Dai Aoki

Publications (162) View all

• Article: Metamagnetic Transition in UCoAl Probed by Thermoelectricity Measurements

  A. Palacio-Morales, A. Pourret, G. Knebel, T. Combier, D. Aoki, H. Harima, J. Flouquet
  [show abstract] [hide abstract]
  ABSTRACT: We report field and temperature dependent measurements of the thermoelectric power (TEP) and the Nernst effect in the itinerant metamagnet UCoAl. The magnetic field is applied along the easy magnetization c-axis in the hexagonal crystal structure. The metamagnetic transition from the paramagnetic phase at zero field to the field induced ferromagnetic (FM) state is of first order at low temperatures and becomes a broad crossover above the critical temperature $T^{\star}_{M} \sim 11$ K. The field-dependence of the TEP reveals that the effective mass of the hole carriers changes significantly at the metamagnetic transition. The TEP experiment reflects the existence of different carrier types in good agreement with band structure calculations and previous Hall effect experiments. According to the temperature dependence of the TEP, no Fermi liquid behavior appears in the paramagnetic state down to 150 mK, but is achieved only in the field induced ferromagnetic state.
  02/2013;
• Article: Evidence of Strong Correlations and Coherence-Incoherence Crossover in the Iron Pnictide Superconductor KFe2As2

  F. Hardy, A. E. Boehmer, D. Aoki, P. Burger, T. Wolf, P. Schweiss, R. Heid, P. Adelmann, Y. X. Yao, G. Kotliar, J. Schmalian, C. Meingast
  [show abstract] [hide abstract]
  ABSTRACT: Using resistivity, heat-capacity, thermal-expansion, and susceptibility measurements we study the normal-state behavior of KFe2As2. We find that both the Sommerfeld coefficient gamma = 103 mJ mol-1 K-2 and the Pauli susceptibility chi = 4x10-4 are strongly enhanced, which confirm the existence of heavy quasiparticles inferred from previous de Haas-van Alphen and ARPES experiments. We discuss this large enhancement using a Gutzwiller slave-boson mean-field calculation, which reveals the proximity of KFe2As2 to an orbital-selective Mott transition. The temperature dependence of the magnetic susceptibility and the thermal expansion provide strong experimental evidence for the existence of a coherence-incoherence crossover, similar to what is found in heavy fermion and ruthenate compounds, due to Hund's coupling between orbitals.
  02/2013;
• Article: Observation of the Meissner-Ochsenfeld Effect and the Absence of the Meissner State in UCoGe.

  C Paulsen, D J Hykel, K Hasselbach, D Aoki
  [show abstract] [hide abstract]
  ABSTRACT: We present low field magnetization and susceptibility measurements made on a single crystal of the ferromagnetic superconductor UCoGe. The interplay between ferromagnetism and superconductivity comes into view in the study of hysteresis along the c axis (easy magnetization axis). The Meissner state (perfect diamagnetism) could not be observed in very low magnetic fields for all three crystallographic directions, implying that the sample is always in the mixed state. Notwithstanding, the Meissner-Ochsenfeld effect (reversible flux expulsion) occurs and is found to be anisotropic. For the c axis in low fields, it is proportional to the bulk magnetization M (and thus to the population of domains) and not to the applied magnetic field H. On a microscopic level, our interpretation of these results implies that flux is expelled independently from each domain proportional to its volume.
  Physical Review Letters 12/2012; 109(23):237001. · 7.37 Impact Factor
• Article: High-field moment polarization in the ferromagnetic superconductor UCoGe

  W. Knafo, T. D. Matsuda, D. Aoki, F. Hardy, G. W. Scheerer, G. Ballon, M. Nardone, A. Zitouni, C. Meingast, J. Flouquet
  [show abstract] [hide abstract]
  ABSTRACT: We report magnetization and magnetoresistivity measurements on the isostructural ferromagnetic superconductors UCoGe and URhGe in magnetic fields up to 60 T and temperatures from 1.5 to 80 K. At low-temperature, a moment polarization in UCoGe in a field $\mu_0\mathbf{H}\parallel\mathbf{b}$ of around 50 T leads to well-defined anomalies in both magnetization and magnetoresistivity. These anomalies vanish in temperatures higher than 30-40 K, where maxima in the magnetic susceptibility and the field-induced variation of the magnetoresistivity are found. A comparison is made between UCoGe and URhGe, where a moment reorientation in a magnetic field $\mu_0\mathbf{H}\parallel\mathbf{b}$ of 12 T leads to field-induced reentrant superconductivity.
  11/2012;
• Article: Temperature dependent tunneling spectroscopy in the heavy fermion CeRu(2)Si(2) and in the antiferromagnet CeRh(2)Si(2).

  A Maldonado, H Suderow, S Vieira, D Aoki, J Flouquet
  [show abstract] [hide abstract]
  ABSTRACT: CeRu(2)Si(2) and CeRh(2)Si(2) are two similar heavy fermion stoichiometric compounds located on the two sides of a magnetic quantum critical phase transition. CeRh(2)Si(2) is an antiferromagnet below T(N) = 36 K with moderate electronic masses whereas CeRu(2)Si(2) is a paramagnetic metal with particularly heavy electrons. Here we present tunneling spectroscopy measurements as a function of temperature (from 0.15 to 45 K). The tunneling conductance at 0.15 K reveals V-shaped dips around the Fermi level in both compounds, which disappear in CeRu(2)Si(2) above the coherence temperature, and in CeRh(2)Si(2) above the Néel temperature. In the latter case, two different kinds of V-shaped tunneling conductance dips are found.
  Journal of Physics Condensed Matter 10/2012; 24(47):475602. · 2.55 Impact Factor