Showing papers by "Hilbert von Löhneysen published in 2002"
TL;DR: It is able to distinguish the influence of both the molecule and the contact to the metal electrodes on the transport properties of the compound system and unambiguously detect an intrinsic property of the molecule.
Abstract: We investigate electronic transport through two types of conjugated molecules. Mechanically controlled break junctions are used to couple thiol end groups of single molecules to two gold electrodes. Current-voltage characteristics ( IVs) of the metal-molecule-metal system are observed. These IVs reproduce the spatial symmetry of the molecules with respect to the direction of current flow. We hereby unambiguously detect an intrinsic property of the molecule and are able to distinguish the influence of both the molecule and the contact to the metal electrodes on the transport properties of the compound system.
831 citations
01 Aug 2002
TL;DR: In this paper, conductance spectra of two types of conjugated molecules are studied in a metal-molecule-metal junction established using the mechanically controlled break-junction technique.
Abstract: We investigate electronic transport through single conjugated molecules, and compare our data to results of quantum chemical calculations. Conductance spectra of two types of molecules are studied in a metal–molecule–metal junction established using the mechanically controlled break-junction technique. We observe a suppressed conductance at low bias, characteristic step-like features at higher voltages, and strong sample-to-sample fluctuations. We develop a quantum-chemical model for our system using DFT calculations, with the electrodes modelled by small clusters. We consider the effects of different geometries of molecule–metal configurations and bonding as well as finite electric field, and are thereby able to account for the phenomenology of the experimental data.
169 citations
TL;DR: In this paper, magnetization and resistivity measurements of a cluster compound in which negatively charged Ga84 entities are strongly bonded to organic ligands and confined in an ionic crystal structure are reported.
Abstract: We report on magnetization and resistivity measurements of a cluster compound in which negatively charged Ga84 entities are strongly bonded to organic ligands and confined in an ionic crystal structure. The macroscopic single crystals have a resistivity in the range of 100 Ωcm at room temperature with a semiconducting-like temperature dependence. They reveal a superconducting transition at T
c≈7.2 K and an upper critical field of B
c2=13.8 T. The presence of the superconducting state in the regularly arranged Ga84 clusters implies an electronic coupling between the individual clusters.
45 citations
TL;DR: In this article, low-temperature measurements of the specific heat, resistivity, and magnetization properties of the itinerant antiferromagnet were reported, and it was shown that the stability of the magnetic state up to 14 T inferred from the invariance of these bulk properties is not consistent with the magnetism expected for a conventional Fermi liquid.
Abstract: We report on low-temperature measurements of the specific heat, resistivity, and magnetization of the itinerant antiferromagnet ${\mathrm{Mn}}_{3}\mathrm{Si}.$ The stability of the magnetic state up to 14 T inferred from the invariance of these bulk properties is incompatible with the itinerant magnetism expected for a conventional Fermi liquid.
43 citations
TL;DR: In this paper, a trimethylsilyl (TMS) self-assembled monolayer on a silicon surface is a self-developing positive resist, which can be patterned with low energy electrons.
Abstract: We show that a trimethylsilyl (TMS) self-assembled monolayer on a silicon surface is a self-developing positive resist, which can be patterned with low energy electrons. Contact angle measurements have been used to quantify the efficiency of the exposure as a function of exposure dose and acceleration voltage. Ash formation was negligible, as a 3-aminopropyltriethoxysilane (APTES) self-assembled monolayer could be formed on the patterned area without an intermediate development stage. APTES/TMS patterns have been visualized with scanning electron microscopy at low energy and atomic force microscopy. The functionality of the patterns has been tested by selective deposition of carbon nanotubes.
42 citations
TL;DR: The displacement pattern of the most anomalous phonons is in principle consistent with dynamic charge stripe formation, however, charge stripes would have to extend along the a axis and not the b axis as inferred from the magnetic fluctuations by Mook et al.
Abstract: The dispersion of the Cu-O bond-stretching and bond-bending vibrations in $\mathrm{YBa}{}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.6}$ has been studied by high resolution inelastic neutron scattering. While the behavior of the bond-bending vibrations can be well accounted for by a simple potential model, the bond-stretching vibrations show a highly anomalous behavior. The displacement pattern of the most anomalous phonons is in principle consistent with dynamic charge stripe formation. However, charge stripes would have to extend along the $a$ axis and not the $b$ axis as inferred from the magnetic fluctuations by Mook et al. [Nature (London) 404, 729 (2000)].
33 citations
TL;DR: In this paper, Shubnikov-de Haas (SdH) and Hall-effect measurements of LaBiPt and CeBiPts reveal the presence of very small Fermi surfaces with hole-like charge carriers for both semimetals.
Abstract: Shubnikov-de Haas (SdH) and Hall-effect measurements of CeBiPt and LaBiPt reveal the presence of very small Fermi surfaces with hole-like charge carriers for both semimetals. Reasonable agreement with band structure calculations is achieved when taking spin-orbit coupling into account and assuming localized Ce 4f electrons. For CeBiPt, we observe a strong temperature dependence of the SdH frequency, i.e., an increase from ~ 35 T at 10 K to ~ 60 T at 0.4 K, indicating a possible hybridization of the f electrons.
20 citations
TL;DR: In this paper, electrical resistivity measurements in an applied magnetic field up to 14 T were performed at low temperatures T on several samples of pyrolytic carbon deposited on a substrate of boron nitride.
17 citations
TL;DR: The low temperature properties of CeCu6−xAux show different behavior at the quantum phase transition depending on how the long-range antiferromagnetic order is suppressed as mentioned in this paper.
Abstract: The low temperature properties of CeCu6−xAux show different behavior at the quantum phase transition depending on how the long-range antiferromagnetic order is suppressed Tuning by hydrostatic pressure p results in a temperature dependence of the specific heat C(T) and the electrical resistivity ρ(T) as C/T=a ln (T 0 /T) and ρ≈ρ0+A′T, while C/T=γ0−a′T05 and ρ≈ρ0+A″T15 when tuned by applying a magnetic field B The magnetic (p,T) and (B,T) phase diagrams as inferred from specific heat, electrical resistivity and neutron diffraction measurements are distinctly different These findings suggest that the spectra of the low-lying magnetic fluctuations near these quantum phase transitions differ
13 citations
TL;DR: In this article, single crystals of Yb 1−x Lu x B 12 (x=0,0.125, 0.5,1) were measured between 1.8 and 300 K and found similar spectral shapes in all compounds studied over the whole temperature range scanned.
Abstract: Single crystals of Yb 1−x Lu x B 12 (x=0,0.125,0.5,1) were measured between 1.8 and 300 K . We found similar spectral shapes in all compounds studied over the whole temperature range scanned. In zero field the spectra alter their appearance around 20, 100 and 150 K . In a longitudinal field of 100 G , which largely suppresses the contribution from 11 B nuclear moments the relaxation rate remained constant up to ∼150 K where it suddenly peaks. These findings exclude magnetic correlations as the origin of muon spin relaxational behavior. The dominant features of the μSR spectra at low temperatures arise from lattice dynamics, probably involving the B 12 clusters, rather than from spin dynamics. Higher temperature features are most likely muon related.
11 citations
TL;DR: In this paper, the ternary Yb compound YbZn2Sb2 was synthesized and properties of the compound were reported, including resistivity, magnetization and specific heat at low temperature and high magnetic field.
Abstract: We report synthesis and properties of the ternary Yb compound YbZn2Sb2. The resistivity, magnetization and specific heat at low temperature and high magnetic field of YbZn2Sb2 are typical of a strongly diamagnetic semimetal. A comparison with isostructural YbZn2As2 suggests that YbZn2Sb2 will undergo a nonmagnetic to magnetic quantum phase transition at high pressure that is driven by a valence transition.
TL;DR: In this paper, the temperature dependence of the perpendicular upper critical field B c2 of a single superconducting Nb film sandwiched between insulating (I) and/or normal-metal layers (N=Cu) is investigated.
Abstract: The temperature dependence of the perpendicular upper critical field B c2⊥ of a single superconducting Nb film (S) sandwiched between insulating (I) and/or normal-metal layers (N=Cu) is investigated. For the ISI configuration, B c2⊥ exhibits the usual linear T -dependence near the transition temperature T c in contrast to the NSI and NSN configurations where a positive curvature of B c2⊥ ( T ) is observed near T c . This demonstrates the influence of the different boundary conditions on the B c2⊥ ( T ) behavior of a single S film in contact with N or I. Deviations from the linear T -dependence are thus attributed to the proximity effect due to the presence of an N–S boundary.
TL;DR: In this paper, the authors present measurements of the specific heat C in the range 0.1 − 2.5 K on polycrystalline samples up to 14.7 kbar.
Abstract: In UGe2, superconductivity in coexistence with ferromagnetism is observed in single crystal (Nature 406 (2000) 587; Phys. Rev. B 63 (2001) 144519) and polycrystalline samples (J. Phys. Condens. Matter 13 (2001) L759) just below the pressure p c ≈16 kbar for which the Curie temperature TC drops to zero. We present measurements of the specific heat C in the range 0.1– 2.5 K on polycrystalline samples up to 14.7 kbar . The linear coefficient of the specific heat, γ=C/T, increases as a function of pressure and is the highest in the p-range where superconductivity is observed. In the superconducting state the AC magnetic susceptibility exhibits strong diamagnetic shielding. The specific heat displays an anomaly that is indicative of a bulk superconducting transition of order 15–20% by volume. The observed properties reflect an intricate interplay of the coexistence of ferromagnetism and superconductivity in UGe2.
TL;DR: In this paper, a change of the local atomic arrangement around the two different Mn sites (Mn I and Mn II ) by the incorporation of carbon into the hexagonal lattice compared to Mn 5 Si 3 was reported.
TL;DR: In this article, the authors investigated the magnetic phase diagram through measurements of the specific heat and the magnetocaloric effect, and they found that the phase diagram is almost unaltered in agreement with an estimate of dTAFQ/dp by Ehrenfest's relation.
Abstract: The intermetallic compound PrPb3 exhibits antiferroquadrupolar (AFQ) ordering at low temperature. We investigated the magnetic phase diagram through measurements of the specific heat and the magnetocaloric effect. In zero magnetic field the specific heat shows a second-order phase transition with an ordering temperature T AFQ =0.39 K in line with earlier results. In addition, we find a further transition at T ∗ =0.35 K for 3 T and T ∗ =0.49 K for 4.5 T, which, however, is only observed upon heating. The strong hysteresis between heating and cooling suggests that this transition is of first order. Under hydrostatic pressure of 6.5 kbar, the phase diagram is almost unaltered in agreement with an estimate of dTAFQ/dp by Ehrenfest's relation.
TL;DR: In this article, the authors investigated the magnetic properties of heavy fermion alloys and found that spin fluctuations are dominated by spin fluctuations which are expected to become critical when tuned by pressure to a quantum critical point (QCP), entering a magnetic ordered state.
Abstract: The electronic properties of heavy fermion alloys are dominated by spin fluctuations which are expected to become critical when tuned by pressure to a quantum critical point (QCP), entering a magnetic ordered state. Apart from the onset of exotic superconductivity, unexpected "normal conducting" behavior is found close to the QCP, which does not seem only to escape the conventional view of metals (Fermi liquids) but also the "conventional view" of an antiferromagnetic quantum phase transition in these f-metals. So far only few compounds have been investigated by neutron scattering to directly reveal the critical fluctuations spectrum. In CeCu59Au01 the fluctuations develop an unusual energy dependence, characterized by an exponent α = 0.75, which persist over the entire Brillouin zone, provoking an unexpected local non Fermi liquid behavior. The same unusual exponent derived from E/T scaling determines the H/T scaling of the uniform magnetization. Recent neutron scattering data in magnetic fields further confirm this picture of nearly free local magnetic moments (modified by α) emerging at the antiferromagnetic QCP in this strongly correlated electron system.
TL;DR: In this article, a breakdown of the Fermi-liquid description may be tuned by either Au-concentration, hydrostatic pressure, uniaxial stress or magnetic field.
TL;DR: In this article, a breakdown of the Fermi-liquid properties may be tuned by Au concentration, hydrostatic pressure, or magnetic field, and the d-electron weak itinerant ferromagnet ZrZn2 was recently found to display superconductivity in coexistence with ferromagneticism.
Abstract: Zero-temperature magnetic phase transitions exhibit an abundance of nearly critical magnetic fluctuations that allow to probe the traditional concepts of the metallic state. For the prototypical heavy-fermion compound, CeCu6−xAux, a breakdown of the Fermi-liquid properties may be tuned by Au concentration, hydrostatic pressure, or magnetic field. The d-electron weak itinerant ferromagnet ZrZn2, on the other hand, was recently found to display superconductivity in coexistence with ferromagnetism.
TL;DR: In this article, it was shown that the second order transition of Ce 7 Ni 3 orders antiferromagnetically near 2.85 K, but this ordering vanishes under pressure for P c ⩾-0.32 GPa where the compound exhibits non Fermi liquid behavior.
Abstract: Ce 7 Ni 3 orders antiferromagnetically near 2 K, but this ordering vanishes under pressure for P c ⩾ 0.32 GPa where the compound exhibits non Fermi liquid behavior. The μSR data on single crystals at ambient pressure give T N =1.85 K and reveal properties typical for a second order transition. Just above T N the paramagnetic spin fluctuations are non-isotropic confirming strong magnetic anisotropy. The μSR signal below T N is basically compatible with an incommensurate spin structure involving all Ce atoms having modulated moments primarily along the c -axis in agreement with neutron results. Details of the signal, however, indicate locally a more complex spin modulation. The maximum local field B μ =0.15 T, confirms comparatively small Ce moments. The neutron data claim a second transition at T N =0.7 K, but the μSR signal shows no change around this temperature. If this transition exists at all, then the change in spatial arrangement of Ce spins must be very small.