Two-dimensional simulations of temperature and current-density distribution in electromigrated structures

We report on the application of a feedback-controlled electromigration technique for the formation of nanometer-sized gaps in mesoscopic gold wires and rings. The effect of current density and temperature, linked via Joule heating, on the resulting gap size is investigated. Experimentally, a good thermal coupling to the substrate turned out to be crucial to reach electrode spacings below 10 nm and to avoid overall melting of the nanostructures. This finding is supported by numerical calculations of the current-density and temperature profiles for structure layouts subjected to electromigration. The numerical method can be used for optimizing the layout so as to predetermine the location where electromigation leads to the formation of a gap.Comment: 9 pages, 6 figure

Signature of frustrated moments in quantum critical CePd1−x_{1-x}Nix_xAl

CePdAl with Ce $4f$ moments forming a distorted kagom\'e network is one of the scarce materials exhibiting Kondo physics and magnetic frustration simultaneously. As a result, antiferromagnetic (AF) order setting in at $T_{\mathrm{N}} = 2.7$~K encompasses only two thirds of the Ce moments. We report measurements of the specific heat, $C$, and the magnetic Gr\"uneisen parameter, $\Gamma_{\rm mag}$, on single crystals of CePd$_{1-x}$Ni$_x$Al with $x\leq 0.16$ at temperatures down to 0.05~K and magnetic fields $B$ up to $8$~T. Field-induced quantum criticality for various concentrations is observed with the critical field decreasing to zero at $x_c\approx 0.15$. Remarkably, two-dimensional (2D) AF quantum criticality of Hertz-Millis-Moriya type arises for $x=0.05$ and $x=0.1$ at the suppression of 3D magnetic order. Furthermore, $\Gamma_{\rm mag}(B)$ shows an additional contribution near $2.5$~T for all concentrations which is ascribed to correlations of the frustrated one third of Ce moments.Comment: 5+2 pages with 4+3 figure

Anomalous Microwave Surface Resistance of CeCu6

We present surface resistance measurements of the archetypical heavy-fermion compound CeCu6 for frequencies between 3.7 and 18 GHz and temperatures from 1.2 to 6 K. The measurements were performed with superconducting stripline resonators that allow simultaneous measurements at multiple frequencies. The surface resistance of CeCu6 exhibits a pronounced decrease below 3 K, in consistence with dc resistivity. The low-temperature frequency dependence of the surface resistance follows a power law with exponent 2/3. While for conventional metals this would be consistent with the anomalous skin effect, we discuss the present situation of a heavy-fermion metal, where this frequency dependence might instead stem from the influence of electronic correlations.Comment: 6 pages, 3 figures, proceedings of SCES 201

Large anisotropic uniaxial pressure dependencies of Tc in single crystalline Ba(Fe0.92Co0.08)2As2

Using high-resolution dilatometry, we study the thermodynamic response of the lattice parameters to superconducting order in a self-flux grown Ba(Fe0.92Co0.08)2As2 single crystal. The uniaxial pressure dependencies of the critical temperature of Tc, calculated using our thermal expansion and specific heat data via the Ehrenfest relation, are found to be quite large and very anisotropic (dTc/dpa = 3.1(1) K/GPa and dTc/dpc = - 7.0(2) K/GPa). Our results show that there is a strong coupling of the c/a ratio to superconducting order, which demonstrates that Tc is far from the optimal value. A surprising similarity with the uniaxial pressure effects in several other layered superconductors is discussed.Comment: 11 pages, 4 Figure

STM-induced surface aggregates on metals and oxidized silicon

We have observed an aggregation of carbon or carbon derivatives on platinum and natively oxidized silicon surfaces during STM measurements in ultra-high vacuum on solvent-cleaned samples previously structured by e-beam lithography. We have imaged the aggregated layer with scanning tunneling microscopy (STM) as well as scanning electron microscopy (SEM). The amount of the aggregated material increases with the number of STM scans and with the tunneling voltage. Film thicknesses of up to 10 nm with five successive STM measurements have been obtained

CePdAl - a Kondo lattice with partial frustration

Magnetic frustration, which is well-defined in insulating systems with localized magnetic moments, yields exotic ground states like spin ices, spin glasses, or spin liquids. In metals magnetic frustration is less well defined because of the incipient delocalization of magnetic moments by the interaction with conduction electrons, viz., the Kondo effect. Hence, the Kondo effect and magnetic frustration are antithetic phenomena. Here we present experimental data of electrical resistivity, magnetization, specific heat and neutron diffraction on CePdAl, which is one of the rare examples of a geometrically frustrated Kondo lattice, demonstrating that the combination of Kondo effect and magnetic frustration leads to an unusual ground state.Comment: 8 pages, 6 figure

Terahertz Conductivity of Heavy-fermion Systems from Time-resolved Spectroscopy

The Drude model describes the free-electron conduction in simple metals, governed by the freedom that the mobile electrons have within the material. In strongly correlated systems, however, a significant deviation of the optical conductivity from the simple metallic Drude behavior is observed. Here, we investigate the optical conductivity of the heavy-fermion system CeCu$_{\mathrm{6-x}}$Au$_{\mathrm{x}}$, using time-resolved, phase-sensitive terahertz spectroscopy. Terahertz electric field creates two types of excitations in heavy-fermion materials: First, the intraband excitations that leave the heavy quasiparticles intact. Second, the resonant interband transitions between the heavy and light parts of the hybridized conduction band that break the Kondo singlet. We find that the Kondo-singlet breaking interband transitions do not create a Drude peak, while the Kondo-retaining intraband excitations yield the expected Drude response; thus, making it possible to separate these two fundamentally different correlated contributions to the optical conductivity.Comment: Published version. $\omega/T$ scaling analysis and appendix added. 12 pages, 10 figure

Terahertz conductivity of heavy-fermion systems from time-resolved spectroscopy

The Drude model describes the free-electron conduction in simple metals, governed by the freedom thatthe mobile electrons have within the material. In strongly correlated systems, however, a significant deviationof the optical conductivity from the simple metallic Drude behavior is observed. Here, we investigate theoptical conductivity of the heavy-fermion system CeCu6−xAux, using time-resolved, phase-sensitive terahertzspectroscopy. The terahertz electric field creates two types of excitations in heavy-fermion materials: First,the intraband excitations that leave the heavy quasiparticles intact. Second, the resonant interband transitionsbetween the heavy and light parts of the hybridized conduction band that break the Kondo singlet. We find that theKondo-singlet-breaking interband transitions do not create a Drude peak, while the Kondo-retaining intrabandexcitations yield the expected Drude response. This makes it possible to separate these two fundamentallydifferent correlated contributions to the optical conductivity