326 research outputs found
Thermoelectric response near a quantum critical point of beta-YbAlB4 and YbRh2Si2: A comparative study
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
Thermal conductivity through the quantum critical point in YbRh2Si2 at very low temperature
The thermal conductivity of YbRh2Si2 has been measured down to very low
temperatures under field in the basal plane. An additional channel for heat
transport appears below 30 mK, both in the antiferromagnetic and paramagnetic
states, respectively below and above the critical field suppressing the
magnetic order. This excludes antiferromagnetic magnons as the origin of this
additional contribution to thermal conductivity. Moreover, this low temperature
contribution prevails a definite conclusion on the validity or violation of the
Wiedemann-Franz law at the field-induced quantum critical point. At high
temperature in the paramagnetic state, the thermal conductivity is sensitive to
ferromagnetic fluctuations, previously observed by NMR or neutron scattering
and required for the occurrence of the sharp electronic spin resonance
fracture.Comment: 11 pages + Supplementary Material
Pressure dependence of the magnetization of URu2Si2
The ground state of URu2Si2 changes from so-called hidden order (HO) to
large-moment antiferromagnetism (LMAF) upon applying hydrostatic pressure in
excess of 14 kbar. We report the dc-magnetization M(B,T,p) of URu2Si2 for
magnetic fields B up to 12 T, temperatures T in the range 2 to 100 K, and
pressure p up to 17 kbar. Remarkably, characteristic scales such as the
coherence temperature T*, the transition temperature T0, and the anisotropy in
the magnetization depend only weakly on the applied pressure. However, the
discontinuity in dM/dT at T0, which measures the magnetocaloric effect,
decreases nearly 50 % upon applying 17 kbar for M and B parallel to the
tetragonal c-axis, while it increases 15-fold for the a-axis. Our findings
suggest that the HO and LMAF phases have an astonishing degree of similarity in
their physical properties, but a key difference is the magnetocaloric effect
near T0 in the basal plane
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E2F4 regulates transcriptional activation in mouse embryonic stem cells independently of the RB family.
E2F transcription factors are central regulators of cell division and cell fate decisions. E2F4 often represents the predominant E2F activity in cells. E2F4 is a transcriptional repressor implicated in cell cycle arrest and whose repressive activity depends on its interaction with members of the RB family. Here we show that E2F4 is important for the proliferation and the survival of mouse embryonic stem cells. In these cells, E2F4 acts in part as a transcriptional activator that promotes the expression of cell cycle genes. This role for E2F4 is independent of the RB family. Furthermore, E2F4 functionally interacts with chromatin regulators associated with gene activation and we observed decreased histone acetylation at the promoters of cell cycle genes and E2F targets upon loss of E2F4 in RB family-mutant cells. Taken together, our findings uncover a non-canonical role for E2F4 that provide insights into the biology of rapidly dividing cells
Observation of anisotropic effect of antiferromagnetic ordering on the superconducting gap in ErNi2B2C
The point-contact (PC) spectra of the Andreev reflection dV/dI curves of the
superconducting rare-earth nickel borocarbide ErNi2B2C (Tc=11 K) have been
analyzed in the "one-gap" and "two-gap" approximations using the generalized
Blonder-Tinkham-Klapwijk (GBTK) model and the Beloborod'ko (BB) model allowing
for the pair-breaking effect of magnetic impurities. Experimental and
calculated curves have been compared not only in shape, but in magnitude as
well, which provide more reliable data for determining the temperature
dependence of the energy gap (or superconducting order parameter) \Delta(T).
The anisotropic effect of antiferromagnetic ordering at T_N =6 K on the
superconducting gap/order parameter has been determined: as the temperature is
lowered, \Delta(T) decreases by 25% in the c-direction and only by 4% in the
ab-plane. It is found that the pair-breaking parameter increases in the
vicinity of the magnetic transitions, the increase being more pronounced in the
c-direction. The efficiency of the models was tested for providing \Delta(T)
data for ErNi2B2C from Andreev reflection spectra.Comment: 16 two column pages, 20 figs., will be published in Fiz. Nizk. Temp.
N10, 2010; V2: added - "Acknowledgement" & "Note added in proof
Multiband superconductivity in the heavy fermion compound PrOs4Sb12
The thermal conductivity of the heavy fermion superconductor PrOs4Sb12 was
measured down to Tc/40 throughout the vortex state. At lowest temperatures and
for magnetic fields H ~ 0.07Hc2, already 40% of the normal state thermal
conductivity is restored. This behaviour (similar to that observed in MgB2) is
a clear signature of multiband superconductivity in this compound.Comment: 12pages, version #1 20\_06\_200
Thermal conductivity in B- and C- phase of UPt_3
Although the superconductivity in UPt_3 is one of the most well studied,
there are still lingering questions about the nodal directions in the B and C
phase in the presence of a magnetic field. Limiting ourselves to the low
temperature regime (T<<Delta(0)), we study the magnetothermal conductivity with
in semiclassical approximation using Volovik's approach. The angular dependence
of the magnetothermal conductivity for an arbitrary field direction should
clarify the nodal structure in UPt_3.Comment: 4 pages, 5 figure
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