arXiv subjects

G. Balakrishnan

Publications and source records attributed to G. Balakrishnan.

At least 145 records · Page 8Linked to original sources

arXiv2010

Magnetic correlations in spin ice Ho2-xYxTi2O7 as revealed by neutron polarization analysis

We present single-crystal neutron-diffuse-scattering measurements with polarization analysis on the spin ice Ho2-xYxTi2O7 (x = 0, 0.3, and 1). At 2 K, the spin flip scattering patterns are typical of a nearest-neighbor spin ice and the effects of Y doping are seen in the width of the pinch points. At low-T, the spin flip patterns are characteristic of a dipolar spin ice and are unaffected by Y doping. In the non-spin flip channel, we observe the signature of strong antiferromagnetic correlations at the same T as the dipolar spin ice correlations appear in the spin flip channel. These experimental observations have been well reproduced by our Monte Carlo simulations.

cond-mat.str-el↗

arXiv2010

Two-gap superconductivity in Lu$_2$Fe$_3$Si$_5$: a transverse-field muon spin rotation study

The superconducting properties of Lu2Fe3Si5 with Tc = 6.1 K have been investigated using low-temperature transverse-field muon spin rotation ($μ$SR) and specific heat measurements. The magnetic penetration depth at zero temperature is 353(1) nm. However, the temperature dependence of the magnetic penetration depth is consistent with a two gap s+s-wave model. Low-temperature specific heat measurements on the same sample also show evidence of two distinct superconducting gaps.

cond-mat.supr-con↗

arXiv2010

Magnetic excitations in the XY-pyrochlore antiferromagnet Er$_2$Ti$_2$O$_7$

The XY-pyrochlore antiferromagnet \ETO\ is studied by heat capacity measurements and electron spin resonance spectroscopy performed on single crystal samples. The magnetic phase diagrams are established for two directions of applied field, $H\parallel [100]$ and $H\parallel [111]$. In the magnetically ordered phase observed below $T_N=1.2$ K, the magnetic excitation spectrum consists of a Goldstone mode acquiring an isotropic gap in an applied field, and another mode with a gap softening in the vicinity of a field-induced phase transition. This second-order transition takes place at a critical field $H_c$ above which the magnetization process is accompanied by a canting of the magnetic moments off their local "easy-planes". The specific heat curves for $H\parallel [100]$ ($H\gg H_c$) are well described by a model presuming a single dispersionless excitation mode with the energy gap obtained from the spectroscopic measurements.

cond-mat.str-el↗

arXiv2010

Kagome staircase compound Co3V2O8 in an applied magnetic field: single-crystal neutron-diffraction study

The magnetic properties of Co3V2O8 have been studied by single-crystal neutron-diffraction. In zero magnetic field, the observed broadening of the magnetic Bragg peaks suggests the presence of disorder both in the low-temperature ferromagnetic and in the higher-temperature antiferromagnetic state. The field dependence of the intensity and position of the magnetic reflections in Co3V2O8 reveals a complex sequence of phase transitions in this Kagome staircase compound. For H//a, a commensurate-incommensurate-commensurate transition is found in a field of 0.072 T in the antiferromagnetic phase at 7.5 K. For H//c at low-temperature, an applied field induces an unusual transformation from a ferromagnetic to an antiferromagnetic state at about 1 T accompanied by a sharp increase in magnetisation.

cond-mat.str-el↗

arXiv2010

Spin dynamics in the frustrated three-dimensional hyperkagom{é} compound $Gd_3 Ga_5 O_{12}$

We present the first neutron inelastic scattering results on the low temperature magnetic state of the three dimensional hyperkagomé compound Gd$_{3}$Ga$_{5}$O$_{12}$ (GGG). GGG is often classified as a strongly frustrated system with a manifold of continuously connected states. However, in contrast to the expectation of a continuum of gap-less excitations above a spin liquid-like ground state our results reveal three distinct inelastic modes found at 0.04(1), 0.12(2) an 0.58(3) meV at 0.06 K. The inelastic modes can be attributed to the magnetic ground state with the lowest and highest energy excitations showing spatial dependencies indicative of dimerized short range antiferromagnetic correlations. Short range correlations, reminiscent of spin liquid-like order, are static within the instrumental resolution (50 $μ$eV) and represent 82% of the spectral weight. Longer range correlations, first observed by Petrenko, et al.\cite{Petrenko1998, develop below 0.14 ~K within the elastic cross section. The short range static correlations and dynamic components survive to high temperatures, comparable to the nearest neighbor exchange interactions. Our results suggest that the ground state of a three dimensional hyperkagomé compound differs distinctly from its frustrated counterparts on a pyrochlore lattice and reveals a juxtaposition of spin liquid order and strong dimerised coupling.

cond-mat.str-el↗

arXiv2010

Muon-spin-spectroscopy study of the penetration depth of FeTe0.5Se0.5

Muon spin spectroscopy measurements have been used to study the superconducting state of FeTe0.5Se0.5. The temperature dependence of the in-plane magnetic penetration depth, lambda_ab(T), is found to be compatible with either a two gap s+s wave or an anisotropic s-wave model. The value for lambda_ab(T) at T=0 K is estimated to be lambda_ab(0)=534(2)nm.

cond-mat.supr-con↗

arXiv2008

Field induced magnetic order in the frustrated magnet Gadolinium Gallium Garnet

Gd3Ga5O12, (GGG), has an extraordinary magnetic phase diagram, where no long range order is found down to 25 mK despite Θ_CW \approx 2 K. However, long range order is induced by an applied field of around 1 T. Motivated by recent theoretical developments and the experimental results for a closely related hyperkagome system, we have performed neutron diffraction measurements on a single crystal sample of GGG in an applied magnetic field. The measurements reveal that the H-T phase diagram of GGG is much more complicated than previously assumed. The application of an external field at low T results in an intensity change for most of the magnetic peaks which can be divided into three distinct sets: ferromagnetic, commensurate antiferromagnetic, and incommensurate antiferromagnetic. The ferromagnetic peaks (e.g. (112), (440) and (220)) have intensities that increase with the field and saturate at high field. The antiferromagnetic reflections have intensities that grow in low fields, reach a maximum at an intermediate field (apart from the (002) peak which shows two local maxima) and then decrease and disappear above 2 T. These AFM peaks appear, disappear and reach maxima in different fields. We conclude that the competition between magnetic interactions and alternative ground states prevents GGG from ordering in zero field. It is, however, on the verge of ordering and an applied magnetic field can be used to crystallise ordered components. The range of ferromagnetic and antiferromagnetic propagation vectors found reflects the complex frustration in GGG.

cond-mat.str-el↗

arXiv2008

Low-temperature magnetic ordering in SrEr$_2$O$_4$

SrEr$_2$O$_4$ has been characterised by low-temperature powder neutron diffraction, as well as single crystal specific heat and magnetisation measurements. Magnetisation measurements show that the magnetic system is highly anisotropic at temperature above ordering. A magnetic field of 280 kOe applied at $T=1.6$ K does not overcome the anisotropic magnetisation and fails to fully saturate the system. Long range antiferromagnetic ordering develops below $T_N=0.75$ K, identified by magnetic Bragg reflections with propagation vector ${\bf k}=0$ and a lambda anomaly in the specific heat. The magnetic structure consists of ferromagnetic chains running along the \textit{c} axis, two adjacent chains being stacked antiferromagnetically. The moments point along the \textit{c} direction, but only one of the two crystallographically in-equivalent Er sites has a sizeable ordered magnetic moment, 4.5 $\rm μ_B$ at 0.55 K. The magnetic properties of SrEr$_2$O$_4$ are discussed in terms of the interplay between the low-dimensionality, competing exchange interactions, dipolar interactions and low lying crystal field levels.

cond-mat.str-el↗

arXiv2008

Spin, orbital ordering and magnetic dynamics of LaVO3: magnetization, heat capacity and neutron scattering studies

We report the results of magnetization, heat capacity and neutron scattering studies of LaVO3 single crystals. From the neutron diffraction studies, it was found that the compound is magnetically ordered with a C-type antiferromagnetic spin structure at about 136 K. In the vicinity of the ordering temperature, we also observed hysteresis in the neutron diffraction data measured on cooling and heating which indicates the first order nature of the phase transition. In the antiferromagnetically ordered phase, the inelastic neutron scattering studies reveal the presence of a temperature independent c axis spin-wave gap of about 6 meV which is similar to that previously reported for the sister compound YVO3.

cond-mat.str-el↗

arXiv2008

Electronic texture of the thermoelectric oxide Na0.75CoO2

From 59Co and 23Na NMR, we demonstrate the impact of the Na+ vacancy ordering on the cobalt electronic states in Na0.75CoO2: at long time scales, there is neither a disproportionation into 75 % Co3+ and 25 % Co4+ states, nor a mixed-valence metal with a uniform Co3.25+ state. Instead, the system adopts an intermediate configuration in which 30 % of the lattice sites form an ordered pattern of localized Co3+ states. Above 180 K, an anomalous mobility of specific Na+ sites is found to coexist with this electronic texture, suggesting that the formation of the latter may contribute to stabilizing the Na+ ordering. Control of the ion doping in these materials thus appears to be crucial for fine-tuning of their thermoelectric properties.

cond-mat.str-el↗

arXiv2007

Elliptical hole pockets in the Fermi surfaces of unhydrated and hydrated sodium cobalt oxides

The surprise discovery of superconductivity below 5K in sodium cobalt oxides when hydrated with water has caught the attention of experimentalists and theorists alike. Most explanations for its occurence have focused heavily on the properties of some small elliptically shaped pockets predicted to be the electronically dominant Fermi surface sheet, but direct attempts to look for them have instead cast serious doubts over their existence. Here we present evidence that these pockets do indeed exist, based on bulk measurements of the electron momentum distribution in unhydrated and hydrated sodium cobalt oxides using the technique of x-ray Compton scattering.

cond-mat.supr-con↗

arXiv2006

Magnetic excitations in the Kagome staircase compounds

Inelastic neutron scattering measurements have been performed on single crystal samples of Co3V2O8 and Ni3V2O8. The magnetic system in these compounds is believed to be frustrated, as the magnetic ions Co2+ with S=3/2 and Ni2+ with S=1 adopt a buckled version of the Kagome lattice. Magnetic excitations have been observed in both samples using a time-of-flight neutron spectrometer. The excitation spectrum is dispersive for both samples and has a considerable gap in the low temperature phases, while the intermediate temperature phases are marked by a significant softening of the excitations energy.

cond-mat.mtrl-sci↗

arXiv2006

Magnetic phase diagrams of the Kagome staircase compounds Co3V2O8 and Ni3V2O8

An extensive low temperature magnetisation study of high quality single crystals of the Kagome staircase compounds Ni3V2O8 and Co3V2O8 has been performed, and the H-T phase diagrams have been determined from these measurements. The magnetisation and susceptibility curves for Co3V2O8 are analysed in terms of their compatibility with the different ferromagnetic and antiferromagnetic structures proposed for this compound. For Ni3V2O8, the phase diagram is extended to magnetic fields higher than previously reported; for a field applied along the a-axis, the low temperature incommensurate phase is found to close at around 90 kOe.

cond-mat.mtrl-sci↗

arXiv2006

Intraplanar Magnetic Excitations in Na0.5CoO2: An Inelastic Neutron Study

Inelastic neutron scattering measurements mapping the in-plane magnetic interactions of Na0.5CoO2 reveal dispersive excitations at points above an energy gap Eg = 11.5(5) meV at the superstructural Bragg reflections. The excitations are highly damped, broadening with increasing energy, and disappear at hw ~ 35 meV, a strong indication that the magnetism is itinerant. Tilting into the ac plane reduces the value of Eg by 25%, suggesting that the dispersion along c is significant and the magnetic correlations are three-dimensional, as seen at the higher doping levels.

cond-mat.str-el↗

arXiv2006

The magnetic field and pressure dependence of the magnetic ordering transition in NaxCoO2 (0.6<x<0.72)

We have measured the magnetic field (H<90 kOe) and pressure (P<10 kbar) dependence of the magnetic ordering temperature, Tmag, in single crystal samples of NaxCoO2 for a range of Na concentrations (0.60<x<0.72). We show that in zero field, Tmag remains constant with decreasing x before magnetic order disappears at x=0.65. Heat capacity and magnetization data show that for x=0.70, Tmag is unchanged in an applied field. In contrast, magnetization data collected under hydrostatic pressure show that Tmag increases from 22.0 K at 1 bar to 25.4 K at 10 kbar. This rise is at odds with the behaviour expected for model spin density wave systems.

cond-mat.str-el↗

arXiv2006

Magnetic resonance in a pyrochlore antiferromagnet Gd2Ti2O7

Electron spin resonance study of frustrated pyrochlore Gd2Ti2O7 is performed in a wide frequency band for a temperature range 0.4-30 K, which covers paramagnetic and magnetically ordered phases. The paramagnetic resonance reveals the spectroscopic g-factor about 2.0 and a temperature dependent linewidth. In ordered phases magnetic resonance spectra are distinctive for a nonplanar cubic (or tetrahedral) antiferromagnet with an isotropic susceptibility. In the high-field saturated phase, weakly-dispersive soft modes are observed and their field evolution is traced.

cond-mat.str-el↗

arXiv2005

Critical points in the Bragg glass phase of a weakly pinned crystal of Ca$_3$Rh$_4$Sn$_{13}$

New experimental data are presented on the scan rate dependence of the magnetization hysteresis width $ΔM(H)$ ($\propto$ critical current density $J_c(H)$) in isothermal $M-H$ scans in a weakly pinned single crystal of Ca$_3$Rh$_4$Sn$_{13}$, which displays second magnetization peak (SMP) anomaly as distinct from the peak effect (PE). We observe an interesting modulation in the field dependence of a parameter which purports to measure the dynamical annealing of the disordered bundles of vortices injected through the sample edges towards the destined equilibrium vortex state at a given $H$. These data, in conjunction with the earlier observations made while studying the thermomagnetic history dependence in $J_c(H)$ in the tracing of the minor hysteresis loops, imply that the partially disordered state heals towards the more ordered state between the peak field of the SMP anomaly and the onset field of the PE. The vortex phase diagram in the given crystal of Ca$_3$Rh$_4$Sn$_{13}$ has been updated in the context of the notion of the phase coexistence of the ordered and disordered regions between the onset field of the SMP anomaly and the spinodal line located just prior to the irreversibility line. A multi-critical point and a critical point in the ($H,T$) region of the Bragg glass phase have been marked in this phase diagram and the observed behaviour is discussed in the light of recent data on multi-critical point in the vortex phase diagram in a single crystal of Nb.

cond-mat.supr-con↗

arXiv2004

Magnetocaloric effect in pyrochlore antiferromagnet Gd2Ti2O7

An adiabatic demagnetization process is studied in Gd2Ti2O7, a geometrically frustrated antiferromagnet on a pyrochlore lattice. In contrast to conventional paramagnetic salts, this compound can exhibit a temperature decrease by a factor of ten in the temperature range below the Curie-Weiss constant. The most efficient cooling is observed in the field interval between 120 and 60 kOe corresponding to a crossover between saturated and spin-liquid phases. This phenomenon indicates that a considerable part of the magnetic entropy survives in the strongly correlated state. According to the theoretical model, this entropy is associated with a macroscopic number of local modes remaining gapless till the saturation field. Monte Carlo simulations on a classical spin model demonstrate good agreement with the experiment. The cooling power of the process is experimentally estimated with a view to possible technical applications. The results for Gd2Ti2O7 are compared to those for Gd3Ga5O12, a well-known material for low temperature magnetic refrigeration.

cond-mat.str-el↗