Interplay between fermi surface topology and ordering in URu2Si2 revealed through abrupt hall coefficient changes in strong magnetic fields

Temperature- and field-dependent measurements of the Hall effect of pure and 4% Rh-doped URu2Si2 reveal low density (0.03 hole/U) high mobility carriers to be unique to the "hidden order" phase and consistent with an itinerant density-wave order parameter. The Fermi surface undergoes a series of abrupt changes as the magnetic field is increased. When combined with existing de Haas-van Alphen data, the Hall data expose a strong interplay between the stability of the "hidden order," the degree of polarization of the Fermi liquid, and the Fermi surface topology.     

Field-angle-dependent specific heat measurements and gap determination of a heavy fermion superconductor URu2Si2

To identify the superconducting gap structure in URu2Si2, we perform field-angle-dependent specific heat measurements for the two principal orientations in addition to field rotations, and a theoretical analysis based on microscopic calculations. The Sommerfeld coefficient gamma(H)'s in the mixed state exhibit a distinctly different field dependence. This comes from point nodes and the substantial Pauli paramagnetic effect of URu2Si2. These two features combined give rise to a consistent picture of superconducting properties, including a possible first order transition of Hc2 at low temperatures.     

Pressure tuned ferromagnetism in CeRu(2)M(2)X (M = Al, Ga; X = B, C)

The temperature (T)-pressure (P) phase diagrams are reported for the tetragonal layered compounds CeRu(2)Al(2)B, CeRu(2)Ga(2)B, and CeRu(2)Ga(2)C, studied by magnetization, specific heat and electrical resistivity. These systems exhibit localized 4f magnetic ordering with ferromagnetic ground states at T(C)?=?12.8?K, 16.3?K, and 17.2?K, respectively. Chemical and applied pressure both increase T(C) in a similar manner. The evolution of properties with chemical and applied pressure suggests that these phase diagrams may be connected in a Doniach-like picture where CeRu(2)Al(2)B is furthest from the possible quantum phase transition and CeRu(2)Ga(2)C is the nearest.     

Fermi surface properties of CeRu2(Si1-xGex)2 in magnetic fields above the metamagnetic transitions

We report de Haas-van Alphen effect measurements on CeRu2(Si(1-x)Gex)2 to reveal electronic structure change over the broad range of chemical pressure from x=0.0 to 1.0. It is found that the Fermi surface properties change drastically across the metamagnetic crossover field (B(m)) but vary smoothly with x from those in magnetic fields above B(m) in CeRu2Si2 to those in the ferromagnetic state in CeRu2Ge2. Implications of the present results are discussed in conjunction with the magnetic phase diagram.     

Hybridization wave as the "hidden order" in URu2Si2

A phenomenological model for the "hidden order" transition in the heavy-Fermion material URu(2)Si(2) is introduced. The hidden order is identified as an incommensurate, momentum-carrying hybridization between the light hole band and the heavy electron band. This modulated hybridization appears after a Fano hybridization at higher temperatures takes place. We focus on the hybridization wave as the order parameter in URu(2)Si(2) and possibly other materials with similar band structures. The model is qualitatively consistent with numerous experimental results obtained from, e.g., neutron scattering and scanning tunneling microscopy. Specifically, we find a gaplike feature in the density of states and the appearance of features at an incommensurate vector Q(*)～0.6π/a(0). Finally, the model allows us to make various predictions which are amenable to current experiments.     

Modulated spin liquid: a new paradigm for URu2Si2

We argue that near a Kondo breakdown critical point, a spin liquid with spatial modulations can form. Unlike its uniform counterpart, we find that this occurs via a second order phase transition. The amount of entropy quenched when ordering is of the same magnitude as for an antiferromagnet. Moreover, the two states are competitive, and at low temperatures are separated by a first order phase transition. The modulated spin liquid we find breaks Z4 symmetry, as recently seen in the hidden order phase of URu2Si2. Based on this, we suggest that the modulated spin liquid is a viable candidate for this unique phase of matter.     

Evidence for a new magnetic field scale in CeCoIn5

The Nernst coefficient of displays two distinct anomalies in magnetic field. The feature detected at Hk approximately 23 T is similar to what is observed in CeRu2Si2 at Hm = 7.8 T where a metamagnetic transition occurs. In CeCoIn5, new frequencies are observed in de Haas-van Alphen oscillations when the field exceeds 23 T where the Dingle temperature decreases by about 30%. Based on the Nernst coefficient anomalies, the magnetic phase diagram of CeCoIn5 is revised.     

Competing ordered phases in URu2Si2: hydrostatic pressure and rhenium substitution

A persistent kink in the pressure dependence of the "hidden order" (HO) transition temperature of URu(2-x)RexSi2 is observed at a critical pressure P(c)=15 kbar for 0 相似文献   

f Electron contribution to the change of electronic structure in CeRu2Si2 with temperature: a Compton scattering study

High resolution Compton profiles have been measured in the single crystal of CeRu(2)Si(2) above and below the Kondo temperature to elucidate the change of the Ce-4f electron from localized to itinerant states. Two-dimensional electron occupation number densities projected on the first Brillouin zone, which are obtained after a series of analyses, clearly specify the difference between itinerant and localized states. The contribution of Ce-4f electrons to the electronic structure is discussed by contrast with a band calculation.     

Flux line lattice melting and the formation of a coherent quasiparticle Bloch state in the ultraclean URu2Si2 superconductor

We find that in the ultraclean heavy-fermion superconductor URu(2)Si(2) (T_{c0}=1.45 K) a distinct flux line lattice melting transition with outstanding characters occurs well below the mean-field upper critical fields. We show that a very small number of carriers with heavy mass in this system results in exceptionally large thermal fluctuations even at sub-Kelvin temperatures, which are witnessed by a sizable region of the flux line liquid phase. The uniqueness is further highlighted by an enhancement of the quasiparticle mean free path below the melting transition, implying a possible formation of a quasiparticle Bloch state in the periodic flux line lattice.     

Sequential spin polarization of the Fermi surface pockets in URu2Si2 and its implications for the hidden order

Using Shubnikov-de Haas oscillations measured in URu2Si2 over a broad range in a magnetic field of 11-45 T, we find a cascade of field-induced Fermi surface changes within the hidden order phase I and further signatures of oscillations within field-induced phases III and V [previously discovered by Kim et al., [Phys. Rev. Lett. 91, 256401 (2003)]. A comparison of kinetic and Zeeman energies indicates a pocket-by-pocket polarization of the Fermi surface leading up to the destruction of the hidden order phase I at ≈35 T. The anisotropy of the Zeeman energy driving the transitions in URu2Si2 points to an itinerant hidden order parameter involving quasiparticles whose spin degrees of freedom depart significantly from those of free electrons.     

Phase transition between hidden and antiferromagnetic order in URu2Si2

We have carried out thermal expansion measurements on the heavy fermion compound URu2Si2. We have discovered a new anomaly that appears only under pressure for both the tetragonal a and c axes. Plotting these temperatures on a temperature-pressure plane, we have constructed a phase diagram, in which there is a first order phase transition boundary separating a hidden and antiferromagnetic order. When the system enters the antiferromagnetic phase from the hidden order, the lattice constant shrinks along the a axis and elongates along the c axis, leading to an increase in the lattice constant ratio of c/a. We conclude that the system lies close to a bicritical point from which the first order phase transition line emanates.     

Field-dependent energy scales in URu(2)Si(2)

At low temperature, macroscopic properties of URu(2)Si(2) display a characteristic energy scale delta(0)(B) which decreases when a magnetic field is applied, and eventually vanishes at an extrapolated value of the field of about 40 T. We have performed inelastic neutron scattering measurements of the magnetic dynamics of URu(2)Si(2) in applied fields along the c axis of intensities up to 12 T. We show that delta(0)(B) is not related to gaps in the magnetic fluctuations spectra. This provides direct evidence of the fact that two distinct energy scales govern the physics of this compound.     

Magnetic ground state and electronic structure of CeRu(2)Al(10)

We present a combined theoretical and experimental study of the electronic structure for CeRu(2)Al(10) based on ab initio band structure calculations and x-ray photoemission spectroscopy (XPS) data. Our calculations were performed for the base unit cell and for the hypothetical unit cell which enables antiferromagnetic ordering. The stability of the magnetic phase was investigated within fixed spin moment calculations. When additional 4f correlations are not included in the LSDA C U approach, CeRu(2)Al(10) exhibits an unstable magnetic configuration with the difference in total energy per unit cell between the weakly magnetic state and the non-magnetic one of the order ~0.3 meV. We found that Coulomb correlations among 4f electrons, when they are included in the LSDA C U approach, stabilize the magnetic structure. In the weakly correlated system (small U) an antiferromagnetic (AFM) ground state with the lowest total energy is preferred. The situation is, however, the opposite when the 4f correlations are strong. In this case the ferromagnetic (FM) ground state is preferred. By comparing our calculations with the experimental data we conclude that the 4f correlations in CeRu(2)Al(10) are weak. We also carried out a structural relaxation of atomic positions within the Cmcm unit cell and we found that the Al atoms exhibit noticeable displacement from their positions known from x-ray diffraction (XRD) analysis.     

Nexus between quantum criticality and phase formation in U(Ru1-xRhx)2Si2

Simplification of the magnetic field-versus-temperature phase diagram and quantum criticality in URu2Si2 dilutely doped with Rh are studied by measuring the magnetization and resistivity in magnetic fields of up to 45 T. For x=4%, the hidden order is completely destroyed, leaving a single field-induced phase II. A correlation between the field dependence of this phase and that of the quantum critical point, combined with the suppression of the T2 coefficient of the resistivity within it, implicates field-tuned quantum criticality as an important factor in phase formation.     

Inflection point in the magnetic field dependence of the ordered moment of URu2Si2 observed by neutron scattering in fields up to 17 T

We have measured the magnetic field dependence of the ordered antiferromagnetic moment and the magnetic excitations in the heavy-fermion superconductor URu2Si2 for fields up to 17 T applied along the tetragonal c axis, using neutron scattering. The decrease of the magnetic intensity of the tiny moment with increasing field does not follow a simple power law, but shows a clear inflection point, indicating that the moment disappears first at the metamagnetic transition at approximately 40 T. This suggests that the moment m is connected to a hidden order parameter psi which belongs to the same irreducible representation breaking time-reversal symmetry. The magnetic excitation gap at the antiferromagnetic zone center Q = (1,0,0) increases continuously with increasing field, while that at Q = (1.4,0,0) is nearly constant. This field dependence is opposite to that of the gap extracted from specific-heat data.     

Non-Fermi-liquid behavior within the ferromagnetic phase in URu2-xRexSi2

The URu2-xRexSi2 system exhibits ferromagnetic order for Re concentrations 0.3 < x < or =1.0. Non-Fermi-liquid (NFL) behavior is observed in the specific heat for 0.15< or = x< or =0.6 [C/T proportional to, -lnT (or T(-0.1))], and also in the power-law T dependence of the electrical resistivity [rhoT proportional to, Tn] with n<2 for 0.15< or = x <0.8, at low T, providing strong evidence that the NFL behavior persists within the ferromagnetic phase. Furthermore, the deviation of the physical properties of URu2-xRexSi2 from Fermi-liquid behavior is most pronounced at the ferromagnetic quantum critical point, and the NFL behavior found in the ferromagnetic phase may be consistent with the Griffiths-McCoy phase model.     

Continuous evolution of the Fermi surface of CeRu2Si2 across the metamagnetic transition

We present new, high resolution Hall effect and magnetoresistance measurements across the metamagnetic transition in the heavy fermion compound CeRu2Si2 . The results, and ambiguities in the interpretation of de Haas-van Alphen data, force us to rethink the notion that the transition is accompanied by an abrupt f-electron localization. Instead, we explain our data assuming a continuous evolution of the Fermi surface, which sees one of the spin-split sheets of the heaviest surface shrink to a point.     

Three-dimensional bulk fermiology of CeRu2Ge2 in the paramagnetic phase by soft x-ray hnu-dependent (700-860 eV) ARPES

By virtue of the soft x-ray angle-resolved photoelectron spectroscopy, the three-dimensional bulk fermiology has been successfully performed for a strongly correlated Ce compound, ferromagnet CeRu2Ge2 in the paramagnetic phase. A clear difference of the Fermi surface topology from either band calculation or de Haas-van Alphen results in the ferromagnetic phase is observed and interpreted by considering the difference of the 4f contribution to the Fermi surfaces in the paramagnetic phase.     

Exotic superconducting properties in the electron-hole-compensated heavy-fermion "Semimetal" URu2Si2

We show that the charge and thermal transport measurements on ultraclean crystals of URu2Si2 reveal a number of unprecedented superconducting properties. The uniqueness is best highlighted by the peculiar field dependence of thermal conductivity including the first-order transition at Hc2 with a reduction of entropy flow. This is a consequence of multiband superconductivity with compensated electronic structure in the hidden order state of this system. We provide strong evidence for a new type of unconventional superconductivity with two distinct gaps having different nodal topology.