Electronic phase separation due to magnetic polaron formation in the semimetallic ferromagnet EuB6_6 - A weakly-nonlinear-transport study

We report measurements of weakly nonlinear electronic transport, as measured by third-harmonic voltage generation $V_{3\omega}$, in the low-carrier density semimetallic ferromagnet EuB$_6$, which exhibits an unusual magnetic ordering with two consecutive transitions at $T_{c_1} = 15.6$\,K and $T_{c_2} = 12.5$\,K. Upon cooling in zero magnetic field through the ferromagnetic transition, the dramatic drop in the linear resistivity at the upper transition $T_{c_1}$ coincides with the onset of nonlinearity, and upon further cooling is followed by a pronounced peak in $V_{3 \omega}$ at the lower transition $T_{c_2}$. Likewise, in the paramagnetic regime, a drop of the material's magnetoresistance $R(H)$ precedes a magnetic-field-induced peak in nonlinear transport. A striking observation is a linear temperature dependence of $V_{3\omega}^{\rm peak}(H)$. We suggest a picture where at the upper transition $T_{c_1}$ the coalescing MP form a conducting path giving rise to a strong decrease in the resistance. The MP formation sets in at around $T^\ast \sim 35$\,K below which these entities are isolated and strongly fluctuating, while growing in number. The MP then start to form links at $T_{c_1}$, where percolative electronic transport is observed. The MP merge and start forming a continuum at the threshold $T_{c_2}$. In the paramagnetic temperature regime $T_{c_1} < T < T^\ast$, MP percolation is induced by a magnetic field, and the threshold accompanied by charge carrier delocalization occurs at a single critical magnetization.Comment: to appear in J. Kor. Phys. Soc (ICM2012 conference contribution

Evidence for electron-phonon interaction in Fe1−x_{1-x}Mx_{x}Sb2_{2} (M=Co, Cr) single crystals

We have measured polarized Raman scattering spectra of the Fe$_{1-x}$Co$_{x}$Sb$_{2}$ and Fe$_{1-x}$Cr$_{x}$Sb$_{2}$ (0$\leq x\leq$0.5) single crystals in the temperature range between 15 K and 300 K. The highest energy $B_{1g}$ symmetry mode shows significant line asymmetry due to phonon mode coupling width electronic background. The coupling constant achieves the highest value at about 40 K and after that it remains temperature independent. Origin of additional mode broadening is pure anharmonic. Below 40 K the coupling is drastically reduced, in agreement with transport properties measurements. Alloying of FeSb$_2$ with Co and Cr produces the B$_{1g}$ mode narrowing, i.e. weakening of the electron-phonon interaction. In the case of A$_{g}$ symmetry modes we have found a significant mode mixing

Kondo insulator SmB6 under strain: surface dominated conduction near room temperature

SmB6 is a strongly correlated mixed-valence Kondo insulator with a newly discovered surface state, proposed to be of non-trivial topological origin. However, the surface state dominates electrical conduction only below T* ~ 4 K limiting its scientific investigation and device application. Here, we report the enhancement of T * in SmB6 under the application of tensile strain. With 0.7% tensile strain we report surface dominated conduction at up to a temperature of 240 K, persisting even after the strain has been removed. This can be explained in the framework of strain-tuned temporal and spatial fluctuations of f-electron configurations, which might be generally applied to other mixed-valence materials. We note that this amount of strain can be indued in epitaxial SmB6 films via substrate in potential device applications.Comment: to appear in Nature Material

Radio Frequency Tunable Oscillator Device Based on SmB6 Microcrystal

Radio frequency tunable oscillators are vital electronic components for signal generation, characterization, and processing. They are often constructed with a resonant circuit and a 'negative' resistor, such as a Gunn-diode, involving complex structure and large footprints. Here we report that a piece of SmB6, 100 micron in size, works as a current-controlled oscillator in the 30 MHz frequency range. SmB6 is a strongly correlated Kondo insulator that was recently found to have a robust surface state likely to be protected by the topology of its electronics structure. We exploit its non-linear dynamics, and demonstrate large AC voltage outputs with frequencies from 20 Hz to 30 MHz by adjusting a small DC bias current. The behaviors of these oscillators agree well with a theoretical model describing the thermal and electronic dynamics of coupled surface and bulk states. With reduced crystal size we anticipate the device to work at higher frequencies, even in the THz regime. This type of oscillator might be realized in other materials with a metallic surface and a semiconducting bulk.Comment: v3 to appear in Physical Review Letter

Strong pinning of vortices by antiferromagnetic domain boundaries in CeCo(In1−x_{1-x}Cdx_x)5_5

We have studied the isothermal magnetization $M(H)$ of CeCo(In$_{1-x}$Cd$_x$)$_5$ with $x$ = 0.0075 and 0.01 down to 50 mK. Pronounced field-history dependent phenomena occur in the coexistence regime of the superconducting and antiferromagnetic phases. At low-fields, a phenomenological model of magnetic-flux entry well explains $M(H)$ implying the dominance of bulk pinning effect. However, unless crystallographic quenched disorder is hysteretic, the asymmetric peak effect (ASPE) which appears at higher fields cannot be explained by the pinning of vortices due to material defects. Also the temperature dependence of the ASPE deviates from the conventional scenario for the peak effect. Comparison of our thermodynamic phase diagrams with those from previous neutron scattering and magnetoresistance experiments indicates that the pinning of vortices takes place at the field-history dependent antiferromagnetic domain boundaries.Comment: 13 pages,4 figures, to be published in New Journal of Physic