arXiv subjects

G. Behr

Publications and source records attributed to G. Behr.

72 records · Page 4Linked to original sources

arXiv2006

Point-contact spectroscopy of the borocarbide superconductor YNi2B2C

Point-contact (PC) spectroscopy measurements on YNi2B2C single crystals in the normal and superconducting (SC) state (T_c=15.4K) for the main crystallographic directions are reported. The PC study reveals the electron-phonon interaction (EPI) function with a dominant maximum around 12meV and a further weak structure (kink or shallow broad maximum) at higher energy at about 50meV. Other phonon maxima at 20, 24 and 32meV specified in the phonon DOS of YNi2B2C by neutron measurements [PRB, V.55, 9058 (1997)] are not resolved in the PC spectra pointing out to the main role of the low energy phonon modes in EPI. Directional study of the SC gap results in Δ_[100]=1.5meV for the a- direction and Δ_[001]=2.4meV along the c-axis which may point to anisotropic and/or multiband behavior. Noteworthy, the critical temperature T_c in all cases corresponds to that of bulk samples. The value 2Δ_[001]/kT_c=3.6 is close to the BCS one of 3.52, and the temperature dependence Δ(T) is BCS-like, while for the a-direction Δ(T) deviates from mean-field BCS behavior above T_c/2. The directional variation in Δcan be attributed to the multiband nature of the SC state in YNi2B2C predicted 10 years ago (PRL, V.80, 1730 (1998)).

cond-mat.supr-con↗

arXiv2006

Point-contact spectroscopy of the antiferromagnetic superconductor HoNi2B2C

The point-contact (PC) spectroscopy study of the electron-phonon(boson) interaction (EP(B)I) spectral function in HoNi2B2C reveals phonon maxima at 16 and 22meV and 34meV. For the first time the pronounced high energy maxima at about 50meV and 100meV were resolved. Additionally, an admixture of a crystalline-electric-field (CEF) excitation with a maximum near 10meV and a `magnetic` peak near 3meV are observed. The contribution of the CEF peak in EP(B)I constant λ_{PC} is evaluated as 20-30%, while contribution of the high frequency modes at 50 and 100meV amounts about 10% for each maxima. PC Andreev reflection measurements below the critical temperature T_c=8.5K reveals two different superconducting (SC) states separated by T_c^*=5.6K which is close to the Neel temperature T_N=5.3K. Below T_c^* the gap Δin HoNi2B2C exhibits a standard single-band BCS-like behavior with 2Δ/kT_c^*=3.9. Above T_c^* the gap features in PC spectra are strongly suppressed pointing to a peculiar SC state between T_c^* and T_c, where incommensurate magnetic order develops.

cond-mat.supr-con↗

arXiv2006

Point-contact spectroscopy of the nickel borocarbide superconductors RNi2B2C (R=Y, Dy, Ho, Er, Tm, Lu)

An overview of the recent efforts in point-contact (PC) spectroscopy of the nickel borocarbide superconductors RNi2B2C in the normal and superconducting (SC) state is given. The results of measurements of the PC electron- boson(phonon) interaction spectral function are presented. Phonon maxima and crystalline-electric-field (CEF) excitations are observed in the PC spectra of compounds with R=Dy, Ho, Er and Tm, while for R=Y a dominant phonon maximum around 12 meV is characteristic. Additionally, non-phonon and non-CEF maxima are observed near 3 meV in R=Ho and near 6 meV in R=Dy. Directional PC study of the SC gap gives evidence for the multi-band nature of superconductivity in R=Y, Lu. At low temperature the SC gap in R=Ho exhibits a standard single-band BCS-like dependence, which vanishes above T_c^*= 5.6K< T_c=8.5K, where a specifc magnetic ordering starts to play a role. For R=Tm (T_c=10.5 K) a decrease of the SC gap is observed below 5 K.

cond-mat.supr-con↗

arXiv2006

One- and Two-Triplon Spectra of a Cuprate Ladder

We have performed inelastic neutron scattering on the near ideal spin-ladder compound La4Sr10Cu24O41, which will serve as a model system for the investigation of doped ladders. A key feature in the analysis was the separation of one-triplon and two-triplon scattering contributions due to an antiphase rung modulation. This enabled extraction of the one-triplon dispersion which has a spin gap of 26.4+/-0.3 meV, and for the first time two-triplon scattering is clearly observed. The exchange constants are determined by fitting the data to be Jleg=186 meV and Jrung=124 meV along the leg and the rung respectively. In addition to the surprisingly large difference between Jleg and Jrung a substantial ring exchange of Jcyc=31 meV was confirmed.

cond-mat.str-el↗

arXiv2006

Suppression of ferromagnetism in CeSi_1.81 under temperature and pressure

We have studied the pressure dependence of the magnetization of single crystalline CeSi_1.81. At ambient pressure ferromagnetism develops below T_C = 9.5 Below ~ 5 K an additional shoulder in low-field hysteresis loops and a metamagnetic crossover around 4 T suggest the appearance of an additional magnetic modulation to the ferromagnetic state. The suppression of the magnetic order in CeSi_1.81 as function of temperature at ambient pressure and as function of pressure at low temperature are in remarkable qualitative agreement. The continuous suppression of the ordered moment at p ~ 13.1 kbar suggests the existence of a ferromagnetic quantum critical point in this material.

cond-mat.str-el↗

arXiv2004

A transition from antiferromagnetism to spin-glass freezing in single crystalline, Dy2PdSi3

We report low temperature (1.8-15 K) ac magnetic susceptibility and isothermal remanent magnetization behavior for the single crystalline form of Dy2PdSi3, crystallizing in a AlB2-derived hexagonal structure, for two orientations. The results, while confirming the existence of two magnetic transitions, one at 8 K and the other at 2.5 K, as inferred from previous dc magnetization studies, suggest that the 2.5K-transition is spin-glass-like, with an interesting sensitivity to the application of an external dc magnetic field. Thus, this compound appears to undergo a transition from antiferromagnetism to spin-glass freezing as the temperature is lowered.

cond-mat.str-el↗

arXiv2004

Novel magnetic behavior of single crystalline Er2PdSi3

We report the results of ac and dc magnetic susceptibility (chi) and electrical resistivity (rho) measurements on the single crystals of Er2PdSi3, crystallizing in an AlB2-derived hexagonal structure, for two orientations H//[0001] and H//[2 -1 -1 0]. For H//[0001], there are apparently two magnetic transitions as revealed by the ac chi data, one close to 7 K attributable to antiferromagnetic ordering and the other around 2 K. However, for H // [2 -1 -1 0], we observe additional features above 7 K (near 11 and 23 K) in the plot of low-field chi(T); also, there is no corresponding anomaly in the rho(T) plot. In this respect, the magnetic behavior of this compound is novel, particularly while compared with other members of this series. The features in ac chi respond differently to the application of a small dc magnetic field for the two directions. As far as low temperature (T= 1.8 and 5 K) isothermal magnetization (M) behaviour is concerned, it exhibits meta-magnetic-like features around 2 kOe saturating at high fields for the former orientation, whereas for the latter, there is no saturation even at 120 kOe. The sign of paramagnetic Curie temperature is different for these two directions. Thus, there is a strong anisotropy in the magnetic behavior. However, interestingly, the rho(T) plots are found to be essentially isotropic, with the data revealing possible formation of magnetic superzone formation below 7 K.

cond-mat.str-el↗

arXiv2004

Point-contact investigations of challenging superconductors: two-band MgB2, antiferromagnetic HoNi2B2C, heavy fermion UPd2Al3, paramagnetic MgCNi3

An overview on recent efforts in point-contact (PC) spectroscopy of title superconductors is given. Distinct phonon features and crystalline-electric-field effects are observed in PC spectra of HoNi2B2C. Results of study of superconducting (SC) gap and excess current versus temperature and magnetic field reflecting specific multi-band electronic structure in MgB2 are presented. The nature of the extremely nonlinear I(V) curves in the antiferromagnetic (AF) and SC state are elucidated for UPd2Al3 break-junctions and MgCNi3 point contacts.

cond-mat.supr-con↗

arXiv2003

Unconventional superconductivity and normal state properties of epsilon-iron at high pressure

Following the discovery of superconductivity in epsilon-iron, subsequent experiments hinted at non-Fermi liquid behaviour of the normal phase and sensitive dependence of the superconducting state on disorder, both signatures of unconventional pairing. We report further resistive measurements under pressure of samples of iron from multiple sources. The normal state resistivity of epsilon-iron varied as rho_0+AT^{5/3} at low temperature over the entire superconducting pressure domain. The superconductivity could be destroyed by mechanical work, and was restored by annealing, demonstrating sensitivity to the residual resistivity rho_0. There is a strong correlation between the rho_0 and A coefficients and the superconducting critical temperature T_c. Within the partial resistive transition there was a significant current dependence, with V(I)=a(I-I_0)+bI^2, with a >> b, possibly indicating flux-flow resistivity, even in the absence of an externally applied magnetic field.

cond-mat.supr-con↗

arXiv2003

Anisotropic spin-glass-like and quasi-one-dimensional magnetic behaviour in an intermetallic compound, Tb2PdSi3

We report temperature dependent ac susceptibility ($χ$) measurements on a high quality single crystal of Tb$_2$PdSi$_3$, crystallizing in a AlB$_2$-derived hexagonal structure. This compound is found to exhibit features attributable to quasi-low-dimensional magnetism at high temperatures and {\it anisotropic} spin-glass-like behavior at low temperatures ($<$ 10 K) with an unusually large frequency dependence of peak temperature in ac $χ$(T). This compound thus presents a novel situation in {\it metallic magnetism}, considering that the former phenomenon is normally encountered only among insulators, whereas the anisotropic spin-glass-like behavior, to our knowledge, has not been known in stoichiometric intermetallic compounds.

cond-mat.str-el↗

arXiv2002

Study of the electron-phonon interaction in metal diborides MeB_2 (Me=Zr, Nb, Ta, Mg) by point-contact spectroscopy

We review investigations of the electron-phonon interaction (EPI) in metal diborides MeB_2 (Me=Zr, Nb, Ta, Mg) by point-contact (PC) spectroscopy. For transition metal compounds the PC EPI functions were recovered and EPI parameter $λ\lesssim 0.1$ were estimated. The data are consistent with the measured surface phonon dispersion curves. The low $λ$ value questions some reports about superconductivity in these compounds. Contrary, EPI in superconducting MgB_2 films manifests also in the PC spectra itself by virtue of an elastic EPI contribution to the excess current determined by the energy dependence of the superconducting order parameter. To analyse the phonon features in the PC spectra of MgB$_2$ a two-band model is exploited and the proximity effect in the {\bf k}-space is suggested.

cond-mat.supr-con↗

arXiv2002

Electron-phonon interaction in transition metal diborides TB_2 (T=Zr, Nb, Ta) studied by point-contact spectroscopy

The electron-phonon interaction (EPI) in transition metal diborides TB_2 (T=Zr, Nb, Ta) is investigated by point-contact (PC) spectroscopy. The PC EPI functions were recovered and the EPI parameters lambda<0.1 were estimated for all three compounds. Common and distinctive features between the EPI functions for those diborides are discussed also in connection with the superconductivity in MgB_2.

cond-mat.supr-con↗

arXiv2002

Crystal growth of MgB2 from Mg-Cu-B melt flux and superconducting properties

A new method for preparation of single crystals of the superconducting intermetallic MgB2 compound from a Mg-Cu-B melt flux is presented. The high vapour pressure of Mg at elevated temperature is a serious challenge of the preparation process. The approximate thermodynamic calculations of the ternary Mg-Cu-B phase diagram show a beneficial effect of Cu, which extends the range of formation of MgB2 to lower temperatures. Within the as-solidified Mg-Cu-B melt flux the MgB2 compound forms plate-like single crystals up to a size of 0.2 x 0.2 x 0.05 mm3 or alternatively rims peritectically grown around MgB4 particles. AC-susceptibility measurements were conducted with specimen selected from different parts of the as-solidified flux containing MgB2 particles. Peritectically formed MgB2-particles display the highest transition temperature of Tc = 39.2 K and a relatively narrow transition width of DTc = 1.3 K. Other sections of the sample exhibit various superconducting transitions from Tc = 39 K to 7.2 K. This variation of Tc is attributed to a finite homogeneity range of the MgB2 compound whereas significant Cu solid solubility in MgB2 can be excluded.

cond-mat.supr-con↗

arXiv2002

Magnetic behavior of single crystalline Ho$_2$PdSi$_3$

The magnetic behavior of single-crystal Ho$_2$PdSi$_3$, crystallizing in an AlB$_2$-derived hexagonal structure, is investigated by magnetic susceptibility ($χ$) and electrical resistivity ($ρ$) measurements along two directions. There is no dramatic anisotropy in the high temperature Curie-Weiss parameter or in the $ρ$ and isothermal magnetization data, though there is a noticeable anisotropy in the magnitude of $ρ$ between two perpendicular orientations. The degree of anisotropy is overall less prominent than in the Gd (which is an S-state ion!) and Tb analogues. A point of emphasis is that this compound undergoes long range magnetic ordering below 8 K as in the case of analogous Gd and Dy compounds. Considering this fact for these compounds with well-localised f-orbital, the spin glass freezing noted for isomorphous U compounds in the recent literature could be attributed to the role of the f-ligand hybridization, rather than just Pd-Si disorder.

cond-mat.str-el↗

arXiv2002

Superconductivity of epsilon-Fe: complete resistive transition

Last year, iron was reported to become superconducting at temperatures below 2K and pressures between 15 and 30 GPa. The evidence presented was a weak resistivity drop, suppressed by a magnetic field above 0.2 T, and a small Meissner signal. However, a compelling demonstration, such as the occurrence of zero resistance, was lacking. Here we report the measurement of a complete resistive transition at 22.2 GPa with an onset slightly above 2 K in two very pure samples of iron, of different origins. The superconductivity appears unusually sensitive to disorder, developing only when the electronic mean free path is above a threshold value, while the normal state resistivity is characteristic of a nearly ferromagnetic metal.

cond-mat.supr-con↗

arXiv2001

Electronic structure and weak electron-phonon coupling in TaB$_2$

We present electronic structure calculations together with resistivity, susceptibility, and specific heat measurements for TaB$_2$ to search for the recently contradictorily reported superconductivity and to study related normal state properties. We ascribe the absence of superconductivity down to 1.5 K for our TaB$_2$ samples to the generally weak electron phonon coupling derived from comparison of the calculated and measured specific heat constants. For the E$_{2g}$ and the B$_{1g}$ $Γ$ point phonons we derive from the calculated deformation potentials very small electron phonon couplings for these modes, opposite to the strong coupling of the E$_{2g}$ mode in MgB$_2$, probably responsible for its high $T_c$. In comparison to MgB$_2$, we discuss the origin of the quite different features in the density of states and of the Fermi surfaces. The differences are mainly due to the strong hybridization between Ta 5$d$ and B 2$p$ states outside the hexagonal basis plane.

cond-mat.supr-con↗

arXiv2000

Electron-Phonon Coupling Origin of the resistivity in YNi_{2}B_{2}C Single Crystals

Resistivity measurements from 4.2 K up to 300 K were made on YNi_{2}B_{2}C single crystals with Tc=15.5 K. The resulting rho(T) curve shows a perfect Bloch-Grueneisen (BG) behavior, with a very small residual resistivity which indicates the low impurity content and the high cristallographic quality of the samples. The value lambda_{tr}=0.53 for the transport electron-phonon coupling constant was obtained by using the high-temperature constant value of d(rho)/dT and the plasma frequency reported in literature. The BG expression for the phononic part of the resistivity rho_{ph}(T) was then used to fit the data in the whole temperature range, by approximating alpha^{2}_{tr}F(Omega) with the experimental phonon spectral density G(Omega) multiplied by a two-step weighting function to be determined by the fit. The resulting fitting curve perfectly agrees with the experimental points. We also solved the real-axis Eliashberg equations in both s- and d-wave symmetries under the approximation alpha^{2}F(Omega)= alpha^{2}_{tr}F(Omega). We found that the value of lambda_{tr} here determined in single-band approximation is quite compatible with Tc and the gap Delta experimentally observed. Finally, we calculated the normalized tunneling conductance, whose comparison with break-junction tunnel data gives indication of the possible s-wave symmetry for the order parameter in YNi_{2}B_{2}C.

cond-mat.supr-con↗

arXiv1999

Resistivity and electron-phonon coupling in YNi$_2$B$_2$C single crystals

In this work, we present the results of precise measurements of the resistivity of YNi$_2$B$_2$C single crystals with $T_{\rm c}$ = 15.5 K as a function of the temperature, and analyze the experimental data in the framework of the Bloch-Grüneisen theory and electron-phonon coupling. The transport electron-phonon spectral function that best fits the resistivity data is then inserted in the real-axis Eliashberg equations, which are directly solved to determine the normalized tunneling conductance both in s- and d-wave symmetry.

cond-mat.supr-con↗