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H. v. Löhneysen

Publications and source records attributed to H. v. Löhneysen.

At least 37 records · Page 2Linked to original sources
arXiv2012

Thermodynamic phase diagram and phase competition in BaFe2(As1-xPx)2 studied by thermal expansion

High-resolution thermal-expansion and specific-heat measurements were performed on single crystalline BaFe2(As1-xPx)2 (0 < x < 0.33, x = 1). The observation of clear anomalies allows to establish the thermodynamic phase diagram which features a small coexistence region of SDW and superconductivity with a steep rise of Tc on the underdoped side. Samples that undergo the tetragonal-orthorhombic structural transition are detwinned in situ, and the response of the sample length to the magneto-structural and superconducting transitions is studied for all three crystallographic directions. It is shown that a reduction of the magnetic order by superconductivity is reflected in all lattice parameters. On the overdoped side, superconductivity affects the lattice parameters in much the same way as the SDW on the underdoped side, suggesting an intimate relation between the two types of order. Moreover, the uniaxial pressure derivatives of Tc are calculated using the Ehrenfest relation and are found to be large and anisotropic. A correspondence between substitution and uniaxial pressure is established, i.e., uniaxial pressure along the b-axis (c-axis) corresponds to a decrease (increase) of the P content. By studying the electronic contribution to the thermal expansion we find evidence for a maximum of the electronic density of states at optimal doping.

cond-mat.supr-con↗
arXiv2011

Complex magneto-elastic properties in the frustrated kagome-staircase compounds (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$

High resolution heat capacity and thermal expansion experiments performed on single crystalline kagome-staircase compounds (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$ are presented. The parent compounds Co$_3$V$_2$O$_8$ and Ni$_3$V$_2$O$_8$ undergo a complex sequence of first- and second-order magnetic phase transitions. The low-temperature ($T<40$ K) magnetic entropy evolves monotonously with the doping content $x$, from the full S=1 Ni$^{2+}$ magnetic entropy in Ni$_3$V$_2$O$_8$ to half of the S=3/2 Co$^{2+}$ magnetic entropy in Co$_3$V$_2$O$_8$. Thermal expansion coefficients $α_i$ ($i = a$, $b$ and $c$) show a strong anisotropy for all (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$ compounds. The low-temperature magnetic distortion indicates that Co-doping (Ni-doping) has similar effects to applying a uniaxial pressures along $a$ or $b$ ($c$). Linear Grüneisen parameters $Γ_i$ are extracted for the three main axes $i$ and exhibit a complex behavior with both temperature and doping. For each axis, $Γ_i$ and $α_i$ exhibit a sign change (at low temperature) at the critical concentration $x_c\simeq0.25$, at which the incommensurate magnetic propagation vector changes. Beyond our study, an understanding of the multiple and complex parameters (magnetic frustration, magnetic anisotropy, mixture of S=1 and S=3/2 ions, etc.) is now necessarily to bring light to the rich magneto-elastic properties of (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$.

cond-mat.str-el↗
arXiv2011

Graphene microwave transistors on sapphire substrates

We have developed metal-oxide graphene field-effect transistors (MOGFETs) on sapphire substrates working at microwave frequencies. For monolayers, we obtain a transit frequency up to ~ 80 GHz for a gate length of 200 nm, and a power gain maximum frequency of about ~ 3 GHz for this specific sample. Given the strongly reduced charge noise for nanostructures on sapphire, the high stability and high performance of this material at low temperature, our MOGFETs on sapphire are well suited for a cryogenic broadband low-noise amplifier.

cond-mat.mes-hall↗
arXiv2010

Towards the identification of a quantum critical line in the (p, B) phase diagram of CeCoIn5

The low-temperature thermal expansion of CeCoIn5 single crystals measured parallel and perpendicular to magnetic fields B oriented along the c axis yields the volume thermal-expansion coefficient $β$. Considerable deviations of $β(T)$ from Fermi-liquid behavior occur already within the superconducting region of the (B, T) phase diagram and become maximal at the upper critical field $B^0_{c2}$. However, $β(T)$ and the Grüneisen parameter $Γ$ are incompatible with a quantum critical point (QCP) at $B^0_{c2}$, but allow for a QCP shielded by superconductivity and extending to negative pressures for $B < B^0_{c2}$. Together with literature data we construct a tentative (p, B, T) phase diagram of CeCoIn5 suggesting a quantum critical line in the (p, B) plane.

cond-mat.str-el↗
arXiv2010

Charge imbalance in superconductors in the low-temperature limit

We explore charge imbalance in mesoscopic normal-metal/superconductor multiterminal structures at very low temperatures. The investigated samples, fabricated by e-beam lithography and shadow evaporation, consist of a superconducting aluminum bar with several copper wires forming tunnel contacts at different distances from each other. We have measured in detail the local and non-local conductance of these structures as a function of the applied bias voltage V, the applied magnetic field B, the temperature T and the contact distance d. From these data the charge-imbalance relaxation length lambda_Q* is derived. The bias-resolved measurements show a transition from dominant elastic scattering close to the energy gap to an inelastic two-stage relaxation at higher bias. We observe a strong suppression of charge imbalance with magnetic field, which can be directly linked to the pair-breaking parameter. In contrast, practically no temperature dependence of the charge-imbalance signal was observed below 0.5 K. These results are relevant for the investigation of other non-local effects such as crossed Andreev reflexion and spin diffusion.

cond-mat.mes-hall↗
arXiv2009

Non-local transport in normal-metal/superconductor hybrid structures: the role of interference and interaction

We have measured local and non-local conductance of mesoscopic normal-metal/superconductor hybrid structures fabricated by e-beam lithography and shadow evaporation. The sample geometry consists of a superconducting aluminum bar with two normal-metal wires forming tunnel contacts to the aluminum at distances of the order of the superconducting coherence length. We observe subgap anomalies in both local and non-local conductance that quickly decay with magnetic field and temperature. For the non-local conductance both positive and negative signs are found as a function of bias conditions, indicating at a competition of crossed Andreev reflection and elastic cotunneling. Our data suggest that the signals are caused by a phase-coherent enhancement of transport rather than dynamical Coulomb blockade.

cond-mat.mes-hall↗
arXiv2009

High-temperature signatures of quantum criticality in heavy fermion systems

We propose a new criterion for distinguishing the Hertz-Millis (HM) and the local quantum critical (LQC) mechanism in heavy fermion systems with a magnetic quantum phase transition (QPT). The criterion is based on our finding that the spin screening of Kondo ions can be completely suppressed by the RKKY coupling to the surrounding magnetic ions even without magnetic ordering and that, consequently, the signature of this suppression can be observed in spectroscopic measurements above the magnetic ordering temperature. We apply the criterion to high-resolution photoemission (UPS) measurements on CeCu$_{6-x}$Au$_{x}$ and conclude that the QPT in this system is dominated by the LQC scenario.

cond-mat.str-el↗
arXiv2008

Signature of quantum criticality in photoemission spectroscopy at elevated temperature

A quantum phase transition (QPT) in a heavy-fermion (HF) compound may destroy the Fermi liquid groundstate. However, the conditions for this breakdown have remained obscure. We report the first direct investigation of heavy quasiparticle formation and breakdown in the canonical system CeCu$_{6-x}$Au$_x$ by ultraviolet photoemission spectroscopy at elevated temperatures without the complications of lattice coherence. Surprisingly, the single-ion Kondo energy scale $T_K$ exhibits an abrupt step near the quantum critical Au concentration of $x_c=0.1$. We show theoretically that this step is expected from a highly non-linear renormalization of the local spin coupling at each Ce site, induced by spin fluctuations on neighboring sites. It provides a general high-temperature indicator for HF quasiparticle breakdown at a QPT.

cond-mat.str-el↗
arXiv2008

Ferromagnetism and Lattice Distortions in the Perovskite YTiO$_3$

The thermodynamic properties of the ferromagnetic perovskite YTiO$_3$ are investigated by thermal expansion, magnetostriction, specific heat, and magnetization measurements. The low-temperature spin-wave contribution to the specific heat, as well as an Arrott plot of the magnetization in the vicinity of the Curie temperature $T_C\simeq27$ K, are consistent with a three-dimensional Heisenberg model of ferromagnetism. However, a magnetic contribution to the thermal expansion persists well above $T_C$, which contrasts with typical three-dimensional Heisenberg ferromagnets, as shown by a comparison with the corresponding model system EuS. The pressure dependences of $T_C$ and of the spontaneous moment $M_s$ are extracted using thermodynamic relationships. They indicate that ferromagnetism is strengthened by uniaxial pressures $\mathbf{p}\parallel \mathbf{a}$ and is weakened by uniaxial pressures $\mathbf{p}\parallel \mathbf{b},\mathbf{c}$ and hydrostatic pressure. Our results show that the distortion along the $a$- and $b$-axes is further increased by the magnetic transition, confirming that ferromagnetism is favored by a large GdFeO$_3$-type distortion. The c-axis results however do not fit into this simple picture, which may be explained by an additional magnetoelastic effect, possibly related to a Jahn-Teller distortion.

cond-mat.str-el↗
arXiv2008

Size Dependence of Current Spin Polarization Through Superconductor/Ferromagnet Nanocontacts

The spin polarization P of the transport current through the interface between superconducting Al and ferromagnetic Fe is determined by means of Andreev reflection at nanostructured point contacts. We observe a systematic decrease of P with decreasing contact resistance. Our data provide evidence for the reduction of P by spin-orbit scattering and thus establish a link between density-of-states and transport spin polarizations.

cond-mat.mes-hall↗
arXiv2008

Heat Capacity and Magnetic Phase Diagram of the Low-Dimensional Antiferromagnet Y$_2$BaCuO$_5$

A study by specific heat of a polycrystalline sample of the low-dimensional magnetic system Y$_2$BaCuO$_5$ is presented. Magnetic fields up to 14 T are applied and permit to extract the ($T$,$H$) phase diagram. Below $μ_0H^*\simeq2$ T, the Néel temperature, associated with a three-dimensional antiferromagnetic long-range ordering, is constant and equals $T_N=15.6$ K. Above $H^*$, $T_N$ increases linearly with $H$ and a field-induced increase of the entropy at $T_N$ is related to the presence of an isosbestic point at $T_X\simeq20$ K, where all the specific heat curves cross. A comparison is made between Y$_2$BaCuO$_5$ and the quasi-two-dimensional magnetic systems BaNi$_{2}$V$_{2}$O$_{8}$, Sr$_2$CuO$_2$Cl$_2$, and Pr$_2$CuO$_4$, for which very similar phase diagrams have been reported. An effective field-induced magnetic anisotropy is proposed to explain these phase diagrams.

cond-mat.str-el↗
arXiv2008

Critical Scaling of the Magnetization and Magnetostriction in the Weak Itinerant Ferromagnet UIr

The weak itinerant ferromagnet UIr is studied by magnetization and magnetostriction measurements. Critical behavior, which surprisingly extends up to several Tesla, is observed at the Curie temperature $T_C\simeq45$ K and is analyzed using Arrott and Maxwell relations. Critical exponents are found that do not match with any of the well-known universality classes. The low-temperature magnetization $M_s\simeq0.5$ $μ_B \cong const.$ below 3 T rises towards higher fields and converges asymptotically around 50 T with the magnetization at $T_C$. From the magnetostriction and magnetization data, we extract the uniaxial pressure dependences of $T_C$, using a new method presented here, and of $M_s$. These results should serve as a basis for understanding spin fluctuations in anisotropic itinerant ferromagnets.

cond-mat.str-el↗
arXiv2005

Experimental Evidence for Crossed Andreev Reflection

We report on electronic transport properties of mesoscopic superconductor-ferromagnet spin-valve structures. Two ferromagnetic iron leads form planar tunnel contacts to a superconducting aluminum wire, where the distance of the two contacts is of the order of the coherence length of the aluminum. We observe a negative non-local resistance which can be explained by crossed Andreev reflection, a process where an electron incident from one of the leads gets reflected as a hole into the other, thereby creating a pair of spatially separated, entangled particles.

cond-mat.mes-hall↗
arXiv2004

Evidence for crossed Andreev reflection in superconductor-ferromagnet hybrid structures

We have measured the non-local resistance of aluminum-iron spin-valve structures fabricated by e-beam lithography and shadow evaporation. The sample geometry consists of an aluminum bar with two or more ferromagnetic wires forming point contacts to the aluminum at varying distances from each other. In the normal state of aluminum, we observe a spin-valve signal which allows us to control the relative orientation of the magnetizations of the ferromagnetic contacts. In the superconducting state, at low temperatures and excitation voltages well below the gap, we observe a spin-dependent non-local resistance which decays on a smaller length scale than the normal-state spin-valve signal. The sign, magnitude and decay length of this signal is consistent with predictions made for crossed Andreev reflection (CAR).

cond-mat.mes-hall↗
arXiv2004

Electrical transport properties of stable single-atom contacts of zinc

We report low-temperature measurements of the electrical transport properties of few-atom contacts of the superconducting metal zinc, arranged with lithographically fabricated mechanically controllable breakjunctions (MCB). The conductance histogram shows several narrow peaks, not regularly observed for multivalent metals. The first peak - corresponding to the single-atom contact - is located slightly below one conductance quantum and is split into two subpeaks, possibly indicating two preferred configurations of the single-atom contact. The current-voltage characteristics in the superconducting state show nonlinearities due to multiple Andreev reflections (MAR). In addition we find nonlinearities in the current-voltage characteristics that we attribute to electronic-interaction effects due to the granular structure of the film. Some of these latter findings are in qualitative agreement with a prediction by Avishai et al. A. Avishai, A. Golub, A. D. Zaikin, Europhys. Lett. {\bf 54}, 640.

cond-mat.mes-hall↗
arXiv2004

Dimensionality effects on non-equilibrium electronic transport in Cu nanobridges

We report on non-equilibrium electronic transport through normal-metal (Cu) nanobridges coupled to large reservoirs at low temperatures. We observe a logarithmic temperature dependence of the zero-bias conductance, as well as a universal scaling behavior of the differential conductance. Our results are explained by electron-electron interactions in diffusive metals in the zero-dimensional limit.

cond-mat.mes-hall↗
arXiv2002

Low-temperature Conductance Measurements On Single Molecules

An experimental protocol which allows to perform conductance spectroscopy on organic molecules at low temperatures (T~30 K) has been developed. This extends the method of mechanically controlled break junctions which has recently demonstrated to be suitable to contact single molecules at room temperature. The conductance data obtained at low T with a conjugated sample molecule show a highly improved data quality with a higher stability, narrower linewidth and substantially reduced noise. Thus the comparability of experimental data with other measurements as well as with theoretical simulations is considerably improved.

cond-mat.mes-hall↗
arXiv2002

Low temperature specific heat of the heavy fermion superconductor PrOs$_4$Sb$_{12}$

We report the magnetic field dependence of the low temperature specific heat of single crystals of the first Pr-based heavy fermion superconductor PrOs$_4$Sb$_{12}$. The low temperature specific heat and the magnetic phase diagram inferred from specific heat, resistivity and magnetisation provide compelling evidence of a doublet ground state and hence superconductivity mediated by quadrupolar fluctuations. This establishes PrOs$_4$Sb$_{12}$ as a very strong contender of superconductive pairing that is neither electron-phonon nor magnetically mediated.

cond-mat.supr-con↗