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ABSTRACT: As ferromagnetism and superconductivity are usually considered to be antagonistic, the discovery of their coexistence in UGe(2), URhGe, UIr and UCoGe has attracted a lot of interest. The mechanism to explain such a state has, however, not yet been fully elucidated. In these compounds superconductivity may be unconventional: Cooper pairs could be formed by electrons with parallel spins and magnetic fluctuations might be involved in the pairing mechanism. URhGe becomes ferromagnetic below a Curie temperature of 9.5 K, with a spontaneous moment aligned to the c-axis. For temperatures below 260 mK and fields lower than 2 T, superconductivity was first observed in 2001. Recently, we discovered a second pocket of superconductivity. This new pocket of superconductivity appears at higher fields applied close to the b-axis, enveloping a sudden magnetic moment rotation transition at H(R) = 12 T. Detailed studies of the field induced metamagnetic transition and superconductivity are presented. The possibility that magnetic fluctuations emerging from a quantum critical point provide the pairing mechanism for superconductivity is discussed.
Journal of Physics Condensed Matter 04/2009; 21(16):164211. · 2.55 Impact Factor
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ABSTRACT: When a pure material is tuned to the point where a continuous phase-transition line is crossed at zero temperature, known as a quantum critical point (QCP), completely new correlated quantum ordered states can form
Nature Physics 05/2007; 3(7):460-463. · 18.97 Impact Factor
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E Bustarret,
C Marcenat,
P Achatz,
J Kacmarcik,
F Lévy, A Huxley,
L Ortéga,
E Bourgeois,
X Blase,
D Débarre,
J Boulmer
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ABSTRACT: Although the local resistivity of semiconducting silicon in its standard crystalline form can be changed by many orders of magnitude by doping with elements, superconductivity has so far never been achieved. Hybrid devices combining silicon's semiconducting properties and superconductivity have therefore remained largely underdeveloped. Here we report that superconductivity can be induced when boron is locally introduced into silicon at concentrations above its equilibrium solubility. For sufficiently high boron doping (typically 100 p.p.m.) silicon becomes metallic. We find that at a higher boron concentration of several per cent, achieved by gas immersion laser doping, silicon becomes superconducting. Electrical resistivity and magnetic susceptibility measurements show that boron-doped silicon (Si:B) made in this way is a superconductor below a transition temperature T(c) approximately 0.35 K, with a critical field of about 0.4 T. Ab initio calculations, corroborated by Raman measurements, strongly suggest that doping is substitutional. The calculated electron-phonon coupling strength is found to be consistent with a conventional phonon-mediated coupling mechanism. Our findings will facilitate the fabrication of new silicon-based superconducting nanostructures and mesoscopic devices with high-quality interfaces.
Nature 12/2006; 444(7118):465-8. · 36.28 Impact Factor
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ABSTRACT: We report inelastic neutron scattering measurements of the phonon spectrum of the pressure-induced ferromagnetic superconductor UGe$_{2}$. No changes of the spectrum were found on cooling down to low temperature. The phonon contribution to the specific heat was estimated from a fit to our data. The excess specific heat previously noted at around $T_{x} \approx$ 30 K is not due to phonons but is well described by the temperature dependence of the magnetic order parameter at the molecular field level. Comment: This paper has been withdrawn by the author due to a crucial error in phonon branches assignement
09/2005;
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ABSTRACT: The interplay of magnetism and unconventional superconductivity (d singlet wave or p triplet wave) in strongly correlated electronic system (SCES) is discussed with recent examples found in heavy fermion compounds. A short presentation is given on the formation of the heavy quasiparticle with the two sources of a local and intersite enhancement for the effective mass. Two cases of the coexistence or repulsion of antiferromagnetism and superconductivity are given with CeIn3 and CeCoIn5. A spectacular example is the emergence of superconductivity in relatively strong itinerant ferromagnets UGe2 and URhGe. The impact of heavy fermion matter among other SCES as organic conductor or high Tc oxide is briefly pointed out. Comment: 22pages, 15 figures
05/2005;
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ABSTRACT: Inelastic neutron scattering was used to study the low-energy magnetic excitations of the ferromagnetic superconductor UGe2. The ferromagnetic fluctuations are of Ising nature with a non-conserved magnetization and have an intermediate behavior between localized and itinerant magnetism.
Physica B: Condensed Matter. 12/2003;
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ABSTRACT: The weak antiferromagnetic order of the heavy-fermion superconductor UPt3 has been investigated by elastic neutron-scattering measurements under applied uniaxial pressure up to 6 kbar along the a and c axes of the hexagonal crystal structure. For p||c the small antiferromagnetically ordered moment of 0.02 7B/U-atom shows a non-linear decrease for increasing pressures and is still not completely suppressed at the maximum applied pressure of 6 kbar. For p||a a significant increase in the magnetic Bragg peak intensity is observed, which suggests an incomplete domain repopulation and confirms the presence of a single-k structure.
Applied Physics A 11/2002; 74:s586-s588. · 1.63 Impact Factor
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ABSTRACT: The discovery of superconductivity at high pressure (albeit over a restricted range) in the ferromagnetic material UGe2 raised the possibility that bulk superconductivity might be found in other ferromagnets. The exact symmetry of the paired state and the dominant mechanism responsible for the pairing, however, remain unidentified. Meanwhile, the conjecture that superconductivity could occur more generally in ferromagnets has been fuelled by the recent observation of a low-temperature transition that suggests an onset of superconductivity in high-quality crystals of the itinerant-ferromagnet ZrZn2 (ref. 2), although the thermodynamic signature of this transition could not be detected. Here we show that the ferromagnet URhGe is superconducting at ambient pressure. In this case, we find the thermodynamic signature of the transition-its form is consistent with a superconducting pairing of a spin-triplet type, although further testing with cleaner samples is needed to confirm this. The combination of superconductivity and ferromagnetism may thus be more common and consequently more important than hitherto realized.
Nature 11/2001; 413(6856):613-6. · 36.28 Impact Factor
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ABSTRACT: The superconducting properties of the heavy fermion UPt
3
have been changed by irradiation with high energy electrons which creates point defects in a reproducible and controled way. Measurements of the residual resistivity, critical temperature, upper critical field and thermal conductivity have been realized on these irradiated samples. The strong suppresion of superconductivity with increasing defect concentration is in agreement with the theory of unconventional superconductivity. However, our thermal conductivity data contradicts the simple predictions derived from the most popular modelsE
1g
and E
2u) of the superconducting order parameter in UPt
3
.
Journal of Low Temperature Physics 08/1999; 116(5):393-405. · 1.19 Impact Factor
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ABSTRACT: We report measurements of the field-induced magnetization density in . The main results of the study are that the magnetic density is located equally at the Ce and Ru sites, and that the distribution of the induced magnetization about the Ce site extends to larger distances than predicted for ions with well localized f electrons. Our measurements also cover the superconducting state, where we do not observe any suppression of the spin susceptibility. In an accompanying structural study (in zero field) of our single crystal we detect a small deviation from the ideal Laves phase structure. These results are discussed in relation to the unusual electronic and magnetic properties of this compound.
Journal of Physics Condensed Matter 12/1998; 9(20):4185. · 2.55 Impact Factor
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ABSTRACT: The Hall resistivity has been measured in single crystal at low temperatures (0.1-4.2 K) for magnetic field -axis up to 26 T. For temperatures , the Hall coefficient goes asymptotically to zero, indicating a compensated-metal ground state. Since the Hall resistivity does not show an anomaly at the metamagnetic crossover of 20 T in the Fermi-liquid state below 0.8 K, a drastic change of the Fermi surface is unlikely at the crossover. A change of the skew scattering contribution to the Hall effect has been observed around 6 and 2 K, i.e. near respectively the magnetic ordering temperature and the temperature below which the Fermi-liquid state appears.
Journal of Physics Condensed Matter 12/1998; 11(1):221. · 2.55 Impact Factor
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ABSTRACT: The weak magnetic order of the heavy-fermion superconductor UPt3 has been investigated by elastic neutron-scattering measurements in magnetic fields up to 12 T along the a and c axes of the hexagonal crystal structure. The small antiferromagnetically ordered moment of 0.02μB/(U atom) shows only a weak dependence on the applied magnetic field and no sign of a domain repopulation for B∥a. In high magnetic fields an increase in the magnetic correlation length is observed for magnetic fields along the c axis.
Physical Review B 07/1998; 58:3186. · 3.69 Impact Factor
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ABSTRACT: We present new measurements of the thermal conductivity of UPt3 down to very low temperatures (16mK) and under magnetic fields (up to 4 T) which cover all the superconducting phases of UPt3. The measurements in zero field are compared with recent theoretical predictions for the thermal conductivity, which is dominated by impurity states at the lowest temperatures studied. The measurements under magnetic field at low temperatures are surprising since they don't show the expected low field square root dependence. The discontinuity of d kappa/dT at Tc changes drastically when passing from the high field low temperature C phase to the low field high temperature A phase : this is related to the change of the symmetry of the superconducting order parameter when crossing the A - C phase transition. Comment: 21 pages, 9 Figures, Latex, to be published in Journal of Low Temperature Physics
02/1997;
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ABSTRACT: We present a study of the surface critical field Hc3(Φ,Θ,T) measured for two needlelike whiskers of UPt3. Dominant surface effects were observed in the angular dependence of the critical field by means of ac-resistivity measurements. These surface superconductivity effects show a surprisingly nonlinear thermal variation of Hc3 contrary to behavior expected from conventional theory, where Hc3/Hc2 = 1.69 is predicted. The ratio Hc3/Hc2 is strongly depressed from its initial value 1.7 when going from the A to the C phase as the temperature is decreased. It seems to remain constant in the C phase for even lower T. Nevertheless, for temperatures close to Tc+ it is possible to describe the angular behavior of Hc3(Θ,Φ) with a standard model by introducing an effective-mass anisotropy of the heavy quasiparticles. These results are compared to recent Hc3 calculations for different representations of the order parameter and seem to provide a direct evidence for the suppression of one component of the order parameter at the surface. The restrictions imposed by these measurements on the choice of the representations of the unconventional order parameter will be discussed by also taking into account the limitations imposed due to the temperature dependence of the basal plane Hc2 modulation. © 1996 The American Physical Society.
Phys. Rev. B. 10/1996; 54(18).
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ABSTRACT: We present muon spin relaxation measurements performed on crystals of the heavy fermion superconductor UPt$_3$. In zero applied field, contrary to a previous report, we do not observe an increase of the internal magnetic field in the lower superconducting phase (the B phase). Our result gives an experimental upper bound of the magnetic field that could be associated with the superconducting state. Comment: 4 pages, REVTeX 3.0, 2 PostScript figures
11/1995;
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J. P. Brison,
N. Keller,
P. Lejay,
J. L. Tholence, A. Huxley,
N. Bernhoeft,
A. I. Buzdin,
B. Fåk,
J. Flouquet,
L. Schmidt,
A. Stepanov,
R. A. Fisher,
N. Phillips,
C. Vettier
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ABSTRACT: We discuss some consequences of the interplay between magnetism and superconductivity in the two heavy fermion systems URu2Si2 and UPt3, notably on the temperature dependence of the specific heat, on possible observation of Larkin-OvchinnikovFulde-Ferrel phase, and on the anisotropy of the upper critical field. We demonstrate that in UPt3, a clear double steep superconducting transition can be obtained reversibly.
Journal of Low Temperature Physics 01/1994; 95(1):145-152. · 1.19 Impact Factor
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ABSTRACT: Two examples of heavy-fermion systems are presented : CePd2Si2, an antiferromagnet with a quantum critical point at PC=28 kbar and UGe2 an itinerant ferromagnet which transits in a paramagnetic phase above PC=16 kbar. In CePd2Si2 the superconductivity domain is centered on PC. Special attention was given to the superconducting and magnetic anomalies at their superconducting and Néel temperatures. In UGe2 superconductivity appears in 9 kbar at a temperature TS, more than two orders of magnitude lower than the Curie temperature; furthermore, it occurs only on the magnetic border (P<PC). Another characteristic temperature TX is detected by resistivity; the zigzag uranium chain of the lattice may favor a supplementary nesting in the majority spin band.
Journal of Magnetism and Magnetic Materials.
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ABSTRACT: We have discovered a low-field metamagnetic transition in UGe2 close to the critical pressure at which the Curie temperature is suppressed to zero. The systematic evolution of the transition with pressure provides a unique opportunity to test theoretical models of metamagnetism.
Physica B Condensed Matter · 1.06 Impact Factor
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ABSTRACT: In UGe2, ZrZn2 and URhGe the co-existence of superconductivity and ferromagnetism appears to arise as a co-operative phenomena rather than as the overlap of two mutually competing orders. In all three compounds the magnetism is in intimate contact with the electronic degrees of freedom while the Curie temperatures are more than an order of magnitude higher than their critical temperatures for superconductivity. The most direct indication that the two orders are conjugate however is that the superconductivity and ferromagnetism are suppressed at the same critical pressure in both UGe2 and ZrZn2. This has motivated the recent theoretical classification of the permitted superconducting order parameter symmetries for such states. In the following we will review the experimental evidence relevant to the identification of the actual symmetries. This is followed by a discussion of the possible states allowed theoretically. Finally we discuss briefly whether the magnetic order can indeed lead to an enhancement of the superconductivity.
Physica C: Superconductivity.
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ABSTRACT: The heavy-fermion superconductor UPt3 is one of the best-studied examples of systems that show unconventional superconductivity. Below a transition temperature of Tc=0.55 K a complex phase diagram with three different superconducting phases is observed as a function of temperature, magnetic field, and pressure. The weak antiferromagnetic order (TN=6 K) with an ordered moment of only 0.02μB/U atom is believed to play a crucial role in the existence of multiple superconducting phases, as it can act as a symmetry-breaking field for the vector order parameter of the unconventional superconductivity. We review recent progress in neutron scattering measurements on the weak antiferromagnetic order as a function of magnetic field, pressure, and Pd doping and on the superconducting flux-line lattice. The relation between the magnetic and superconducting properties is discussed.
Physica B: Condensed Matter.
