arXiv subjects

R. Klingeler

Publications and source records attributed to R. Klingeler.

At least 55 records · Page 3Linked to original sources

arXiv2017

Electronic structure and magnetic properties of the strong-rung spin-1 ladder compound Rb$_3$Ni$_2$(NO$_3$)$_7$

Small single crystals of Rb$_3$Ni$_2$(NO$_3$)$_7$ were obtained by crystallization from anhydrous nitric acid solution of rubidium nitrate and nickel nitrate hexahydrate. The basic elements of the crystal structure of this new compound are isolated spin-1 two-leg ladders of Ni$^{2+}$-ions connected by (NO$_3$)$^-$ groups. The experimental data show the absence of long range magnetic order at T $\geq 2$~K. LDA+U calculations and the detailed analysis of the experimental data, i.e. of the magnetic susceptibility, the specific heat in magnetic fields up to 9~T, the magnetization, and of the high-frequency electron spin resonance data, enable quantitative estimates of the relevant parameters of the $S=1$ ladders in Rb$_3$Ni$_2$(NO$_3$)$_7$ . The rung-coupling $J_1 = 10.5$~K, the leg-coupling $J_2=1.6$~K, and the uniaxial anisotropy $|A| = 179$~GHz are obtained. The scenario of spin liquid quantum ground state is further corroborated by quantum Monte Carlo simulations of the magnetic susceptibility.

cond-mat.mtrl-sci↗

arXiv2016

Hollow carbon sphere/metal oxide nanocomposite anodes for lithium-ion batteries

Hollow carbon spheres (HCS) covered with metal oxide nanoparticles (SnO2 and MnO2, respectively) were successfully synthesized and investigated regarding their potential as anode materials for lithium-ion batteries. Raman spectroscopy shows a high degree of graphitization for the HCS host structure. The mesoporous nature of the nanocomposites is confirmed by Brunauer-Emmett-Teller analysis. For both metal oxides under study, the metal oxide functionalization of HCS yields a significant increase of electrochemical performance. The charge capacity of HCS/SnO2 is 370 mAh/g after 45 cycles (266 mAh/g in HCS/MnO2) which clearly exceeds the value of 188 mAh/g in pristine HCS. Remarkably, the data imply excellent long term cycling stability after 100 cycles in both cases. The results hence show that mesoporous HCS/metal oxide nanocomposites enable exploiting the potential of metal oxide anode materials in Lithium-ion batteries by providing a HCS host structure which is both conductive and stable enough to accommodate big volume change effects.

cond-mat.mtrl-sci↗

arXiv2016

Microwave-assisted hydrothermal synthesis of NH4V3O8 microcrystals with controllable morphology

Water-free NH4V3O8 microcrystals have been successfully synthesized by a microwave-assisted hydrothermal method. The products were characterized by means of X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, thermal gravimetric analysis, cyclic voltammetry, and galvanostatic cycling. The results show phase-pure products whose particle size and morphology can be tailored by varying the reaction conditions, i.e., reaction temperature, synthesis duration, and initial pH value. For instance, at low pH (2.5 to 3), flower-like agglomerates with primary particles of 20 to 30 microm length are found, while at pH = 5.5 single microplates with hexagonal outline (30 to 40 microm) prevail. The sample with the comparably highest specific surface area (11 m2/g) was studied regarding its electrochemical performance. It shows an extraordinary initial discharge capacity of 378 mA h g-1 at 10 mA g-1, which corresponds to the intercalation of 4.2 Li+/f.u.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic anisotropy and the phase diagram of chiral MnSb2O6

The magnetic phase diagram and low-energy magnon excitations of structurally and magnetically chiral MnSb2O6 are reported. The specific heat and the static magnetization are investigated in magnetic fields up to 9 and 30 T, respectively, while the dynamicmagnetic properties are probed by X-band as well as tunable high-frequency electron spin-resonance spectroscopy. Below TN = 11.5 K, we observe antiferromagnetic resonance modes which imply small but finite planar anisotropy showing up in a zero-field splitting of 20 GHz. The data are well described by means of an easy-plane two-sublattice model with the anisotropy field BA = 0.02 T. The exchange field BE = 13 T is obtained from the saturation field derived from the pulsed-field magnetization. A crucial role of the small anisotropy for the spin structure is reflected by competing antiferromagnetic phases appearing, at T = 2 K, in small magnetic fields at BC1 $\approx$ 0.5 T and BC2 = 0.9 T. We discuss the results in terms of spin reorientation and of small magnetic fields favoring helical spin structure over the cycloidal ground state which, at B = 0, is stabilized by the planar anisotropy. Above TN, short-range magnetic correlations up to >=60 K and magnetic entropy changes well above TN reflect the frustrated triangular arrangement of Mn2+ ions in MnSb2O6.

cond-mat.str-el↗

arXiv2016

Nematicity in LaFeAsO$_{1-x}$F$_x$

Orbital ordering has recently emerged as another important state in iron based superconductors, and its role for superconductivity as well as its connection to magnetic order and orthorhombic lattice distortion are heavily debated. In order to search for signatures of this so-called nematic phase in oxypnictides, we revisit the normal state properties of the pnictide superconductor LaFeAsO$_{1-x}$F$_x$ with a focus on resistivity, Nernst effect, thermal expansion, and $^{75}$As NMR data. The transport properties at the underdoped level $x=0.05$ exhibit pronounced anomalies at about the same temperature where undoped LaFeAsO develops long-range nematic ordering, i.e. at about 160 K. Furthermore, the $^{75}$As-NMR spin-lattice relaxation rate $1/(T_1T)$ reveals a progressive slowing down of spin fluctuations. Yet, long-range magnetic order and also a detectable orthorhombic lattice distortion are absent. Thus, we conclude from the data that short-range orbital-nematic ordering or a slowly fluctuating form of it sets in near 160 K. Remarkably, all anomalies in the transport and also the indications of slow spin fluctuations disappear close to optimal doping $x=0.1$ which suggests that in LaFeAsO$_{1-x}$F$_x$ the nematic phase actually competes with superconductivity.

cond-mat.supr-con↗

arXiv2014

Specific heat of K071Na0.29Fe2As2 at very low temperatures

A commercially available calorimeter has been used to investigate the specific heat of a high-quality \kn\ single crystal. The addenda heat capacity of the calorimeter is determined in the temperature range $0.02 \, \mathrm{K} \leq T \leq 0.54 \, \mathrm{K}$. The data of the \kn\ crystal imply the presence of a large $T^2$ contribution to the specific heat which gives evidence of $d$-wave order parameter symmetry in the superconducting state. To improve the measurements, a novel design for a calorimeter with a paramagnetic temperature sensor is presented. It promises a temperature resolution of $ΔT \approx 0.1 \, \mathrm{μK}$ and an addenda heat capacity less than $200 \, \mathrm{pJ/K}$ at $ T < 100 \, \mathrm{mK}$.

cond-mat.supr-con↗

arXiv2014

Unusual spin fluctuations and magnetic frustration in olivine and non-olivine LiCoPO$_4$ detected by $^{31}$P and $^7$Li nuclear magnetic resonance

We report $^{31}$P and $^{7}$Li nuclear magnetic resonance (NMR) studies in new non-olivine LiZnPO$_4$-type LiCoPO$_4^\text{tetra}$ microcrystals, where the Co$^{2+}$ ions are tetrahedrally coordinated. Olivine LiCoPO$_4$, which was directly transformed from LiCoPO$_4^\text{tetra}$ by an annealing process, was also studied and compared. The uniform bulk magnetic susceptibility and the $^{31}$P Knight shift obey the Curie-Weiss law for both materials with a high spin Co$^{2+}$ ($3d^7$, $S=3/2$), but the Weiss temperature $Θ$ and the effective magnetic moment $μ_\text{eff}$ are considerably smaller in LiCoPO$_4^\text{tetra}$. The spin-lattice relaxation rate $T_1^{-1}$ reveals a quite different nature of the spin dynamics in the paramagnetic state of both materials. Our NMR results imply that strong geometrical spin frustration occurs in tetrahedrally coordinated LiCoPO$_4$, which may lead to the incommensurate magnetic ordering.

cond-mat.str-el↗

arXiv2014

Weak ferromagnetic spin and charge stripe order in La[sub 5/3]Sr[sub 1/3]NiO[sub 4]

We present magnetization and specific heat data of a La5/3Sr1/3NiO4 single crystal in high magnetic fields. From the charge and spin stripe ordering temperatures, as well as a magnetic low temperature transition, we have constructed the electronic phase diagram for fields up to 14 Tesla. While the charge stripe ordering temperature TCO is independent of the magnetic field, there is a significant shift of the spin stripe ordering temperature TSO of about 1.5 K/ Tesla, if the magnetic fields are applied parallel to the NiO2-planes. The specific heat measurements indicate a large anomalous entropy change at TCO. In contrast, no significant entropy change is observed at the spin stripe transition. The high field magnetization experiments reveal the presence of in-plane weak ferromagnetic moments in the charge stripe ordered phase. From a phenomenological analysis, the magnetic correlation length of these moments is determined. We suggest that the weak ferromagnetism is due either to the presence of bond-centered charge stripes or to double exchange interactions across site-centered charge stripes.

cond-mat.str-el↗

arXiv2014

Magnetisation of hole-doped CuO2 spin chains in Sr14-xCaxCu24O41

We report on magnetisation measurements of Sr14-xCaxCu24O41, with 0 <= x <= 12, in magnetic fields up to 16 T. The low temperature magnetic response of the CuO2 spin chains changes strongly upon doping. For x = 0, the ground state with nearly independent dimers is confirmed. Reduction of the number of holes in the chains through Ca-doping leads to an additional contribution to the magnetisation, which depends linearly on the magnetic field. Remarkably, the slope of this linear contribution increases with the Ca content. We argue that antiferromagnetic spin chains do not account for this behaviour but that the hole dynamics might be involved.

cond-mat.str-el↗

arXiv2013

Evidence of d-wave Superconductivity in K_(1-x)Na_xFe_2As_2 (x = 0, 0.1) Single Crystals from Low-Temperature Specific Heat Measurements

From the measurement and analysis of the specific heat of high-quality K_(1-x)Na_xFe_2As_2 single crystals we establish the presence of large T^2 contributions with coefficients alpha_sc ~ 30 mJ/mol K^3 at low-T for both x=0 and 0.1. Together with the observed square root field behavior of the specific heat in the superconducting state both findings evidence d-wave superconductivity on almost all Fermi surface sheets with an average gap amplitude of Delta_0 in the range of 0.4 - 0.8 meV. The derived Delta_0 and the observed T_c agree well with the values calculated within the Eliashberg theory, adopting a spin-fluctuation mediated pairing in the intermediate coupling regime.

cond-mat.supr-con↗

arXiv2012

Spin-state transition, magnetism and local crystal structure in Eu_{1-x}Ca_xCoO_{3-d}

The doping series Eu1-xCaxCoO3-d provides a rather peculiar way to study the spin-state transition in cobalt-based complex oxides since partial substitution of Eu3+ ions by Ca2+ ions does not increase the mean valence state of cobalt but is accompanied by appearance of oxygen vacancies in the ratio d \sim x/2. In the parent compound EuCoO3, the low spin (LS)-high spin (HS) transition takes place at temperatures so high that the chemical decomposition prevents its direct observation. The substitution of Eu3+ for Ca2+ in this system shifts the LS-HS transition to lower temperatures. The energy gap associated with this transition in octahedrally-coordinated Co3+ ions changes from 1940 K in EuCoO3 to 1540 K in Eu0.9Ca0.1CoO2.95 and 1050 K in Eu0.8Ca0.2CoO2.9. Besides, each O2- vacancy reduces the local coordination of two neighboring Co3+ ions from octahedral to pyramidal thereby locally creating magnetically active sites which couple into dimers. These dimers at low temperatures form another gapped magnetic system with very different energy scale, D~3 K, on the background of intrinsically non-magnetic lattice of octahedrally-coordinated low-spin Co3+ ions.

cond-mat.str-el↗

arXiv2012

Pr magnetism and its interplay with the Fe spin density wave in PrFeAsO

We have studied the magnetism of the Pr3+ ions in PrFeAsO_1-xF_x (x = 0; 0.15) and its interaction with the Fe magnetic order (for x = 0). Specific heat data confirm the presence of a first excited crystal electric field (CEF) level around 3.5 meV in the undoped compound PrFeAsO. This finding is in agreement with recent neutron scattering experiments. The doped compound is found to have a much lower first CEF splitting of about 2.0 meV. The Pr ordering in PrFeAsO gives rise to large anomalies in the specific heat and the thermal expansion coefficient. In addition, a field-induced transition is found at low temperatures that is most pronounced for the magnetostriction coefficient. This transition, which is absent in the doped compound, is attributed to a reversal of the Fe spin canting as the antiferromagnetic Pr order is destroyed by the external magnetic field.

cond-mat.supr-con↗

arXiv2012

Weak ferrimagnetism and multiple magnetization reversal in α-Cr3(PO4)2

The chromium(II) orthophosphate α-Cr3(PO4)2 is a weak ferrimagnet with the Curie temperature TC = 29 K confirmed by a λ-type peak in specific heat. Dominant antiferromagnetic interactions in this system are characterized by the Weiss temperature Θ = - 96 K, indicating an intermediate frustration ratio |Θ|/TC ~ 3. In its magnetically ordered states α-Cr3(PO4)2 exhibits a remarkable sequence of temperature-induced magnetization reversals sensitive to the protocol of measurements, i.e. either field-cooled or zero-field-cooled regimes. The reduction of the effective magnetic moment 4.5 μB/Cr2+, as compared to the spin-only moment 4.9 μB/Cr2+, cannot be ascribed to the occurence of the low-spin state in any crystallographic site of the Jahn-Teller active 3d4 Cr2+ ions. X-ray absorption spectra at the K-edge indicate divalent chromium and unravel the high-spin state of these ions at the L2,3-edges. Weak ferrimagnetism and multiple magnetization reversal phenomena seen in this compound could be ascribed to incomplete cancellation and distortion of partial spontaneous magnetization functions of Cr2+ in its six crystallographically inequivalent positions.

cond-mat.mtrl-sci↗

arXiv2011

Orthogonal spin arrangement as possible ground state of three - dimensional Shastry - Sutherland network in Ba3Cu3In4O12

The Ba3Cu3In4O12 stands for unique topology of the magnetic subsystem. It consists of rotated by 90\Box$^b$ relative to each other "paper-chain" columns made of vertex-sharing CuIO4 and CuIIO4 planar units. The overall pattern of the copper ions is that of a three-dimensional Shastry-Sutherland network. At high temperatures, the magnetic susceptibility follows the Curie-Weiss law with positive Weiss temperature indicating strong predominance of ferromagnetic coupling. At low temperatures, however, this compound reaches the antiferromagnetically ordered state and experiences non-trivial succession of two spin-flop and two spin-flip transitions reaching full saturation in modest magnetic fields. Here we show that the possible ground state in Ba3Cu3In4O12 can be three-dimensional orthogonal arrangement of the Cu2+ (S = 1/2) magnetic moments forming three virtually independent antiferromagnetic subsystems. In this arrangement, favored by anisotropic exchange interactions, the quantum fluctuations provide the coupling between three mutually orthogonal magnetic subsystems resulting in an impressive "order by disorder" effect.

cond-mat.mtrl-sci↗

arXiv2011

Magnetic properties of the low-dimensional spin-1/2 magnet α-Cu_2As_2O_7

In this work we study the interplay between the crystal structure and magnetism of the pyroarsenate α-Cu_2As_2O_7 by means of magnetization, heat capacity, electron spin resonance and nuclear magnetic resonance measurements as well as density functional theory (DFT) calculations and quantum Monte Carlo (QMC) simulations. The data reveal that the magnetic Cu-O chains in the crystal structure represent a realization of a quasi-one dimensional (1D) coupled alternating spin-1/2 Heisenberg chain model with relevant pathways through non-magnetic AsO_4 tetrahedra. Owing to residual 3D interactions antiferromagnetic long range ordering at T_N\simeq10K takes place. Application of external magnetic field B along the magnetically easy axis induces the transition to a spin-flop phase at B_{SF}~1.7T (2K). The experimental data suggest that substantial quantum spin fluctuations take place at low magnetic fields in the ordered state. DFT calculations confirm the quasi-one-dimensional nature of the spin lattice, with the leading coupling J_1 within the structural dimers. QMC fits to the magnetic susceptibility evaluate J_1=164K, the weaker intrachain coupling J'_1/J_1 = 0.55, and the effective interchain coupling J_{ic1}/J_1 = 0.20.

cond-mat.str-el↗

arXiv2011

Calorimetric study of the superconducting and normal state properties of Ca(Fe1-xCox)2As2

We present a calorimetric study on single crystals of Ca(Fe1-xCox)2As2 (x = 0, 0.032, 0.051, 0.056, 0.063, and 0.146). The combined first order spin-density wave/structural transition occurs in the parent CaFe2As2 compound at 168 K and gradually shifts to lower temperature for low doping levels (x = 0.032 and x = 0.051). It is completely suppressed upon higher doping x = 0.056. Simultaneously, superconductivity appears at lower temperature with a transition temperature around Tc = 14.1 K for Ca(Fe0.937Co0.063)2As2. The phase diagram of Ca(Fe0.937Co0.063)2As2 has been derived and the upper critical field is found to be H(c) c2 = 11.5

cond-mat.supr-con↗

arXiv2011

Saturation field of frustrated chain cuprates: broad regions of predominant interchain coupling

An efficient and precise thermodynamic method to extract the interchain coupling (IC) of spatially anisotropic 2D or 3D spin-1/2 systems from their empirical saturation field H_s (T=0) is proposed. Using density-matrix renormalization group, hard-core boson, and spin-wave theory we study how H_s is affected by an antiferromagnetic (AFM) IC between frustrated chains described in the J_1-J_2-spin model with ferromagnetic 1st and AFM 2nd neighbor in-chain exchange. A complex 3D-phase diagram has been found. For Li2CuO2 and Y2Ca2Cu5O10, we show that H_s is solely determined by the IC and predict H_s approx 61 T for the latter.Using H_s approx 55 T from our high-field pulsed measurements one reads out a weak IC for Li2CuO2 close to that from neutron scattering.

cond-mat.str-el↗

arXiv2011

High-field electron spin resonance spectroscopy study of GdO$_{1-x}$F$_{x}$FeAs superconductors

We report a detailed investigation of GdO$_{1-x}$F$_{x}$FeAs (x = 0, 0.07 and 0.14) samples by means of high-field/high-frequency electron spin resonance (HF-ESR) together with measurements of thermodynamic and transport properties. The parent GdOFeAs compound exhibits Fe long-range magnetic order below 128 K, whereas both doped samples do not show such order and are superconducting with T$_c$ = 20 K (x = 0.07) and T$_c$ = 45 K (x = 0.14). The Gd$^{3+}$ HF-ESR reveals an appreciable exchange coupling between Gd and Fe moments, through which the static magnetic order is clearly seen in the parent compound. Owing to this coupling, HF-ESR can probe sensitively the evolution of the magnetism in the FeAs planes upon F doping. It is found that in both superconducting samples, where the Fe long-range order is absent, there are short-range, static on the ESR time scale magnetic correlations between Fe spins. Their occurrence on a large doping scale may be indicative of the ground states' coexistence.

cond-mat.str-el↗