arXiv subjects

H. Winkler

Publications and source records attributed to H. Winkler.

29 records · Page 2Linked to original sources

arXiv2015

Destruction of Kondo effect in cubic heavy fermion compound Ce3Pd20Si6

How ground states of quantum matter transform between one another reveals deep insights into the mechanisms stabilizing them. Correspondingly, quantum phase transitions are explored in numerous materials classes, with heavy fermion compounds being among the most prominent ones. Recent studies in an anisotropic heavy fermion compound have shown that different types of transitions are induced by variations of chemical or external pressure [1-3], raising the question of the extent to which heavy fermion quantum criticality is universal. To make progress, it is essential to broaden both the materials basis and the microscopic parameter variety. Here, we identify a cubic heavy fermion material as exhibiting a field-induced quantum phase transition, and show how the material can be used to explore one extreme of the dimensionality axis. The transition between two different ordered phases is accompanied by an abrupt change of Fermi surface, reminiscent of what happens across the field-induced antiferromagnetic to paramagnetic transition in the anisotropic YbRh2Si2. This finding leads to a materials-based global phase diagram -- a precondition for a unified theoretical description.

cond-mat.str-el↗

arXiv2014

Thermopower enhancement by encapsulating cerium in clathrate cages

The increasing worldwide energy consumption calls for the design of more efficient energy systems. Thermoelectrics could be used to convert waste heat back to useful electric energy if only more efficient materials were available. The ideal thermoelectric material combines high electrical conductivity and thermopower with low thermal conductivity. In this regard, the intermetallic type-I clathrates show promise with their exceedingly low lattice thermal conductivities [1]. Here we report the successful incorporation of cerium as guest atom into the clathrate crystal structure. In many simpler intermetallic compounds, this rare earth element is known to lead, via the Kondo interaction, to strong correlation phenomena including the ocurrence of giant thermopowers at low temperatures [2]. Indeed, we observe a 50% enhancement of the thermopower compared to a rare earth-free reference material. Importantly, this enhancement occurs at high temperatures and we suggest that a `rattling' enhanced Kondo interaction [3] underlies this effect.

cond-mat.str-el↗

arXiv2012

Anisotropic optical conductivity of the putative Kondo insulator CeRu$_4$Sn$_6$

Kondo insulators and in particular their non-cubic representatives have remained poorly understood. Here we report on the development of an anisotropic energy pseudogap in the tetragonal compound CeRu$_4$Sn$_6$ employing optical reflectivity measurements in broad frequency and temperature ranges, and local density approximation plus dynamical mean field theory calculations. The calculations provide evidence for a Kondo insulator-like response within the $a-a$ plane and a more metallic response along the c axis and qualitatively reproduce the experimental observations, helping to identify their origin.

cond-mat.str-el↗

arXiv2012

High-spin low-spin transition

Temperature dependent nuclear inelastic-scattering (NIS) of synchrotron radiation was applied to investigate both spin states of the spin-crossover complex [Fe(tpa)(NCS)(2)] (tpa = tris(2-pyridylmethyl)amine). A remarkable increase of the iron-ligand bond stretching upon spin crossover has unambiguously been identified by comparing the measured NIS spectra with theoretical simulations based on density-functional calculations.

physics.chem-ph↗

arXiv2012

Interpretation of Mössbauer spectra in the energy and time domain with neural networks

An artificial neural network for extracting reasonable and fast estimates of hyperfine parameters from Mössbauer spectra in the energy or time domain is outlined. First promising results for determining the asymmetry of the electric field gradient at the nucleus of a diamagnetic iron center as derived with different types of neural networks are reported.

physics.chem-ph↗

arXiv2012

Pressure-induced changes of the vibrational modes of spin-crossover complexes studied by nuclear resonance scattering of synchrotron radiation

Nuclear inelastic scattering (NIS) spectra were recorded for the spin-crossover complexes STP and ETP (STP = [Fe(1,1,1-trisf[N-(2-pyridylmethyl)-N-methylamino]methylg- ethane)](ClO4)2 and ETP = [Fe(1,1,1-trisf[N-(2-pyridylmethyl)-N-methylamino]methylg-butane)](ClO4)2) at 30 K and at room temperature and also at ambient pressure and applied pressure (up to 2.6 GPa). Spin transition from the high-spin (HS) to the low-spin (LS) state was observed by lowering temperature and also by applying pressure at room temperature and has been assigned to the hardening of iron-bond stretching modes due to the smaller volume in the LS isomer.

physics.chem-ph↗

arXiv2012

Mössbauer, nuclear inelastic scattering and density functional studies on the second metastable state of Na2[Fe(CN)5NO]$\cdot$2H2O

The structure of the light-induced metastable state SII of Na2[Fe(CN)5NO]$\cdot$2H2O 14 was investigated by transmission Mössbauer spectroscopy (TMS) in the temperature range 15 between 85 and 135 K, nuclear inelastic scattering (NIS) at 98 K using synchrotron 16 radiation and density functional theory (DFT) calculations. The DFT and TMS results 17 strongly support the view that the NO group in SII takes a side-on molecular orientation 18 and, further, is dynamically displaced from one eclipsed, via a staggered, to a second 19 eclipsed orientation. The population conditions for generating SII are optimal for 20 measurements by TMS, yet they are modest for accumulating NIS spectra. Optimization 21 of population conditions for NIS measurements is discussed and new NIS experiments on 22 SII are proposed.

physics.chem-ph↗

arXiv2010

Zeros of nonpositive type of generalized Nevanlinna functions with one negative square

A generalized Nevanlinna function $Q(z)$ with one negative square has precisely one generalized zero of nonpositive type in the closed extended upper halfplane. The fractional linear transformation defined by $Q_τ(z)=(Q(z)-τ)/(1+τQ(z))$, $τ\in \mathbb{R} \cup \{\infty\}$, is a generalized Nevanlinna function with one negative square. Its generalized zero of nonpositive type $α(τ)$ as a function of $τ$ defines a path in the closed upper halfplane. Various properties of this path are studied in detail.

math.FA↗

arXiv2007

Testing thermal reprocessing in AGN accretion discs

The thermal reprocessing hypothesis in AGN, where EUV/X-ray photons are reprocessed by the accretion disc into optical/UV photons, predicts wavelength-dependent time delays between the optical continuum at different wavelengths. Recent photometric monitoring by Sergeev et al. has shown that the time-delay is observed in 14 AGN, and generally seen to increase with increasing wavelength, as predicted in the reprocessing scenario. We fit the observed time delays and optical spectral energy distribution using a disc reprocessing model. The model delivers estimates for the nuclear reddening, the product of black hole mass times accretion rate, and the distance to each object. However, the distances at face value give H_0 = 44 +/- 5 km/s/Mpc - a factor of 1.6 smaller than generally accepted. We discuss the implications of this on the reprocessing model.

astro-ph↗

arXiv2000

UBVRI photometry of Southern Sky BL Lacs

Seventeen southern sky BL Lacs were observed in UBVRI using the CCD Camera on the 1.0m telescope at the South African Astronomical Observatory (SAAO) in Aug and Nov 1999. The analyses of all the seventeen sources are now complete, and are available via anonymous ftp (ftp pukrs1.puk.ac.za/pub/Blazars). A few examples of our results are however given in this paper. Whereas PKS 2005-489 and PKS 2155-304 apear to have been in a high state, PKS 0048-097 and PKS 0521-365 showed evidence of variability on a time-scale of a few days, with the amplitude of variability increasing towards short wavelengths. This is consistent with observations of gamma-ray BL Lacs, which show similar behaviour in optical and X-rays.

astro-ph↗

arXiv2000

Limits on magnetic field strength of the extended nebula of PSR B1706-44 from optical, X-ray and TeV observations

The detection of unpulsed TeV emission from PSR B1706-44 raised the question if this VHE emission results from inverse Compton scattering of multi-TeV electrons on the CMBR and other soft photon fields. This hints at the existence of an unseen synchrotron nebula, which is larger than the observed 1 arcminute compact X-ray nebula. In a search for synchrotron counterpart of the TeV nebula, we have taken V-band CCD images of the region around the PSR B1706-44 pulsar position. By rejecting all point source contributions down to the diffuse sky limit, we were able to search for such extended plerionic synchrotron emission on scale sizes limited by the angular resolution of the CANGAROO imaging TeV observations. By combining the V-band flux limits with the observed TeV flux, we were able to obtain upper limits for the magnetic field strength as a function of the radius of the assumed TeV nebula. Assuming that the 2 arcminute radio plerion also defines the size of the TeV nebula, we constrain the steady state particle spectral index to the values between 4 and 5.5, with a field strength limit of about 3x10^-6 G. This compares with an index of 3.6+/-0.6 inside the 1 arcminute compact X-ray nebula.

astro-ph↗