Giant Carrier Mobility in Single Crystals of FeSb2

We report the giant carrier mobility in single crystals of FeSb2. Nonlinear field dependence of Hall resistivity is well described with the two-carrier model. Maximum mobility values in high mobility band reach ~10^5 cm^2/Vs at 8 K, and are ~10^2 cm^2/Vs at the room temperature. Our results point to a class of materials with promising potential for applications in solid state electronics.Comment: 5 pages, 3 figures. Applied Physics Letters (in press

Probing many-body localization in a disordered quantum magnet

Quantum states cohere and interfere. Quantum systems composed of many atoms arranged imperfectly rarely display these properties. Here we demonstrate an exception in a disordered quantum magnet that divides itself into nearly isolated subsystems. We probe these coherent clusters of spins by driving the system beyond its linear response regime at a single frequency and measuring the resulting "hole" in the overall linear spectral response. The Fano shape of the hole encodes the incoherent lifetime as well as coherent mixing of the localized excitations. For the disordered Ising magnet, $\mathrm{LiHo_{0.045}Y_{0.955}F_4}$, the quality factor $Q$ for spectral holes can be as high as 100,000. We tune the dynamics of the quantum degrees of freedom by sweeping the Fano mixing parameter $q$ through zero via the amplitude of the ac pump as well as a static external transverse field. The zero-crossing of $q$ is associated with a dissipationless response at the drive frequency, implying that the off-diagonal matrix element for the two-level system also undergoes a zero-crossing. The identification of localized two-level systems in a dense and disordered dipolar-coupled spin system represents a solid state implementation of many-body localization, pushing the search forward for qubits emerging from strongly-interacting, disordered, many-body systems.Comment: 22 pages, 6 figure

Anisotropy in magnetic and transport properties of Fe1-xCoxSb2

Anisotropic magnetic and electronic transport measurements were carried out on large single crystals of Fe1-xCoxSb2 (0<= x <=1). The semiconducting state of FeSb2 evolves into metallic and weakly ferromagnetic by substitution of Fe with Co for x<0.5. Further doping induces structural transformation from orthorhombic Pnnm structure of FeSb2 to monoclinic P21/c structure of CoSb2 where semiconducting and diamagnetic ground state is restored again. Large magnetoresistance and anisotropy in electronic transport were observed.Comment: 7 pages, 6 figure

Quantum and Classical Glass Transitions in LiHoxY1−xF4Li Ho_x Y_{1-x} F_4

When performed in the proper low field, low frequency limits, measurements of the dynamics and the nonlinear susceptibility in the model Ising magnet in transverse field, $\text{LiHo}_x\text{Y}_{1-x}\text{F}_4$, prove the existence of a spin glass transition for $x$ = 0.167 and 0.198. The classical behavior tracks for the two concentrations, but the behavior in the quantum regime at large transverse fields differs because of the competing effects of quantum entanglement and random fields.Comment: 5 pages, 5 figures. Updated figure 3 with corrected calibration information for thermometr

Anomalous low temperature state of CeOs4Sb12: Magnetic field and La-impurity study

Specific heat for single crystalline samples of Ce1-xLaxOs4Sb12 at zero-field and magnetic fields to 14 T is reported. Our results confirm enhanced value of the electronic specific heat coefficient in the paramagnetic state. They provide arguments for the intrinsic origin of the 1.1 K anomaly. This transition leads to opening of the gap at the Fermi surface. This low temperature state of CeOs4Sb12 is extremely sensitive to chemical impurities. 2% of La substituted for Ce suppresses the transition and reduces the electronic specific heat coefficient. The magnetic field response of the specific heat is also anomalous.Comment: 4 pages, 3 figure

Competing topological and Kondo insulator phases on a honeycomb lattice

We investigate the competition between the spin-orbit interaction of itinerant electrons and their Kondo coupling with local moments densely distributed on the honeycomb lattice. We find that the model at half-filling displays a quantum phase transition between topological and Kondo insulators at a nonzero Kondo coupling. In the Kondo-screened case, tuning the electron concentration can lead to a new topological insulator phase. The results suggest that the heavy-fermion phase diagram contains a new regime with a competition among topological, Kondo-coherent and magnetic states, and that the regime may be especially relevant to Kondo lattice systems with $5d$-conduction electrons. Finally, we discuss the implications of our results in the context of the recent experiments on SmB$_6$ implicating the surface states of a topological insulator, as well as the existing experiments on the phase transitions in SmB$_6$ under pressure and in CeNiSn under chemical pressure.Comment: (v3) Published version including the main text (5 pages + 4 figures) and a supplementary material discussing the effects of quantum fluctuations of the slave bosons and antiferromagnetic ordering of the local moments on the transitions among the Kondo, magnetic and topological state

Scaling of magnetic fluctuations near a quantum phase transition

We use inelastic neutron scattering to measure the magnetic fluctuations in a single crystal of the heavy fermion alloy CeCu_5.9Au_0.1 close to the antiferromagnetic quantum critical point. The energy and temperature-dependent spectra obey (E/T) scaling at Q near (1,0,0). The neutron data and earlier bulk susceptibility are consistent with the form 1/X ~ f(Q)+(-iE+bT)^a, with an anomalous exponent a=0.8. We confirm the earlier observation of quasi-low dimensionality and show how both the magnetic fluctuations and the thermodynamics can be understood in terms of a quantum Lifshitz point.Comment: Latex file with two postscript figure