arXiv subjects

A. V. Sologubenko

Publications and source records attributed to A. V. Sologubenko.

At least 19 recordsLinked to original sources

arXiv2009

Spinon localization in the heat transport of the spin-1/2 ladder compound (C$_5$H$_{12}$N)$_2$CuBr$_4$

We present experiments on the magnetic field-dependent thermal transport in the spin-1/2 ladder system (C$_5$H$_{12}$N)$_2$CuBr$_4$. The thermal conductivity $κ(B)$ is only weakly affected by the field-induced transitions between the gapless Luttinger-liquid state realized for $B_{c1}< B < B_{c2}$ and the gapped states, suggesting the absence of a direct contribution of the spin excitations to the heat transport. We observe, however, that the thermal conductivity is strongly suppressed by the magnetic field deeply within the Luttinger-liquid state. These surprising observations are discussed in terms of localization of spinons within finite ladder segments and spinon-phonon umklapp scattering of the predominantly phononic heat transport.

cond-mat.str-el↗

arXiv2008

Large thermomagnetic effects in weakly disordered Heisenberg chains

The interplay of different scattering mechanisms can lead to novel effects in transport. We show theoretically that the interplay of weak impurity and Umklapp scattering in spin-1/2 chains leads to a pronounced dip in the magnetic field dependence of the thermal conductivity $κ$ at a magnetic field $B \sim T$. In sufficiently clean samples, the reduction of the magnetic contribution to heat transport can easily become larger than 50% and the effect is predicted to exist even in samples with a large exchange coupling, J >> B, where the field-induced magnetization is small. Qualitatively, our theory might explain dips at $B \sim T$ observed in recent heat transport measurements on copper pyrazine dinitrate, but a fully quantitative description is not possible within our model.

cond-mat.str-el↗

arXiv2008

Magneto-thermal evidence of a partial gap at the Fermi surface of UPt_2Si_2

Motivated by the observation of a giant Nernst effect in URu_2Si_2, the thermoelectric response of the related system UPt_2Si_2 was investigated using thermal and electric transport properties such as the Nernst and Seebeck effects, thermal conductivity, Hall effect and electrical resitivity. Unlike URu_2Si_2, UPt_2Si_2 is neither superconducting nor exhibits a ``hidden-order'' state. Nevertheless a pronounced Nernst effect anomaly is found to coincide with the onset of the antiferromagnetic order in UPt_2Si_2. Although the absolute values are substantially lower, its appearance and characteristics can favorably be compared to the giant Nernst effect in URu_2Si_2 indicating the common feature of a partial Fermi surface gap.

cond-mat.str-el↗

arXiv2008

Field-dependent thermal transport in the Haldane chain compound NENP

We present a study of the magnetic field-dependent thermal transport in the spin S=1 chain material Ni(C2H8N2)2NO2(ClO4) (NENP). The measured thermal conductivity is found to be very sensitive to the field-induced changes in the spin excitation spectrum. The magnetic contribution to the total heat conductivity is analyzed in terms of a quasiparticle model, and we obtain a temperature and momentum independent mean free path. This implies that the motion of quasiparticles is effectively three dimensional despite the tiny interchain coupling.

cond-mat.str-el↗

arXiv2007

Nernst Effect in NdBa_2[Cu_{1-y}Ni_y]_3O_{7-δ}

In NdBa_2[Cu_{1-y}Ni_y]_3O_{7-δ}, magnetic Ni-impurities suppress Tc but at the same time the pseudogap is strongly enhanced. This unique feature makes it an ideal system to study possible relations between the anomalous Nernst effect, superconductivity and the pseudogap. We present Nernst effect measurements on a series of optimally doped (O_7) and underdoped (O_{6.8}) samples with Ni contents ranging from y=0 to 0.12. In all samples an onset of the Nernst signal is found at T^ν> Tc. For the optimally doped samples T^νand Tc decrease simultaneously with increasing Ni content. The underdoped samples show a different behavior, i.e. the onset of the Nernst signal is hardly affected by increasing the Ni content from y=0 to 0.03. Irrespective of the oxygen content, T^νclearly does not track the enhanced pseudogap temperature T*.

cond-mat.supr-con↗

arXiv2007

Magnetothermal transport in the spin-1/2 chains of copper pyrazine dinitrate

We present experiments on the thermal transport in the spin-1/2 chain compound copper pyrazine dinitrate Cu(C_4 H_4 N_2)(NO_3)_2. The heat conductivity shows a surprisingly strong dependence on the applied magnetic field B, characterized at low temperatures by two main features. The first one appearing at low B is a characteristic dip located at mu_B B ~ k_B T, that may arise from Umklapp scattering. The second one is a plateau-like feature in the quantum critical regime, mu_B |B-B_c| < k_B T, where B_c is the saturation field at T=0. The latter feature clearly points towards a momentum and field independent mean free path of the spin excitations, contrary to theoretical expectations.

cond-mat.str-el↗

arXiv2006

Thermal conductivity of Al-doped MgB2: Impurity scattering and the validity of the Wiedemann-Franz law

We report data on the thermal conductivity K(T,H) along the basal plane of the hexagonal crystal structure of superconducting Mg{1-y}Al{y}B2 with y=0.02 and 0.07 at temperatures between 0.5 and 50 K and in external magnetic fields between 0 and 70 kOe. The substitution of Al for Mg leads to a substantial reduction of the heat transport via electronic quasiparticles. The analysis of the K(T,H) data implies that the Al impurities provoke an enhancement of the intraband scattering rate, almost equal in magnitude for both the sigma- and the pi band of electronic excitations. This is in contrast with conclusions drawn from analogous data sets for material in which carbon replaces boron and where mainly the intraband scattering rate of the sigma band is enhanced. Our complete data set, including additional results of measurements of the low-temperature thermal conductivity of pure MgB2, confirms the validity of the Wiedemann-Franz law for both pure and doped MgB2.

cond-mat.supr-con↗

arXiv2006

Thermal conductivity via magnetic excitations in spin-chain materials

We discuss the recent progress and the current status of experimental investigations of spin-mediated energy transport in spin-chain and spin-ladder materials with antiferromagnetic coupling. We briefly outline the central results of theoretical studies on the subject but focus mainly on recent experimental results that were obtained on materials which may be regarded as adequate physical realizations of the idealized theoretical model systems. Some open questions and unsettled issues are also addressed.

cond-mat.str-el↗

arXiv2006

Spin thermal conductivity of the Haldane chain compound Y2BaNiO5

We have measured the thermal conductivity of the spin S=1 chain compound Y2BaNiO5. Analyzing the anisotropy of the thermal transport allows to identify a definite spin-mediated thermal conductivity κ_s along the chain direction. The calculated spin-related energy diffusion constant D_E(T) shows a broad peak around 120 K. Close to room temperature, D_E(T) approaches the theoretically predicted high-temperature value, while scattering of spin excitations by magnetic impurities seems to be the major limiting factor of κ_s at low temperature.

cond-mat.str-el↗

arXiv2004

Influence of carbon substitution on the heat transport in single crystalline MgB2

We report data on the thermal conductivity κ(T,H) in the basal plane of hexagonal single-crystalline and superconducting Mg(B_{1-x}C_x)_2 (x= 0.03, 0.06) at temperatures between 0.5 and 50 K, and in external magnetic fields H between 0 and 50 kOe. The substitution of carbon for boron leads to a considerable reduction of the electronic heat transport, while the phonon thermal conductivity seems to be much less sensitive to impurities. The introduction of carbon enhances mostly the intraband scattering in the σ-band. In contrast to the previously observed anomalous behavior of pure MgB$_2$, the Wiedemann-Franz law is valid for Mg(B_0.94 C_0.06)_2 at low temperatures.

cond-mat.supr-con↗

arXiv2004

Anisotropic field dependence of the magnetic transition in Cu2Te2O5Br2

We present the results of measurements of the thermal conductivity of Cu2Te2O5Br2, a compound where tetrahedra of Cu^{2+} ions carrying S=1/2 spins form chains along the c-axis of the tetragonal crystal structure. The thermal conductivity kappa was measured along both the c- and the a-direction as a function of temperature between 3 and 300 K and in external magnetic fields H up to 69 kOe, oriented both parallel and perpendicular to the c-axis. Distinct features of kappa(T) were observed in the vicinity of T_N=11.4 K in zero magnetic field. These features are unaltered in external fields which are parallel to the c-axis, but are more pronounced when a field is applied perpendicularly to the c-axis. The transition temperature increases upon enhancing the external field, but only if the field is oriented along the a-axis.

cond-mat.str-el↗

arXiv2003

Ambivalence of the anisotropy of the vortex lattice in an anisotropic type-II superconductor

We present a geometry-based discussion of possible vortex configurations in the mixed state of anisotropic type-II superconductors. It is shown that, if energy considerations assign six nearest neighbors to each vortex, two distinct modifications of the vortex lattice are possible. It is expected that certain conditions lead to a first order phase transition from one modification of the vortex lattice to the other upon varying the external magnetic field.

cond-mat.supr-con↗

arXiv2003

Diffusive energy transport in the S=1 Haldane chain compound AgVP2S6

We present the results of measurements of the thermal conductivity $κ$ of the spin S=1 chain compound AgVP_2S_6 in the temperature range between 2 and 300 K and with the heat flow directed either along or perpendicular to the chain direction. The analysis of the anisotropy of the heat transport allowed for the identification of a small but non-negligible magnon contribution $κ_m$ along the chains, superimposed on the dominant phonon contribution $κ_ph$. At temperatures above about 100 K the energy diffusion constant D_E(T), calculated from the $κ_m(T)$ data, exhibits similar features as the spin diffusion constant D_S(T), previously measured by NMR. In this regime, the behaviour of both transport parameters is consistent with a diffusion process that is caused by interactions inherent to one-dimensional S=1 spin systems.

cond-mat.str-el↗

arXiv2003

Unusual magnetic field-induced phase transition in the mixed state of superconducting NbSe2

The thermal conductivity κin the basal plane of single-crystalline hexagonal NbSe2 has been measured as a function of magnetic field H, oriented both along and perpendicular to the c-axis, at several temperatures below T_c. With the magnetic field in the basal plane and oriented parallel to the heat flux we observed, in fields well below H_c2, an unexpected hysteretic behavior of κ(H) with all the generic features of a first order phase transition. The transition is not manifest in the κ(H) curves, if H is still in the basal plane but oriented perpendicularly to the heat-flux direction. The origin of the transition is not yet understood.

cond-mat.supr-con↗

arXiv2003

Universal behavior of spin-mediated energy transport in S=1/2 chain cuprates: BaCu2Si2O7 as an example

The thermal conductivity of the spin-1/2 chain cuprate BaCu2Si2O7 was measured along different crystallographic directions in the temperature region between 0.5 and 300 K. The thermal conductivity along the chain direction considerably exceeds that along perpendicular directions. Near the antiferromagnetic transition at T_N = 9.2 K the data indicates enhanced scattering of phonons by critical fluctuations in the spin system. A comparison of the data above T_N with available results on similar materials reveals similarities in the main features of the temperature dependence of the mean free path of itinerant spin excitations. This universal behavior is most likely caused by the spin-lattice interaction.

cond-mat.str-el↗

arXiv2002

Thermal conductivity of single crystalline MgB_2

The ab-plane thermal conductivity $κ$ of single-crystalline hexagonal MgB_2 has been measured as a function of magnetic field H with orientations both parallel and perpendicular to the c-axis and at temperatures between 0.5 and 300 K. In the mixed state, $κ(H)$ measured at constant temperatures reveals features that are not typical for common type-II superconductors. The observed behavior may be associated with the field-induced reduction of two superconducting energy gaps, significantly different in magnitude. A nonlinear temperature dependence of the electronic thermal conductivity is observed in the field-induced normal state at low temperatures. This behavior is at variance with the Wiedemann-Franz law, and suggests an unexpected instability of the electronic subsystem in the normal state at T ~ 1 K.

cond-mat.supr-con↗

arXiv2002

Temperature dependence and anisotropy of the bulk upper critical field H_c2 of MgB_2

The bulk upper critical field H_{c2}(T) of superconducting MgB_2 and its anisotropy are established by analyzing experimental data on the temperature and magnetic field dependences of the ab-plane thermal conductivity of a single-crystalline sample in external magnetic fields oriented both parallel (H_{c2}^c) and perpendicular (H_{c2}^{ab}) to the c axis of the hexagonal lattice. From numerical fits we deduce the anisotropy ratio γ_0 = H_{c2}^{ab}(0) / H_{c2}^c(0)=4.2 at T=0 K. Both the values and the temperature dependences of H_{c2}^c and H_{c2}^{ab} are distinctly different from previous claims based on measurements of the electrical resistivity.

cond-mat.supr-con↗

arXiv2001

Thermal and electrical transport in single crystalline MgB_2

We present the results of measurements of the thermal and electrical conductivity in the basal plane of single-crystalline MgB_2 between 2 and 300 K. The analysis of the temperature dependence of the thermal conductivity gives supporting evidence for two different gaps on different sheets of the Fermi surface in the superconducting state. The zero-temperature values of the two gaps are D_1 (0) = 5.3 meV and D_2 (0) = 1.65 meV.

cond-mat.supr-con↗