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K. Singh

Publications and source records attributed to K. Singh.

At least 37 records · Page 2Linked to original sources
arXiv2014

Multiglass properties and magnetoelectric coupling in uniaxial anisotropic spin-cluster-glass Fe2TiO5

The compound, Fe2TiO5 (FTO)is a well-known uniaxial anisotropic spin-glass insulator with two successive glassy freezing temperatures i.e. transverse (TTF= 9K) and longitudinal (TLF= 55 K). In this article, we present the results of measurements of complex dielectric behavior, electric polarization as a function of temperature (T), in addition to characterization by magnetic susceptibility and heat-capacity, primarily to explore magnetoelectric (ME) coupling and multiglass properties in uniaxial anisotropic spin cluster-glass FTO. The existence of two magnetic transitions is reflected in the isothermal magnetodielectric (MD) behavior in the sense that the sign of MD is different in the T regime T TTF.The data in addition provide evidence for the glassy dynamics of electric-dipoles; interestingly, this occurs at much higher temperature (~100-150 K) than TLF with high remnant polarization at 10 K( 4000 micro C/m2) attributable to short-range magnetic correlations, thereby offering a route to attain ME coupling above 77 K.

cond-mat.mtrl-sci↗
arXiv2014

Symmetry Classification and Exact Solutions of a Variable Coefficient Nonlinear Space-Time Fractional Burgers' Equation

In this paper, we investigate the symmetry properties of a variable coefficient nonlinear space-time fractional Burgers' equation. Fractional Lie symmetries and corresponding infinitesimal generators are obtained. With the help of the infinitesimal generators some group invariant solutions are deduced. Further, some exact solutions of fractional Burgers' equation are generated by the invariant subspace method.

math.AP↗
arXiv2014

Impact of the various spin and orbital ordering processes on multiferroic properties of orthovanadate DyVO3

The orthovanadate DyVO3 crystal, known to exhibit multiple structural, spin and orbital ordering transitions, is presently investigated on the basis of magnetization, heat capacity, resistivity, dielectric and polarization measurements. Our main result is experimental evidence for the existence of multiferroicity below a high TC of 108 K over a wide temperature range including different spin-orbital ordered states. The onset of ferroelectricity is found to coincide with the antiferromagnetic C-type spin ordering transition taking place at 108 K, which indicates that DyVO3 belongs to type II multiferroics exhibiting a coupling between magnetism and ferroelectricity. Some anomalies detected on the temperature dependence of electric polarization are discussed with respect to the nature of the spin-orbital ordered states of the V sublattice and the degree of spin alignment in the Dy sublattice. The orthovanadates RVO3 (R = rare earth or Y) form an important new category for searching for high-TC multiferroics.

cond-mat.str-el↗
arXiv2014

Multiferroicity and magneto-electric effect in Gd$_2$BaNiO$_5$

We report the observation of electric polarization in the magnetically ordered state of the Haldane chain compound, Gd$_2$BaNiO$_5$, with strongly correlated magnetic and dielectric properties. The results of dc magnetic susceptibility and heat capacity measurements indicate two magnetic transitions,one corresponding to the anti-ferromagnetic order at T$_N$ around 55 K and the other to spin-reorientation transition at T$_{SR}$ around 24 K. The dielectric permittivity ($ε'$) and loss (tan$δ$) also exhibit anomalies in the vicinity of T$_{SR}$ and T$_N$ respectively. Below the spin-reorientation transition, concurrently magnetic-field-induced spin-flop and the meta-electric transitions are observed at a critical magnetic field in isothermal magnetization and magneto-dielectric results respectively. Another interesting finding is that the change in dielectric constant changes its sign at the critical magnetic field. The origin of the observed magneto-electric effect is discussed on the basis of spin-phonon coupling.

cond-mat.mtrl-sci↗
arXiv2013

Gigantic magnetic field polarization and magnetoelectric coupling in a ferrimagnetic oxide CaBaCo4O7

The single crystal study of CaBaCo4O7, a non collinear ferrimagnet (TC=64K), with a polar orthorhombic space group (Pbn21) between 4 K and 293 K, shows the appearance below TC of a large electric polarization along its c axis, reaching 17mC.m-2 at 10K. At 62.5K, a magnetic field driven giant variation of polarization, P(9T)-P(0T)=8mC.m-2, is observed. Moreover, the present magnetoelectric measurements, are fully consistent with the m'm2' magnetic point group, strongly supporting that this oxide is also ferrotoroidic. This ferrimagnetic oxide, which belongs to the '114' structural family, opens an avenue for the search of new magnetoelectrics.

cond-mat.mtrl-sci↗
arXiv2012

Ordering process and ferroelectricity in a spinel derived from FeV2O4

The spinel FeV2O4 is known to exhibit peculiar physical properties, which is generally ascribed to the unusual presence of two cations showing a pronounced interplay between spin, orbital and lattice degrees of freedom (Fe2+ and V3+ on the tetrahedral and octahedral sites, respectively). The present work reports on an experimental re-investigation of this material based on a broad combination of techniques, including x-ray diffraction, energy dispersive and Mössbauer spectroscopies, as well as magnetization, heat capacity, dielectric and polarization measurements. Special attention was firstly paid to establish the exact cationic composition of the investigated samples, which was found to be Fe1.18V1.82O4. All the physical properties were found to point out a complex ordering process with a structural transition at TS = 138 K, followed by two successive magnetostructural transitions at TN1 = 111 K and TN2 = 56 K. This latter transition marking the appearance of electric polarization, magnetization data were analysed in details to discuss the nature of the magnetic state at T< TN2. An overall interpretation of the sequence of transitions was proposed, taking into account two spin couplings, as well as the Jahn-Teller effects and the mechanism of spin-orbit stabilization. Finally, the origin of ferroelectricity in Fe1.18V1.82O4 is discussed on the basis of recent models.

cond-mat.mtrl-sci↗
arXiv2011

An air shower array for LOFAR: LORA

LOFAR is a new form of radio telescope which can detect radio emission from air showers induced by very high-energy cosmic rays. It can also look for radio emission from particle cascades on the Moon induced by ultra high-energy cosmic rays or neutrinos. To complement the radio detection, we are setting up a small particle detector array LORA (LOfar Radboud Air shower array) within an area of $\sim 300$ m diameter in the LOFAR core. It will help in triggering and confirming the radio detection of air showers with the LOFAR antennas. In this paper, we present a short overview about LORA and discuss its current status.

astro-ph.IM↗
arXiv2010

A new limit on the Ultra-High-Energy Cosmic-Ray flux with the Westerbork Synthesis Radio Telescope

A particle cascade (shower) in a dielectric, for example as initiated by an ultra-high energy cosmic ray, will have an excess of electrons which will emit coherent Čerenkov radiation, known as the Askaryan effect. In this work we study the case in which such a particle shower occurs in a medium just below its surface. We show, for the first time, that the radiation transmitted through the surface is independent of the depth of the shower below the surface when observed from far away, apart from trivial absorption effects. As a direct application we use the recent results of the NuMoon project, where a limit on the neutrino flux for energies above $10^{22}$\,eV was set using the Westerbork Synthesis Radio Telescope by measuring pulsed radio emission from the Moon, to set a limit on the flux of ultra-high-energy cosmic rays.

astro-ph.HE↗
arXiv2010

Constraints on the flux of Ultra-High Energy neutrinos from WSRT observations

Ultra-high energy (UHE) neutrinos and cosmic rays initiate particle cascades underneath the Moon's surface. These cascades have a negative charge excess and radiate Cherenkov radio emission in a process known as the Askaryan effect. The optimal frequency window for observation of these pulses with radio telescopes on the Earth is around 150 MHz. By observing the Moon with the Westerbork Synthesis Radio Telescope array we are able to set a new limit on the UHE neutrino flux. The PuMa II backend is used to monitor the Moon in 4 frequency bands between 113 and 175 MHz with a sampling frequency of 40 MHz. The narrowband radio interference is digitally filtered out and the dispersive effect of the Earth's ionosphere is compensated for. A trigger system is implemented to search for short pulses. By inserting simulated pulses in the raw data, the detection efficiency for pulses of various strength is calculated. With 47.6 hours of observation time, we are able to set a limit on the UHE neutrino flux. This new limit is an order of magnitude lower than existing limits. In the near future, the digital radio array LOFAR will be used to achieve an even lower limit.

astro-ph.HE↗
arXiv2009

Lateral Distribution of the Radio Signal in Extensive Air Showers Measured with LOPES

The antenna array LOPES is set up at the location of the KASCADE-Grande extensive air shower experiment in Karlsruhe, Germany and aims to measure and investigate radio pulses from Extensive Air Showers. The coincident measurements allow us to reconstruct the electric field strength at observation level in dependence of general EAS parameters. In the present work, the lateral distribution of the radio signal in air showers is studied in detail. It is found that the lateral distributions of the electric field strengths in individual EAS can be described by an exponential function. For about 20% of the events a flattening towards the shower axis is observed, preferentially for showers with large inclination angle. The estimated scale parameters R0 describing the slope of the lateral profiles range between 100 and 200 m. No evidence for a direct correlation of R0 with shower parameters like azimuth angle, geomagnetic angle, or primary energy can be found. This indicates that the lateral profile is an intrinsic property of the radio emission during the shower development which makes the radio detection technique suitable for large scale applications.

astro-ph.HE↗
arXiv2009

Improved flux limits for neutrinos with energies above 10$^{22}$ eV from observations with the Westerbork Synthesis Radio Telescope

Particle cascades initiated by ultra-high energy (UHE) neutrinos in the lunar regolith will emit an electromagnetic pulse with a time duration of the order of nano seconds through a process known as the Askaryan effect. It has been shown that in an observing window around 150 MHz there is a maximum chance for detecting this radiation with radio telescopes commonly used in astronomy. In 50 hours of observation time with the Westerbork Synthesis Radio Telescope array we have set a new limit on the flux of neutrinos, summed over all flavors, with energies in excess of $4\times10^{22}$ eV.

astro-ph.HE↗
arXiv2009

Air Shower Measurements with LOFAR

Air showers from cosmic rays emit short, intense radio pulses. LOFAR is a new radio telescope, that is being built in the Netherlands and Europe. Designed primarily as a radio interferometer, the core of LOFAR will have a high density of radio antennas, which will be extremely well calibrated. This makes LOFAR a unique tool for the study of the radio properties of single air showers. Triggering on the radio emission from air showers means detecting a short radio pulse and discriminating real events from radio interference. At LOFAR we plan to search for pulses in the digital data stream - either from single antennas or from already beam-formed data - and calculate several parameters characterizing the pulse shape to pick out real events in a second stage. In addition, we will have a small scintillator array to test and confirm the performance of the radio only trigger.

astro-ph.IM↗
arXiv2008

Status report of the NuMoon experiment

We show that at wavelengths comparable to the length of the shower produced by an Ultra-High Energy cosmic ray or neutrino, radio signals are an extremely efficient way to detect these particles. First results are presented of an analysis of 20 hours of observation data for NuMoon project using the Westerbork Synthesis Radio Telescope to search for short radio pulses from the Moon. A limit on the neutrino flux is set that is a factor four better than the current one (based on FORTE).

astro-ph↗
arXiv2008

The NuMoon experiment: first results

The NuMoon project uses the Westerbork Synthesis Radio Telescope to search for short radio pulses from the Moon. These pulses are created when an ultra high energy cosmic ray or neutrino initiates a particle cascade inside the Moon's regolith. The cascade has a negative charge excess and moves faster than the local speed of light, which causes coherent Cherenkov radiation to be emitted. With 100 hours of data, a limit on the neutrino flux can be set that is an order of magnitude better than the current one (based on FORTE). We present an analysis of the first 10 hours of data.

astro-ph↗
arXiv2006

Quantitative conditions do not guarantee the validity of the adiabatic approximation

In this letter, we point out that the widely used quantitative conditions in the adiabatic theorem are insufficient in that they do not guarantee the validity of the adiabatic approximation. We also reexamine the inconsistency issue raised by Marzlin and Sanders (Phys. Rev. Lett. 93, 160408, 2004) and elucidate the underlying cause.

quant-ph↗
arXiv2005

A note on the geometric phase in adiabatic approximation

The adiabatic theorem shows that the instantaneous eigenstate is a good approximation of the exact solution for a quantum system in adiabatic evolution. One may therefore expect that the geometric phase calculated by using the eigenstate should be also a good approximation of exact geometric phase. However, we find that the former phase may differ appreciably from the latter if the evolution time is large enough.

quant-ph↗
arXiv2003

Geometric phases for non-degenerate and degenerate mixed states

This paper focuses on the geometric phase of general mixed states under unitary evolution. Here we analyze both non-degenerate as well as degenerate states. Starting with the non-degenerate case, we show that the usual procedure of subtracting the dynamical phase from the total phase to yield the geometric phase for pure states, does not hold for mixed states. To this end, we furnish an expression for the geometric phase that is gauge invariant. The parallelity conditions are shown to be easily derivable from this expression. We also extend our formalism to states that exhibit degeneracies. Here with the holonomy taking on a non-abelian character, we provide an expression for the geometric phase that is manifestly gauge invariant. As in the case of the non-degenerate case, the form also displays the parallelity conditions clearly. Finally, we furnish explicit examples of the geometric phases for both the non-degenerate as well as degenerate mixed states.

quant-ph↗