G. Lapertot

Publications (65)108.14 Total impact

• Article: Magnetic Polarization and Fermi Surface Instability: Case of YbRh2Si2

  A. Pourret, G. Knebel, G. Lapertot, T. D. Matsuda, J. Flouquet
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  ABSTRACT: We report thermoelectric and resitivity measurements of antiferromagnetic heavy fermion compound YRh2Si2 at low temperatures down and under high magnetic field. At low temperature, the thermoelectric power and the resistivity present several distinct anomalies as a function of field around H_0 ~ 9.5 T when the magnetic polarization reaches a critical value. The anomalies are accompanied with a change of sign from negative at low magnetic field to positive at high field (H>H_0) and are resulting from a Lifshitz-type topological transition of the Fermi surface. A logarithmic divergence of S/T at T \to 0 K just above H_0 (H=11.5 T) is quite comparable to the well known divergence of S/T in the temperature range above the antiferromagnetic order at H=0 T referred to as non Fermi liquid behavior. The transition will be compared to the well characterized Fermi surface change in CeRu2Si2 at its pseudo-metamagnetic transition.
  02/2013;
• Article: Evidence for Three Fluctuation Channels in the Spin Resonance of the Unconventional Superconductor CeCoIn_{5}

  S Raymond, K Kaneko, A Hiess, P Steffens, G Lapertot
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  ABSTRACT: Polarized inelastic neutron scattering under a magnetic field is used to get a microscopic insight into the spin resonance of the heavy fermion superconductor CeCoIn_{5}. The resonance line shape is found to depend on the neutron polarization: Some of the spectral weight is common to the two polarization channels while the remaining part is distributed equally between them. This is evidence for the spin resonance being a degenerate mode with three fluctuation channels: A Zeeman split contribution and an additional longitudinal mode.
  Physical Review Letters 12/2012; 109(23):237210. · 7.37 Impact Factor
• Article: Thermoelectric Response Near a Quantum Critical Point of β-YbAlB_{4} and YbRh_{2}Si_{2}: A Comparative Study.

  Y Machida, K Tomokuni, C Ogura, K Izawa, K Kuga, S Nakatsuji, G Lapertot, G Knebel, J-P Brison, J Flouquet
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  ABSTRACT: The thermoelectric coefficients have been measured down to a very low temperature for the Yb-based heavy-fermion compounds β-YbAlB_{4} and YbRh_{2}Si_{2}, often considered as model systems for the local quantum criticality case. We observe a striking difference in the behavior of the Seebeck coefficient S in the vicinity of their respective quantum critical point (QCP). Approaching the critical field, S/T is enhanced in β-YbAlB_{4}, but drastically reduced in YbRh_{2}Si_{2}. The ratio of thermopower to specific heat remains constant for β-YbAlB_{4}, but it is significantly reduced near the QCP in YbRh_{2}Si_{2}. In both systems, on the other hand, the Nernst coefficient shows a diverging behavior near the QCP. The interplay between valence and magnetic quantum criticality and the additional possibility of a Lifshitz transition crossing the critical field under magnetic field are discussed as the origin of the different behaviors of these compounds.
  Physical Review Letters 10/2012; 109(15):156405. · 7.37 Impact Factor
• Article: Verification of the Wiedemann-Franz law in YbRh2Si2 at a quantum critical point

  Y. Machida, K. Tomokuni, K. Izawa, G. Lapertot, G. Knebel, J. -P. Brison, J. Flouquet
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  ABSTRACT: The thermal conductivity measurements have been performed on the heavy-fermion compound YbRh2Si2 down to 0.04 K and under magnetic fields through a quantum critical point (QCP) at Bc = 0.66 T || c-axis. In the limit as T -> 0, we find that the Wiedemann-Franz law is satisfied within experimental error at the QCP despite the destruction of the standard signature of Fermi liquid. Our results place strong constraints on models that attempt to describe the nature of unconventional quantum criticality of YbRh2Si2.
  10/2012;
• Article: (p,T,H) phase diagram of Heavy Fermion systems : Some systematics and some surprises from ytterbium

  D. Braithwaite, A. Fernandez-Pañella, E. Colombier, B. Salce, G. Knebel, G. Lapertot, V. Balédent, J. -P. Rueff, L. Paolasini, R. Verbeni, J. Flouquet
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  ABSTRACT: Pressure is the cleanest way to tune heavy fermion systems to a quantum phase transition in order to study the rich physics and competing phases, and the comparison between ytterbium and cerium systems is particularly fruitful. We briefly review the mechanisms in play and show some examples of expected and unexpected behaviour. We emphasize the importance of the valence changes under pressure and show how modern synchrotron techniques can accurately determine this, including at low temperature.
  06/2012;
• Article: Valence Instability of YbCu$_2$Si$_2$ through its quantum critical point

  A. Fernandez-Panella, V. Balédent, D. Braithwaite, Luigi Paolasini, Roberto Verbeni, G. Lapertot, Jean-Pascal Rueff
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  ABSTRACT: We report Resonant inelastic x-ray scattering measurements (RIXS) in YbCu$_2$Si$_2$ at the Yb L$_{3}$ edge under high pressure (up to 22 GPa) and at low temperatures (down to 7 K) with emphasis on the vicinity of the transition to a magnetic ordered state. We find a continuous valence change towards the trivalent state with increasing pressure but with a pronounced change of slope close to the critical pressure. Even at 22 GPa the Yb$^{+3}$ state is not fully achieved. The pressure where this feature is observed decreases as the temperature is reduced to 9 GPa at 7K, a value close to the critical pressure (\itshape{p\normalfont{$_c$}}\normalfont $\approx$ 7.5 GPa) where magnetic order occurs. The decrease in the valence with decreasing temperature previously reported at ambient pressure is confirmed and is found to be enhanced at higher pressures. We also compare the f electron occupancy between YbCu$_2$Si$_2$ and its Ce-counterpart, CeCu$_2$Si$_2$.
  03/2012;
• Source

  Available from: Jacques Flouquet

  Article: Thermoelectric response near a quantum critical point of beta-YbAlB4 and YbRh2Si2: A comparative study

  Y. Machida, K. Tomokuni, C. Ogura, K. Izawa, K. Kuga, S. Nakatsuji, G. Lapertot, G. Knebel, J. -P. Brison, J. Flouquet
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  ABSTRACT: The thermoelectric coefficients have been measured on the Yb-based heavy fermion compounds beta-YbAlB4 and YbRh2Si2 down to a very low temperature. We observe a striking difference in the behavior of the Seebeck coefficient, S in the vicinity of the Quantum Critical Point (QCP) in the two systems. As the critical field is approached, S/T enhances in beta-YbAlB4 but is drastically reduced in YbRh2Si2. While in the former system, the ratio of thermopower-to-specific heat remains constant, it drastically drops near the QCP in YbRh2Si2. In both systems, on the other hand, the Nernst coefficient shows a diverging behavior near the QCP. The results provide a new window to the way various energy scales of the system behave and eventually vanish near a QCP.
  02/2012;
• Source

  Available from: Jacques Flouquet

  Article: Ferromagnetism in YbCu2Si2 at high pressure

  A. Fernandez-Pañella, D. Braithwaite, B. Salce, G. Lapertot, J. Flouquet
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  ABSTRACT: We demonstrate from detailed ac-susceptibility and calorimetry studies under hydrostatic pressure that YbCu2Si2 probably orders ferromagnetically at high pressure. The (p,H,T) phase diagram, shows that the transition temperature increases with pressure, but also with an applied magnetic field. We suggest that many ytterbium systems may show a trend towards ferromagnetism and we discuss the possible reasons for this. We also examine the implications, including the potential of YbCu2Si2 and other Yb compounds for further studies of the rich physical properties that may occur near a ferromagnetic critical point.
  10/2011;
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  Available from: Jacques Flouquet

  Article: Evolution of the spin resonance of CeCoIn$_{5}$ as a function of magnetic field and La substitution

  S. Raymond, J. Panarin, G. Lapertot, J. Flouquet
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  ABSTRACT: We report the evolution of the spin resonance in CeCoIn$_{5}$ as a function of magnetic field and lanthanum substitution. In both cases, the resonance peak position shifts to lower energy and the lineshape broadens. For La doping, it is found that the ratio $\Omega_{res}/k_{B}T_{c}$ is almost constant as a function of $x$. Under magnetic field the decrease of the excitation energy is similar for H// [1,$\bar{1}$,0] and [1,1,1] and faster than the decrease of $T_{c}(H)$. The Zeeman effect found for the field applied along [1,$\bar{1}$,0] corresponds to the ground state magnetic moment.
  06/2011;
• Source

  Available from: Jacques Flouquet

  Article: Effects of La non-magnetic impurities on the spin resonance of CeCoIn$_{5}$

  J. Panarin, S. Raymond, G. Lapertot, J-M Mignot, J. Flouquet
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  ABSTRACT: The influence of La non magnetic impurities on the spin dynamics of CeCoIn$_{5}$ was studied by inelastic neutron scattering. In La-substituted systems, the spin resonance peak (observed at $\Omega_{res}=0.55 meV$ in the pure system) is shifted to lower energies but the ratio $\Omega_{res}/k_{B}T_{c}$ remains unchanged. The excitation broadens till it reaches 0.3 meV equal to the value of the quasi-elastic signal in the normal state. The evolution of La substitution is compared with the evolution of the magnetic resonance in Ni and Zn substituted YBa$_{2}$Cu$_{3}$O$_{7}$.
  01/2011;
• Article: Thermodynamic phase diagram of Fe (Se_ {0.5} Te_ {0.5}) single crystals in fields up to 28 tesla

  T. Klein, D. Braithwaite, A. Demuer, W. Knafo, G. Lapertot, C. Marcenat, P. Rodière, I. Sheikin, P. Strobel, A. Sulpice, P. Toulemonde
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  ABSTRACT: We report on specific heat (Cp), transport, Hall probe, and penetration depth measurements performed on Fe(Se0.5Te0.5) single crystals (Tc∼14 K). The thermodynamic upper critical field Hc2 lines has been deduced from Cp measurements up to 28 T for both H∥c and H∥ab, and compared to the lines deduced from transport measurements (up to 55 T in pulsed magnetic fields). We show that this thermodynamic Hc2 line presents a very strong downward curvature for T→Tc which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirms that Hc2 is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to T/Tc∼0.99 (for H∥ab) which is the consequence of a very small value of the coherence length ξc(0)∼4 Å [and ξab(0)∼15 Å], confirming the strong renormalization of the effective mass (as compared to DMFT calculations) previously observed in ARPES measurements [ A. Tamai, A. Y. Ganin, E. Rozbicki, J. Bacsa, W. Meevasana, P. D. C. King, M. Caffio, R. Schaub, S. Margadonna, K. Prassides, M. J. Rosseinsky and F. Baumberger Phys. Rev. Lett. 104 097002 (2010)]. Hc1 measurements lead to λab(0)=430±50 nm and λc(0)=1600±200 nm and the corresponding anisotropy is approximatively temperature independent (∼4), being close to the anisotropy of Hc2 for T→Tc. The temperature dependence of both λ(∝T2) and the electronic contribution to the specific heat confirm the nonconventional coupling mechanism in this system.
  Phys. Rev. B. 11/2010; 82(18).
• Source

  Available from: ArXiv

  Article: Thermodynamic phase diagram of Fe(Se$_{0.5}$Te$_{0.5}$) single crystals up to 28 Tesla

  T. Klein, D. Braithwaite, A. Demuer, W. Knafo, G. Lapertot, C. Marcenat, P. Rodieere, I. Sheikin, P. Strobel, A. Sulpice, P. Toulemonde
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  ABSTRACT: We report on specific heat ($C_p$), transport, Hall probe and penetration depth measurements performed on Fe(Se$_{0.5}$Te$_{0.5}$) single crystals ($T_c \sim 14$ K). The thermodynamic upper critical field $H_{c2}$ lines has been deduced from $C_p$ measurements up to 28 T for both $H\|c$ and $H\|ab$, and compared to the lines deduced from transport measurements (up to 55 T in pulsed magnetic fields). We show that this {\it thermodynamic} $H_{c2}$ line presents a very strong downward curvature for $T \rightarrow T_c$ which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirm that $H_{c2}$ is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to $T/T_c \sim 0.99$ (for $H\|ab$) which is the consequence of a very small value of the coherence length $\xi_c(0) \sim 4 \AA$ (and $\xi_{ab}(0) \sim 15 \AA$), confirming the strong renormalisation of the effective mass (as compared to DMFT calculations) previously observed in ARPES measurements [Phys. Rev. Lett. 104, 097002 (2010)]. $H_{c1}$ measurements lead to $\lambda_{ab}(0) = 430 \pm 50$ nm and $\lambda_c(0) = 1600 \pm 200$ nm and the corresponding anisotropy is approximatively temperature independent ($\sim 4$), being close to the anisotropy of $H_{c2}$ for $T\rightarrow T_c$. The temperature dependence of both $\lambda$ ($\propto T^2$) and the electronic contribution to the specific heat confirm the non conventional coupling mechanism in this system. Comment: 10 pages, 5 figures
  10/2010;
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  Available from: ArXiv

  Article: Field evolution of coexisting superconducting and magnetic orders in CeCoIn5.

  G Koutroulakis, M D Stewart, V F Mitrović, M Horvatić, C Berthier, G Lapertot, J Flouquet
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  ABSTRACT: We present nuclear magnetic resonance (NMR) measurements on the three distinct In sites of CeCoIn5 with a magnetic field applied in the [100] direction. We identify the microscopic nature of the long range magnetic order (LRO) stabilized at low temperatures in fields above 10.2 T while still in the superconducting (SC) state. We infer that the ordered moment is oriented along the c axis and map its field evolution. The study of the field dependence of the NMR shift for the different In sites indicates that the LRO likely coexists with a modulated SC phase, possibly that predicted by Fulde, Ferrell, Larkin, and Ovchinnikov. Furthermore, we discern a field region dominated by strong spin fluctuations where static LRO is absent and propose a revised phase diagram.
  Physical Review Letters 02/2010; 104(8):087001. · 7.37 Impact Factor
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  Available from: Jacques Flouquet

  Article: Observation of the J-sheet of the Fermi surface of YbRh2Si2

  A. B. Sutton, P. M. C. Rourke, V. Taufour, A. McCollam, G. Lapertot, G. Knebel, J. Flouquet, S. R. Julian
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  ABSTRACT: We report a previously unobserved quantum oscillation frequency in YbRh2Si2. This is the first quantum oscillation that can definitely be assigned to the larger of the two major sheets of the Fermi surface predicted by band structure calculations. Previously observed frequencies were interpreted in terms of a large Fermi surface, which includes the Yb 4f-hole in the Fermi volume, that has been so strongly spin-split that it closely resembles the small Fermi surface, which does not include the Yb 4f -hole in the Fermi volume. The new frequency can also be incorporated into this picture, however there are some indications that the a need for more advanced energy band calculations. Comment: 5 pages, 3 figures, submitted to Physica Status Solidi on 24 September 2009, fixed lingering issues with figures
  10/2009;
• Source

  Available from: Jacques Flouquet

  Article: Evolution of the spin resonance in CeCoIn$_5$ under magnetic field

  J. Panarin, S. Raymond, G. Lapertot, J. Flouquet
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  ABSTRACT: The evolution of the magnetic excitation spectrum of the unconventional superconductor CeCoIn$_{5}$ was studied by inelastic neutron scattering as a function of magnetic field applied in the [1, -1, 0] direction. The spin resonance characteristic of the superconducting states was observed up to about $H_{c2}$/2 : it is still centered at (1/2, 1/2, 1/2) when the magnetic field increases while its characteristic energy decreases and its lineshape substantially broadens. Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Japan
  08/2009;
• Article: High-pressure transport and microcalorimetry studies on high quality YbCu_ {2} Si_ {2} single crystals

  E. Colombier, D. Braithwaite, G. Lapertot, B. Salce, G. Knebel
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  ABSTRACT: We report electrical transport and calorimetry studies on high quality single crystals of YbCu2Si2 under pressure in diamond and Bridgman anvil pressure cells with liquid pressure-transmitting media. At ambient pressure, in the best samples we find residual resistivities of less than 0.5 μΩ cm. We have also determined the anisotropy of the temperature dependence of the resistivity and shown that this can account for sample-dependent differences of the resistivity. We confirm the previously reported transition to a magnetically ordered ground state with pressure and we have precisely determined the (p,T) phase diagram using pure samples, good hydrostatic pressure conditions, and calorimetry measurements. Above 8.8 GPa we see a clear signature of the transition in the specific heat. We discuss the low-temperature resistivity and specific-heat behaviors in respect to the universal Kadowaki Woods ratio, to other ytterbium-based strongly correlated systems, and to the cerium-based counterpart, CeCu2Si2.
  Phys. Rev. B. 06/2009; 79(24).
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  Available from: arxiv.org

  Article: Superconducting and normal phases of FeSe single crystals at high pressure.

  D Braithwaite, B Salce, G Lapertot, F Bourdarot, C Marin, D Aoki, M Hanfland
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  ABSTRACT: We report on the synthesis of superconducting single crystals of FeSe and their characterization by x-ray diffraction, magnetization and resistivity. We have performed ac susceptibility measurements under high pressure in a hydrostatic liquid argon medium up to 14 GPa and we find that T(C) increases up to 33-36 K in all samples, but with slightly different pressure dependences on different samples. Above 12 GPa no traces of superconductivity are found in any sample. We have also performed a room temperature high pressure x-ray diffraction study up to 12 GPa on a powder sample, and we find that, between 8.5 and 12 GPa, the tetragonal PbO structure undergoes a structural transition to a hexagonal structure. This transition results in a volume decrease of about 16% and is accompanied by the appearance of an intermediate, probably orthorhombic, phase.
  Journal of Physics Condensed Matter 06/2009; 21(23):232202. · 2.55 Impact Factor
• Source

  Available from: Zhiwei Hu

  Article: Crystal-field ground state of the non-centrosymmetric superconductor CePt3Si: a combined polarized soft X-ray absorption and polarized neutron study

  T. Willers, B. Fak, N. Hollmann, P. O. Koerner, Z Hu, A Tanaka, D. Schmitz, M. Enderle, G. Lapertot, L. H. Tjeng, A Severing
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  ABSTRACT: We determined the crystal-field split Hund's rule ground state of the non-centrosymmetric heavy fermion superconductor CePt3Si with polarization dependent soft X-ray absorption spectroscopy (XAS) and polarized neutron scattering. We are also able to give the sequence of the crystal-field states from the temperature evolution of the linear dichroic signal in the XAS. The quantitative analysis of the XAS temperature dependence together with the neutron transition energies complete the identification of the crystal-field level scheme. Comment: 6 pages, 5 figures, submitted to PRB
  05/2009;
• Article: Multigap superconductivity in heavy fermion systems

  G Seyfarth, J-P Brison, M-A Méasson, J Flouquet, G Lapertot, D Aoki
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  ABSTRACT: Heavy fermion superconductors are largely studied for their unconventional ground states. We present results on the skutterudite compound PrOs4Sb12 (Tc ≈ 1.8K), and on the heavy-fermion CeCoIn5 (Tc ≈ 2.3K), where both the low field and the low temperature behaviour of their thermal conductivity yield evidence for multigap superconductivity.
  Journal of Physics Conference Series 03/2009; 150(5):052229.
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  Available from: Jacques Flouquet

  Article: Magnetic-field dependence of the YbRh2Si2 Fermi surface.

  P M C Rourke, A McCollam, G Lapertot, G Knebel, J Flouquet, S R Julian
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  ABSTRACT: Magnetic-field-induced changes of the Fermi surface play a central role in theories of the exotic quantum criticality of YbRh2Si2. We have carried out de Haas-van Alphen measurements in the magnetic-field range 8 T < or = H < or = 16 T, and directly observe field dependence of the extremal Fermi surface areas. Our data support the theory that a low-field "large" Fermi surface, including the Yb 4f quasihole, is increasingly spin split until a majority-spin branch undergoes a Lifshitz transition and disappears at H0 approximately 10 T, without requiring 4f localization at H0.
  Physical Review Letters 12/2008; 101(23):237205. · 7.37 Impact Factor