudogap, which becomes less pronounced with doping. Its
relation to the high-Tc superconductivity has been a key issue, and the
possible rotational symmetry breaking in this state has been reported
suggesting a peculiar electronic ordering preceding the superconducting
pairing. Here we provide direct evidence for a systematic evolution of the
pseudogap phase via angle-resolved photoemission spectroscopy on another family
of high-Tc superconductor, iron-pnictides. We observe the P-substitution
evolution of pseudogap in BaFe2(As1-xPx)2, which develops well above the
magnetostructural transitions and persists above the nonmagnetic
superconducting dome, showing a notable similarity with cuprates. In addition,
the pseudogap formation is accompanied by inequivalent energy shifts in zx/yz
orbitals of iron atoms, indicative of a peculiar iron orbital ordering which
breaks the fourfold rotational symmetry.
05/2013;
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[show abstract]
[hide abstract]
ABSTRACT: The NMR results in iron pnictides BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 are
analyzed based on the self-consistent renormalization (SCR) spin fluctuation
theory. The temperature dependence of the NMR relaxation rate (T_1)^{-1} as
well as the electrical resistivity is well reproduced by a SCR model where
two-dimensional antiferromagnetic (AF) spin fluctuations are dominant. The
successful description of the crossover feature from non-Fermi liquid to Fermi
liquid behavior strongly suggests that low-lying spin fluctuations in
BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 possess an itinerant AF nature, and that
chemical substitution in the two compounds tunes the distance of these systems
to an AF quantum critical point. The close relationship between spin
fluctuations and superconductivity is discussed compared with the other
unconventional superconductors, cuprate and heavy fermion superconductors. In
addition, it is suggested that magnetism and lattice instability in these
pnictides are strongly linked via orbital degrees of freedom.
05/2013;
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[show abstract]
[hide abstract]
ABSTRACT: Whether a quantum critical point (QCP) lies beneath the superconducting dome
has been a long-standing issue that remains unresolved in many classes of
unconventional superconductors, notably cuprates, heavy fermion compounds and
most recently iron-pnictides. The existence of a QCP may offer a route to
understand: the origin of their anomalous non-Fermi liquid properties, the
microscopic coexistence between unconventional superconductivity and magnetic
or some exotic order, and ultimately the mechanism of superconductivity itself.
The isovalent substituted iron-pnictide BaFe$_2$(As$_{1-x}$P$_x$)$_2$ offers a
new platform for the study of quantum criticality, providing a unique
opportunity to study the evolution of the electronic properties in a wide range
of the phase diagram. Recent experiments in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ have
provided the first clear and unambiguous evidence of a second order quantum
phase transition lying beneath the superconducting dome.
04/2013;
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[show abstract]
[hide abstract]
ABSTRACT: Temperature-dependent inter-plane resistivity, $\rho _c(T)$, was measured for
the iron-based superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ over a broad
isoelectron phosphorus substitution range from $x$=0 to $x$=0.60, from
non-superconducting parent compound to heavily overdoped superconducting
composition with $T_c\approx 10~K$. The features due to structural and magnetic
transitions are clearly resolved in $\rho _c(T)$ of the underdoped crystals. A
characteristic maximum in $\rho _c(T)$, found in the parent BaFe$_2$As$_2$ at
around 200~K, moves rapidly with phosphorus substitution to high temperatures.
At the optimal doping, the inter-plane resistivity shows $T$-linear temperature
dependence without any cross-over anomalies, similar to the previously reported
in-plane resistivity. This observation is in stark contrast with dissimilar
temperature dependences found at optimal doping in electron-doped
Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. Our finding suggests that despite similar
values of the resistivity and its anisotropy, the temperature dependent
transport in the normal state is very different in electron and isoelectron
doped compounds. Similar temperature dependence of both in-plane and
inter-plane resistivities, in which the dominant contributions are coming from
different parts of the Fermi surface, suggests that scattering is the same on
the whole Fermi surface. Since magnetic fluctuations are expected to be much
stronger on the quasi-nested sheets, this observation may point to the
importance of the inter-orbital scattering between different sheets.
02/2013;
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T. Yoshida,
S. Ideta,
T. Shimojima,
W. Malaeb,
K. Shinada,
H. Suzuki,
I. Nishi,
A. Fujimori,
K. Ishizaka,
S. Shin, [......],
T. Ito,
K. Kihou,
C. H. Lee,
A. Iyo,
H. Eisaki,
H. Ikeda,
R. Arita,
T. Saito,
S. Onari,
H. Kontani
[show abstract]
[hide abstract]
ABSTRACT: In the iron pnictide superconductors, two distinct unconventional mechanisms
of superconductivity have been put forth: One is mediated by spin fluctuations
leading to the s+- state with sign change of superconducting gap between the
hole and electron bands, and the other is orbital fluctuations which favor the
s++ state without sign reversal. Here we report direct observation of peculiar
momentum-dependent anisotropy in the superconducting gap from angle-resolved
photoemission spectroscopy (ARPES) in BaFe2(As1-xPx)2 (Tc=30 K). The large
anisotropy found only in the electron Fermi surface (FS) and the nearly
isotropic gap on the entire hole FSs are together consistent with modified s+-
gap with nodal loops, which can be theoretically reproduced by considering both
spin and orbital fluctuations whose competition generates the gap modulation.
This indicates that these two fluctuations are nearly equally important to the
high-Tc superconductivity in this system.
01/2013;
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S K Goh,
Y Mizukami,
H Shishido,
D Watanabe,
S Yasumoto,
M Shimozawa,
M Yamashita,
T Terashima,
Y Yanase,
T Shibauchi,
A I Buzdin, Y Matsuda
[show abstract]
[hide abstract]
ABSTRACT: We report a highly unusual angular variation of the upper critical field (H_{c2}) in epitaxial superlattices CeCoIn_{5}(n)/YbCoIn_{5}(5), formed by alternating layers of n and a 5 unit-cell thick heavy-fermion superconductor CeCoIn_{5} with a strong Pauli effect and normal metal YbCoIn_{5}, respectively. For the n=3 superlattice, H_{c2}(θ) changes smoothly as a function of the field angle θ. However, close to the superconducting transition temperature, H_{c2}(θ) exhibits a cusp near the parallel field (θ=0°). This cusp behavior disappears for n=4 and 5 superlattices. This sudden disappearance suggests the relative dominance of the orbital depairing effect in the n=3 superlattice, which may be due to the suppression of the Pauli effect in a system with local inversion symmetry breaking. Taking into account the temperature dependence of H_{c2}(θ) as well, our results suggest that some exotic superconducting states, including a helical superconducting state, might be realized at high magnetic fields.
Physical Review Letters 10/2012; 109(15):157006. · 7.37 Impact Factor
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D Watanabe,
M Yamashita,
S Tonegawa,
Y Oshima,
H M Yamamoto,
R Kato,
I Sheikin,
K Behnia,
T Terashima,
S Uji,
T Shibauchi, Y Matsuda
[show abstract]
[hide abstract]
ABSTRACT: In Mott insulators, the strong electron-electron Coulomb repulsion localizes electrons. In dimensions greater than one, their spins are usually ordered antiferromagnetically at low temperatures. Geometrical frustrations can destroy this long-range order, leading to exotic quantum spin liquid states. However, their magnetic ground states have been a long-standing mystery. Here we show that a quantum spin liquid state in the organic Mott insulator EtMe(3)Sb[Pd(dmit)(2)](2) (where Et is C(2)H(5)-, Me is CH(3)-, and dmit is 1,3-dithiole-2-thione-4,5-dithiolate) with two-dimensional triangular lattice has Pauli-paramagnetic-like low-energy excitations, which are a hallmark of itinerant fermions. Our torque magnetometry down to low temperatures (30 mK) up to high fields (32 T) reveals distinct residual paramagnetic susceptibility comparable to that in a half-filled two-dimensional metal, demonstrating the magnetically gapless nature of the ground state. Moreover, our results are robust against deuteration, pointing toward the emergence of an extended 'quantum critical phase', in which low-energy spin excitations behave as in paramagnetic metals with Fermi surface, despite the frozen charge degree of freedom.
Nature Communications 09/2012; 3:1090. · 7.40 Impact Factor
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K Okazaki,
Y Ota,
Y Kotani,
W Malaeb,
Y Ishida,
T Shimojima,
T Kiss,
S Watanabe,
C-T Chen,
K Kihou, [......],
H Fukazawa,
Y Kohori,
K Hashimoto,
T Shibauchi, Y Matsuda,
H Ikeda,
H Miyahara,
R Arita,
A Chainani,
S Shin
[show abstract]
[hide abstract]
ABSTRACT: In iron-pnictide superconductivity, the interband interaction between the hole and electron Fermi surfaces (FSs) is believed to play an important role. However, KFe(2)As(2) has three zone-centered hole FSs and no electron FS but still exhibits superconductivity. Our ultrahigh-resolution laser angle-resolved photoemission spectroscopy unveils that KFe(2)As(2) is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional gap with "octet-line nodes" on the middle FS, and an almost-zero gap on the outer FS. This gap structure may arise from the frustration between competing pairing interactions on the hole FSs causing the eightfold sign reversal. Our results suggest that the A(1g) superconducting symmetry is universal in iron-pnictides, in spite of the variety of gap functions.
Science 09/2012; 337(6100):1314-7. · 31.20 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: To study the nature of partially substituted Yb-ions in a Ce-based Kondo
lattice, we fabricated high quality Ce_{1-x}Yb_xCoIn_5 epitaxial thin films
using molecular beam epitaxy. We find that the Yb-substitution leads to a
linear decrease of the unit cell volume, indicating that Yb-ions are divalent
forming Kondo-holes in Ce_{1-x}Yb_xCoIn_5, and leads to a strong suppression of
the superconductivity and Kondo coherence. These results, combined with the
measurements of Hall effect, indicate that Yb-ions act as nonmagnetic impurity
scatters in the coherent Kondo lattice without serious suppression of the
antiferromagnetic fluctuations. These are in stark contrast to previous studies
performed using bulk single crystals, which claim the importance of valence
fluctuations of Yb-ions. The present work also highlights the suitability of
epitaxial films in the study of the impurity effect on the Kondo lattice.
08/2012;
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S. K. Goh,
Y. Mizukami,
H. Shishido,
D. Watanabe,
S. Yasumoto,
M. Shimozawa,
M. Yamashita,
T. Terashima,
Y. Yanase,
T. Shibauchi,
A. I. Buzdin, Y. Matsuda
[show abstract]
[hide abstract]
ABSTRACT: We report a highly unusual angular variation of the upper critical field
(H_c2) in epitaxial superlattices CeCoIn_5(n)/YbCoIn_5(5), formed by
alternating layers of n and a 5 unit-cell thick heavy-fermion superconductor
CeCoIn_5 with a strong Pauli effect and normal metal YbCoIn_5, respectively.
For the n=3 superlattice, H_{c2}(\theta) changes smoothly as a function of the
field angle \theta. However, close to the superconducting transition
temperature, H_{c2}(\theta) exhibits a cusp near the parallel field (\theta=0
deg). This cusp behavior disappears for n=4 and 5 superlattices. This sudden
disappearance suggests the relative dominance of the orbital depairing effect
in the n=3 superlattice, which may be due to the suppression of the Pauli
effect in a system with local inversion symmetry breaking. Taking into account
the temperature dependence of H_{c2}(\theta) as well, our results suggest that
some exotic superconducting states, including a helical superconducting state,
might be realized at high magnetic fields.
07/2012;
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S Tonegawa,
K Hashimoto,
K Ikada,
Y-H Lin,
H Shishido,
Y Haga,
T D Matsuda,
E Yamamoto,
Y Onuki,
H Ikeda, Y Matsuda,
T Shibauchi
[show abstract]
[hide abstract]
ABSTRACT: We report the first observation of cyclotron resonance in the hidden-order phase of ultraclean URu2Si2 crystals, which allows the full determination of angle-dependent electron-mass structure of the main Fermi-surface sheets. We find an anomalous splitting of the sharpest resonance line under in-plane magnetic-field rotation. This is most naturally explained by the domain formation, which breaks the fourfold rotational symmetry of the underlying tetragonal lattice. The results reveal the emergence of an in-plane mass anisotropy with hot spots along the [110] direction, which can account for the anisotropic in-plane magnetic susceptibility reported recently. This is consistent with the "nematic" Fermi liquid state, in which itinerant electrons have unidirectional correlations.
Physical Review Letters 07/2012; 109(3):036401. · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Despite more than a quarter century of research, the nature of the
second-order phase transition in the heavy-fermion metal URu$_2$Si$_2$ remains
enigmatic. The key question is which symmetry is being broken below this
"hidden order" transition. We review the recent progress on this issue,
particularly focusing on the thermodynamic evidence from very sensitive
micro-cantilever magnetic torque measurements that the fourfold rotational
symmetry of the underlying tetragonal crystal is broken. The angle dependence
of the torque under in-plane field rotation exhibits the twofold oscillation
term, which sets in just below the transition temperature. This observation
restricts the symmetry of the hidden order parameter to the $E^{+}$- or
$E^{-}$-type, depending on whether the time reversal symmetry is preserved or
not.
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 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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S. Kasahara,
H. J. Shi,
K. Hashimoto,
S. Tonegawa,
Y. Mizukami,
T. Shibauchi,
K. Sugimoto,
T. Fukuda,
T. Terashima,
Andriy H. Nevidomskyy, Y. Matsuda
[show abstract]
[hide abstract]
ABSTRACT: Strongly interacting electrons can exhibit novel collective phases, among
which the electronic nematic phases are perhaps the most surprising as they
spontaneously break rotational symmetry of the underlying crystal lattice. The
electron nematicity has been recently observed in the iron-pnictide and cuprate
high-temperature superconductors. Whether such a tendency of electrons to
self-organise unidirectionally has a common feature in these superconductors
is, however, a highly controversial issue. In the cuprates, the nematicity has
been suggested as a possible source of the pseudogap phase, whilst in the
iron-pnictides, it has been commonly associated with the
tetragonal-to-orthorhombic structural phase transition at $T_s$. Here, we
provide the first thermodynamic evidence in BaFe2(As1-xPx)2 that the nematicity
develops well above the structural transition and persists to the nonmagnetic
superconducting regime, resulting in a new phase diagram strikingly similar to
the pseudogap phase diagram in the cuprates. Our highly sensitive magnetic
anisotropy measurements using microcantilever torque-magnetometry under
in-plane field rotation reveal pronounced two-fold oscillations, which break
the tetragonal symmetry. Combined with complementary high-resolution
synchrotron X-ray and resistivity measurements, our results consistently
identify two distinct temperatures - one at $T^{\ast}$, signifying a true
nematic transition, and the other at $T_s (< T^{\ast})$, which we show to be
not a true phase transition, but rather what we refer to as a "meta-nematic
transition", in analogy to the well-known metamagnetic transition in the theory
of magnetism. Our observation of the extended nematic phase above the
superconducting dome establishes that the nematicity has primarily an
electronic origin, inherent in the normal state of high-temperature
superconductors.
07/2012;
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K Hashimoto,
K Cho,
T Shibauchi,
S Kasahara,
Y Mizukami,
R Katsumata,
Y Tsuruhara,
T Terashima,
H Ikeda,
M A Tanatar,
H Kitano,
N Salovich,
R W Giannetta,
P Walmsley,
A Carrington,
R Prozorov, Y Matsuda
[show abstract]
[hide abstract]
ABSTRACT: In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λ(L). In two-dimensional systems, this ratio n/m* (~1/λ(L)(2)) determines the effective Fermi temperature, T(F). We report a sharp peak in the x-dependence of λ(L) at zero temperature in clean samples of BaFe(2)(As(1)(-x)P(x))(2) at the optimum composition x = 0.30, where the superconducting transition temperature T(c) reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of T(c)/T(F) at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.
Science 06/2012; 336(6088):1554-7. · 31.20 Impact Factor
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S Kasahara,
H J Shi,
K Hashimoto,
S Tonegawa,
Y Mizukami,
T Shibauchi,
K Sugimoto,
T Fukuda,
T Terashima,
Andriy H Nevidomskyy, Y Matsuda
[show abstract]
[hide abstract]
ABSTRACT: Electronic nematicity, a unidirectional self-organized state that breaks the rotational symmetry of the underlying lattice, has been observed in the iron pnictide and copper oxide high-temperature superconductors. Whether nematicity plays an equally important role in these two systems is highly controversial. In iron pnictides, the nematicity has usually been associated with the tetragonal-to-orthorhombic structural transition at temperature T(s). Although recent experiments have provided hints of nematicity, they were performed either in the low-temperature orthorhombic phase or in the tetragonal phase under uniaxial strain, both of which break the 90° rotational C(4) symmetry. Therefore, the question remains open whether the nematicity can exist above T(s) without an external driving force. Here we report magnetic torque measurements of the isovalent-doping system BaFe(2)(As(1-x)P(x))(2), showing that the nematicity develops well above T(s) and, moreover, persists to the non-magnetic superconducting regime, resulting in a phase diagram similar to the pseudogap phase diagram of the copper oxides. By combining these results with synchrotron X-ray measurements, we identify two distinct temperatures-one at T*, signifying a true nematic transition, and the other at T(s) (<T*), which we show not to be a true phase transition, but rather what we refer to as a 'meta-nematic transition', in analogy to the well-known meta-magnetic transition in the theory of magnetism.
Nature 06/2012; 486(7403):382-5. · 36.28 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Novel electronic states resulting from entangled spin and orbital degrees of
freedom are hallmarks of strongly correlated f-electron systems. A spectacular
example is the so-called 'hidden-order' phase transition in the heavy-electron
metal URu2Si2, which is characterized by the huge amount of entropy lost at
T_{HO}=17.5K. However, no evidence of magnetic/structural phase transition has
been found below T_{HO} so far. The origin of the hidden-order phase transition
has been a long-standing mystery in condensed matter physics. Here, based on a
first-principles theoretical approach, we examine the complete set of multipole
correlations allowed in this material. The results uncover that the
hidden-order parameter is a rank-5 multipole (dotriacontapole) order with
'nematic' E^- symmetry, which exhibits staggered pseudospin moments along the
[110] direction. This naturally provides comprehensive explanations of all key
features in the hidden-order phase including anisotropic magnetic excitations,
nearly degenerate antiferromagnetic-ordered state, and spontaneous
rotational-symmetry breaking.
04/2012;
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[show abstract]
[hide abstract]
ABSTRACT: High-precision measurements of magnetic penetration depth λ in clean single crystals of LiFeAs and LiFeP superconductors reveal contrasting behaviors. In LiFeAs the low-temperature λ(T) shows a flat dependence indicative of a fully gapped state, which is consistent with previous studies. In contrast, LiFeP exhibits a T-linear dependence of superfluid density infinity λ(-2), indicating a nodal superconducting order parameter. A systematic comparison of quasiparticle excitations in the 1111, 122, and 111 families of iron-pnictide superconductors implies that the nodal state is induced when the pnictogen height from the iron plane decreases below a threshold value of ~1.33 Å.
Physical Review Letters 01/2012; 108(4):047003. · 7.37 Impact Factor
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C Putzke,
A I Coldea,
I Guillamón,
D Vignolles,
A McCollam,
D Leboeuf,
M D Watson,
I I Mazin,
S Kasahara,
T Terashima,
T Shibauchi, Y Matsuda,
A Carrington
[show abstract]
[hide abstract]
ABSTRACT: We report a de Haas-van Alphen oscillation study of the 111 iron pnictide superconductors LiFeAs with T(c) ≈ 18 K and LiFeP with T(c) ≈ 5 K. We find that for both compounds the Fermi surface topology is in good agreement with density functional band-structure calculations and has almost nested electron and hole bands. The effective masses generally show significant enhancement, up to ~3 for LiFeP and ~5 for LiFeAs. However, one hole Fermi surface in LiFeP shows a very small enhancement, as compared with its other sheets. This difference probably results from k-dependent coupling to spin fluctuations and may be the origin of the different nodal and nodeless superconducting gap structures in LiFeP and LiFeAs, respectively.
Physical Review Letters 01/2012; 108(4):047002. · 7.37 Impact Factor
