Angle-resolved photoemission spectroscopy observation of anomalous electronic states in EuFe2As(2-x)P(x).

We used angle-resolved photoemission spectroscopy to investigate the electronic structure and the Fermi surface of EuFe2As2, EuFe2As1.4P0.6 and EuFe2P2. We observed doubled core level peaks associated with the pnictide atoms. Using K atoms evaporated at the surface to affect the surface quality, we show that one component of these doubled peaks is related to a surface state. Nevertheless, strong electronic dispersion along the c-axis, especially pronounced in EuFe2P2, is observed for at least one band, thus indicating that the Fe states, albeit probably affected at the surface, do not form pure two-dimensional surface states. We determine the evolution of the Fermi surface as a function of the P content and reveal that the hole Fermi surface pockets enlarge with increasing P content. We also show that the spectral weight near the Fermi level of EuFe2P2 is reduced as compared to that of EuFe2As2 and EuFe2As1.4P0.6. Finally, we identify the electronic states associated with the Eu(2+) f states and show an anomalous jump in EuFe2P2.

Crystal growth, structure, and physical properties of Ln(Cu,Al)12 (Ln = Y, Ce, Pr, Sm, and Yb) and Ln(Cu, Ga)12 (Ln = Y, Gd-Er, and Yb).

Single crystals of Ln(Cu,Al)12 and Ln(Cu,Ga)12 compounds (Ln = Y, Ce-Nd, Sm, Gd-Ho, and Yb for Al and Ln = Y, Gd-Er, Yb for Ga) have been grown by flux-growth methods and characterized by means of single-crystal x-ray diffraction, complemented with microprobe analysis, magnetic susceptibility, resistivity and heat capacity measurements. Ln(Cu,Ga)12 and Ln(Cu,Al)12 of the ThMn12 structure type crystallize in the tetragonal I4/mmm space group with lattice parameters a approximately 8.59 Šand c approximately 5.15 Šand a approximately 8.75 Šand c approximately 5.13 Šfor Ga and Al containing compounds, respectively. For aluminium containing compounds, magnetic susceptibility data show Curie-Weiss paramagnetism in the Ce and Pr analogues down to 50 K with no magnetic ordering down to 3 K, whereas the Yb analogue shows a temperature-independent Pauli paramagnetism. Sm(Cu,Al)12 orders antiferromagnetically at T(N)approximately 5 K and interestingly exhibits Curie-Weiss behaviour down to 10 K with no Van Vleck contribution to the susceptibility. Specific heat data show that Ce(Cu,Al)12 is a heavy fermion antiferromagnet with T(N) approximately 2 K and with an electronic specific heat coefficient γ0 as large as 390 mJ K2 mol(-1). In addition, this is the first report of Pr(Cu,Al)12 and Sm(Cu,Al)12 showing an enhanced mass (approximately 80 and 120 mJ K(2) mol(-1)). For Ga containing analogues, magnetic susceptibility data also show the expected Curie-Weiss behaviour from Gd to Er, with the Yb analogue being once again a Pauli paramagnet. The antiferromagnetic transition temperatures range over 12.5, 13.5, 6.7, and 3.4 K for Gd, Tb, Dy, and Er. Metallic behaviour is observed down to 3 K for all Ga and Al analogues. A large positive magnetoresistance up to 150% at 9 T is also observed for Dy(Cu,Ga)12. The structure, magnetic, and transport properties of these compounds will be discussed.

Crystal growth, structure, and physical properties of Ln(Ag, Al, Si)2 (Ln = Ce and Gd).

Single crystals of CeM2 and GdM2 (M = Ag, Al, and Si) were grown by the flux growth technique and characterized by means of single crystal x-ray diffraction, magnetic susceptibility, resistivity, and heat capacity measurements. CeM2 and GdM2 crystallize in the tetragonal I4(1)/amd space group with the α-ThSi2 structure type with lattice parameters a ~4.2 Å and c ~14.4 Å. Curie-Weiss behavior is observed for both analogues with CeM2 ordering first ferromagnetically at 11 K with a second antiferromagnetic transition at 8.8 K while GdM2 orders antiferromagnetically at 24 K. Heat capacity measurements on CeM2 show two magnetic transitions at 10.8 and 8.8 K with an electronic specific heat coefficient, γ(0), of ~53 mJ K(-2) mol(-1). The entropy at the magnetic transition is less than the expected Rln2 for CeM2, reinforcing the assertions of an enhanced mass state and Kondo behavior being observed in the resistivity.

Role of f electrons in the Fermi surface of the heavy fermion superconductor beta-YbAlB4.

We present a detailed quantum oscillation study of the Fermi surface of the recently discovered Yb-based heavy fermion superconductor beta-YbAlB4. We compare the data, obtained at fields from 10 to 45 T, to band structure calculations performed using the local density approximation. Analysis of the data suggests that f holes participate in the Fermi surface up to the highest magnetic fields studied. We comment on the significance of these findings for the unconventional superconducting properties of this material.

Field-induced Fermi surface reconstruction and adiabatic continuity between antiferromagnetism and the hidden-order state in URu2Si2.

Shubnikov-de Haas oscillations reveal at high fields an abrupt reconstruction of the Fermi surface within the hidden-order (HO) phase of URu2Si2. Taken together with reported Hall effect results, this implies an increase in the effective carrier density and suggests that the field suppression of the HO state is ultimately related to destabilizing a gap in the spectrum of itinerant quasiparticles. While hydrostatic pressure favors antiferromagnetism in detriment to the HO state, it has a modest effect on the complex H-T phase diagram. Instead of phase separation between HO and antiferromagnetism our observations indicate adiabatic continuity between both orderings with field and pressure changing their relative weight.

Probing the quantum critical behavior of CeCoIn5 via Hall effect measurements.

We present highly sensitive Hall effect measurements of the heavy fermion compound CeCoIn5 down to temperatures of 55 mK. A pronounced dip in the differential Hall coefficient | partial differential rho(xy)/ partial differential H| at low temperature and above the upper critical field of superconductivity, H(c2), is attributed to critical spin fluctuations associated with the departure from Landau Fermi liquid behavior. This identification is strongly supported by a systematic suppression of this feature at elevated pressures. The resulting crossover line in the field-temperature phase diagram favors a field induced quantum critical point at mu(0)H(qc) approximately 4.1 T below H(c2)(T=0) suggesting related, yet separate, critical fields.

Superconductivity in CeCoIn5-xSnx: veil over an ordered state or novel quantum critical point?

Measurements of specific heat and electrical resistivity in magnetic fields up to 9 T along [001] and temperatures down to 50 mK of Sn-substituted CeCoIn5 are reported. The maximal -ln(T) divergence of the specific heat at the upper critical field Hc2 down to the lowest temperature characteristic of non-Fermi-liquid systems at the quantum critical point (QCP), the universal scaling of the Sommerfeld coefficient, and agreement of the data with spin-fluctuation theory provide strong evidence for quantum criticality at Hc2 for all x< or =0.12 in CeCoIn5-xSnx. These results indicate the "accidental" coincidence of the QCP located near Hc2 in pure CeCoIn5, in actuality, constitute a novel quantum critical point associated with unconventional superconductivity.

Possible Fulde-Ferrell-Larkin-Ovchinnikov superconducting state in CeCoIn5.

We report specific heat measurements of the heavy fermion superconductor CeCoIn5 in the vicinity of the superconducting critical field H(c2), with magnetic fields in the [110], [100], and [001] directions, and at temperatures down to 50 mK. The superconducting phase transition changes from second to first order for fields above 10 T for H parallel [110] and H parallel [100]. In the same range of magnetic fields, we observe a second specific heat anomaly within the superconducting state. We interpret this anomaly as a signature of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting state. We obtain similar results for H parallel [001], with the FFLO state occupying a smaller part of the phase diagram.

Entropy of vortex cores near the superconductor-insulator transition in an underdoped cuprate.

We present a study of Nernst effect in underdoped La(2-x)Sr(x)CuO4 in magnetic fields as high as 28 T. At high fields, a sizable Nernst signal was found to persist in the presence of a field-induced nonmetallic resistivity. By simultaneously measuring resistivity and the Nernst coefficient, we extract the entropy of vortex cores in the vicinity of this field-induced superconductor-insulator transition. Moreover, the temperature dependence of the thermoelectric Hall angle provides strong constraints on the possible origins of the finite Nernst signal above T(c), as recently discovered by Xu et al. [Nature (London) 406, 486 (2000)].