Interplay of strongly correlated electrons and localized Ising moments in one-dimension

We study the ground state properties of the one-dimensional quarter-filled strongly correlated electronic chain coupled by $J$ to another chain of antiferromagnetic Ising moments. We focus on the case where the large Coulomb interactions localize the charges on every other site. Both the electronic spins and the Ising moments interact antiferromagnetically within each chain by $J_{\rm eff}$ and $J'$, respectively. Since the number of electrons is half as that of the Ising moments the period of magnetic correlation of these two chains are incommensurate. In the presence of $J$, the frustration of $J_{\rm eff}$ and $J'$ arises, which may lead the system to the intriguing magneto-electric effect.Comment: 8pages 6figure

The importance of thermal disorder and electronic occupation for the T-dependence of the optical conductivity in FeSi and MnSi

The spectral weight (SW) for optical transitions in FeSi and MnSi are calculated as function of temperature by means of LMTO-LDA band calculations. The main effects, caused by structural disorder and electronic Fermi-Dirac distribution, act oppositely on the T-dependence of the SW, while the variation of the magnetic moment in MnSi has only a minor effect. The calculations agree with the experimental findings of an increasing SW in FeSi and a decreasing SW in MnSi as function of T. The results can be understood from the change of the bandstructure with disorder.Comment: (5 pages, 4 figures

Thermoelectric properties of junctions between metal and strongly correlated semiconductor

We propose a junction of metal and rare-earth compound semiconductor as the basis for a possible efficient low-temperature thermoelectric device. If an overlayer of rare earth atoms differing from the bulk is placed at the interface, very high values of the figure of merit ZT can be reached at low temperature. This is due to sharp variation of the transmission coefficient of carriers across the junction at a narrow energy range, which is intrinsically linked to the localized character of the overlayer f-orbital.Comment: RevTeX 3.0, 4 pages, 3 postscript figures. To be published in Applied Physics Letter

Switchable Hardening of a Ferromagnet at Fixed Temperature

The intended use of a magnetic material, from information storage to power conversion, depends crucially on its domain structure, traditionally crafted during materials synthesis. By contrast, we show that an external magnetic field applied transverse to the preferred magnetization of a model disordered uniaxial ferromagnet is an isothermal regulator of domain pinning. At elevated temperatures, near the transition into the paramagnet, modest transverse fields increase the pinning, stabilize the domain structure, and harden the magnet, until a point where the field induces quantum tunneling of the domain walls and softens the magnet. At low temperatures, tunneling completely dominates the domain dynamics and provides an interpretation of the quantum phase transition in highly disordered magnets as a localization/delocalization transition for domain walls. While the energy scales of the rare earth ferromagnet studied here restrict the effects to cryogenic temperatures, the principles discovered are general and should be applicable to existing classes of highly anisotropic ferromagnets with ordering at room temperature or above.Comment: 10 pages, 4 figure

Dynamical mean field theory of correlated gap formation in Pu monochalcogenides

The correlated Kondo insulator state of the plutonium monochalcogenides is investigated using the dynamical mean field theory (DMFT) and the local density approximation +U (LDA+U). The DMFT-dynamical fluctuations lead to a correlated insulator state at elevated temperature, in sharp contrast to the static LDA+U approach that fails to reproduce both the insulating behavior and anomalous lattice constant. The DMFT conversely predicts the experimentally observed anomalous increase of the gap with pressure and explains the lattice constant very well.Comment: 4 pages, 4 figure

Temperature and Field Dependence of Magnetic Domains in La1.2_{1.2}Sr1.8_{1.8}Mn2_2O7_7

Colossal magnetoresistance and field-induced ferromagnetism are well documented in manganite compounds. Since domain wall resistance contributes to magnetoresistance, data on the temperature and magnetic field dependence of the ferromagnetic domain structure are required for a full understanding of the magnetoresistive effect. Here we show, using cryogenic Magnetic Force Microscopy, domain structures for the layered manganite La$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ as a function of temperature and magnetic field. Domain walls are suppressed close to the Curie temperature T$_C$, and appear either via the application of a c-axis magnetic field, or by decreasing the temperature further. At temperatures well below T$_C$, new domain walls, stable at zero field, can be formed by the application of a c-axis field. Magnetic structures are seen also at temperatures above T$_C$: these features are attributed to inclusions of additional Ruddleston-Popper manganite phases. Low-temperature domain walls are nucleated by these ferromagnetic inclusions.Comment: 6 figure

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

Insulator-to-metal phase transition in Yb-based Kondo insulators

The periodic Anderson lattice model for the crystalline electric field (CEF)split 4f quartet states is used to describe the Yb-based Kondo insulators/semiconductors. In the slave-boson mean-field approximation, we derive the hybridized quasiparticle bands, and find that decreasing the hybridization difference of the two CEF quartets may induce an insulator-to-metal phase transition. The resulting metallic phase has a hole and an electron Fermi pockets. Such a phase transition may be realized experimentally by applying pressure, reducing the difference in hybridization of the two CEF quartets.Comment: 5 pages, 3 figure

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