K. Priyabrat Pandey - CQT

K. Priyabrat Pandey
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K. Priyabrat Pandey

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Physics - Atomic Physics (12)
High Energy Physics - Theory (9)
General Relativity and Quantum Cosmology (9)
Physics - Optics (8)
High Energy Physics - Phenomenology (7)
Cosmology and Nongalactic Astrophysics (6)
Solar and Stellar Astrophysics (5)
Astrophysics (5)
Physics - Materials Science (4)
High Energy Physics - Lattice (2)
Quantum Physics (2)
Mathematical Physics (1)
Mathematics - Mathematical Physics (1)
Computer Science - Networking and Internet Architecture (1)
Physics - Biological Physics (1)
Physics - Medical Physics (1)

Publications Authored By K. Priyabrat Pandey

In this work we theoretically study a five level M-system whose two unpopulated ground states are coupled by a micro-wave (MW) field. The key feature which makes M-systems more efficient in comparison to closed loop $\Lambda$ systems is the absence of MW field induced population transfer even at high intensities of the later. We examine lineshape of probe absorption as a function of its detuning in the presence of both control and MW fields. Read More

We study coherent population oscillations (CPO) in an odd isotope of the two-electron atom ${\rm ^{173}Yb} $. The experiments are done using magnetic sublevels of the $ F_g = 5/2 \rightarrow F_e = 3/2 $ hyperfine transition of the $ {\rm {^1S_0} \rightarrow {^3P_1}} $ intercombination line. The experiments are done both with and without a magnetic field. Read More

Affiliations: 1Centre for Quantum Technologies, National University of Singapore, 2School of Physical and Mathematical Sciences, Nanyang Technological University, 3Centre for Quantum Technologies, National University of Singapore, 4Centre for Quantum Technologies, National University of Singapore

In the presence of an external static magnetic field, an atomic gas becomes optically active, showing magneto-optical rotation. In the saturated regime, the coherences among the excited substates give a nonlinear contribution to the rotation of the light polarization. In contrast with the linear magneto-optical rotation, the nonlinear counterpart is insensitive to Doppler broadening. Read More

We describe an experimental apparatus capable of achieving a high loading rate of strontium atoms in a magneto-optical trap operating in a high vacuum environment. A key innovation of this setup is a two dimensional magneto-optical trap deflector located after a Zeeman slower. We find a loading rate of 6x10^9/s whereas the lifetime of the magnetically trapped atoms in the 3P2 state is 54s. Read More

We study the impact of the extra density fluctuations induced by primordial magnetic fields on the reionization history in the redshift range: $6 < z < 10$. We perform a comprehensive MCMC physical analysis allowing the variation of parameters related to primordial magnetic fields (strength, $B_0$, and power-spectrum index $n_{\scriptscriptstyle \rm B}$), reionization, and $\Lambda$CDM cosmological model. We find that magnetic field strengths in the range: $B_0 \simeq 0. Read More

The compression behavior of delafossite compound CuCrO2 has been investigated by in-situ x-ray diffraction and Raman spectroscopic measurements upto 23.2 and 34 GPa respectively. X-ray diffraction data shows the stability of ambient rhombohedral structure upto ~ 23 GPa. Read More

We obtain axionic charged primordial black holes on a vacuum created gravitational pair of $(3{\bar 3})$-brane by the Kalb-Ramond field on a $D_4$-brane in presence of a background open string metric. The new geometries on an emergent pair of $(3{\bar 3})$-brane universe is shown to be influenced by the non-perturbative quantum effects underlying a geometric torsion in a second order formalism. The presence of small extra dimensions transverse to the pair in the formalism ensures dynamical scalar fields hidden to a $3$-brane or an anti 3-brane universe. Read More

A geometric torsion dynamics leading to an effective curvature in a second order formalism on a D4-brane is revisited with a renewed interest. We obtain two effective $AdS_4$ brane geometries on a vacuum created pair of $(D{\bar D})_3$-brane. One of them is shown to describe an AdS Schwarzschild spinning black hole and the other is shown to describe a spinning black hole bound state. Read More

Affiliations: 1PAP, 2CQT, 3CQT, 4CQT, 5LKB, 6CQT, PAP, INLN, 7CQT, PAP, INLN

We investigate the transient coherent transmission of light through an optically thick cold stron-tium gas. We observe a coherent superflash just after an abrupt probe extinction, with peak intensity more than three times the incident one. We show that this coherent superflash is a direct signature of the cooperative forward emission of the atoms. Read More

We revisit an effective space-time torsion curvature in a second order formalism, underlying the non-linear $U(1)$ gauge dynamics, of a two form on a $D_4$-brane in type IIA superstring theory. The formalism incorporates the significance of a global NS two form into the theory via its perturbative coupling to a dynamical two form. In particular, we explore the non-linear gauge dynamics on a $D_4$-brane in presence of a non-trivial background metric. Read More

There has been a recent controversy about the high pressure polymorphism of Hafnium (Hf). Unlike, the earlier known {\alpha} $\rightarrow$ {\omega} structural transition at 38 $\pm$ 8 GPa, Hrubiak et al (2012) did not observe it till 51 GPa. We have reinvestigated the room temperature phase diagram of Hf, employing x-ray diffraction (XRD) and DFT based first principles calculations. Read More

Injection locking is a well known and commonly used method for coherent light amplification. Usually injection locking is done with a single-frequency seeding beam. In this work we show that injection locking may also be achieved in the case of multi-frequency seeding beam when slave laser provides sufficient frequency filtering. Read More

We investigate some of the quantum gravity effects on a vacuum created pair of $(D{\bar D})_3$-brane by a non-linear $U(1)$ gauge theory on a $D_4$-brane. In particular we obtain a four dimensional quantum Kerr(Newman) black hole in an effective torsion curvature formalism sourced by a two form dynamics in the world-volume of a $D_4$-brane on $S^1$. Interestingly the event horizon is found to be independent of a non-linear electric charge and the $4D$ quantum black hole is shown to describe a degenerate vacua in string theory. Read More

We demonstrate extremely narrow resonances for polarization rotation in an atomic vapor. The resonances are created using a strong control laser on the same transition, which polarizes the atoms due to optical pumping among the magnetic sublevels. As the power in the control laser is increased, successively higher-order nested polarization rotation resonances are created, with progressively narrower linewidths. Read More

We revisit a non-perturbative space-time curvature theory, underlying a two form U(1) gauge dynamics, on a D4-brane. In particular, two different gauge choices for a two form are explored underlying the dynamics of a geometric torsion in a second order formalism. We obtain two non-extremal quantum Kerr geometries in five dimensions on a pair of $(D{\bar D})_4$-brane in a type IIA superstring theory. Read More

We obtain a Schwarzschild and a Reissner-Nordstrom emergent black holes, by exploring the torsion dynamics in a generalized curvature formulation, underlying an effective D4-brane on S1. It is shown that a constant effective metric, sourced by a background fluctuation in B2-potential, on a D3-brane receives a dynamical quantum correction in presence of an electric charge. Read More

The U(1) gauge dynamics on a D4-brane is revisited, with a two form, to construct an effective curvature theory in a second order formalism. We exploit the local degrees in a two form, and modify its dynamics in a gauge invariant way, to incorporate a non-perturbative metric fluctuation in an effective D4-brane. Interestingly, the near horizon D4-brane is shown to describe an asymptotic Anti de Sitter (AdS) in a semi-classical regime. Read More

The well known Lambda-system using two ground state hyperfine levels, F=1 and F=2 of 5S1/2 and one hyperfine level, F=2 of excited state of 5P3/2 of 87Rb has been recently studied using two counter- propagating control lasers [1]. The experiment shows conversion of electromagnetically induced transparency (EIT) into electromagnetically induced absorption (EIA) because the doubly driven Lambda-system forms various sub-systems. We here present detailed theoretical study of the different possible sub-systems created by this configuration. Read More

From previous studies of the effect of primordial magnetic fields on early structure formation, we know that the presence of primordial magnetic fields during early structure formation could induce more perturbations at small scales (at present 1-10 Mpc/h) as compared to the usual LCDM theory. Matter power spectrum over these scales are effectively probed by cosmological observables such as shear correlation and Ly-alpha clouds, In this paper we discuss the implications of primordial magnetic fields on the distribution of Ly-alpha clouds. We simulate the line of sight density fluctuation including the contribution coming from the primordial magnetic fields. Read More

Co /CoO/Co polycrystalline film was grown on Si (001) substrate and magnetic properties have been investigated using in-situ magneto-optic Kerr effect during growth of the sample. Magnetic anisotropy with easy axis perpendicular to the film surface has been observed in top Co layer, whereas bottom layer was found to be soft with in-plane magnetization without any influence of top layer. Ex-situ in-plane and out-of-plane diffraction measurements revealed that the growth of Co on oxidized interface takes place with preferential orientation of c-axis perpendicular to the film plane, which results in the observed perpendicular magnetic anisotropy. Read More

The existence of primordial magnetic fields can induce matter perturbations with additional power at small scales as compared to the usual $\Lambda$CDM model. We study its implication within the context of two-point shear correlation function from gravitational lensing. We show that primordial magnetic field can leave its imprints on the shear correlation function at angular scales $\lesssim \hbox{a few arcmin}$. Read More

A mobile Ad-hoc network (MANET) is a dynamic multi hop wireless network established by a group of nodes in which there is no central administration. Due to mobility of nodes and dynamic network topology, the routing is one of the most important challenges in ad-hoc networks. Several routing algorithms for MANETs have been proposed by the researchers which have been classified into various categories, however, the most prominent categories are proactive, reactive and hybrid. Read More

Microscopic images from the biopsy samples of cervical cancer, the current "gold standard" for histopathology analysis, are found to be segregated into differing classes in their correlation properties. Correlation domains clearly indicate increasing cellular clustering in different grades of pre-cancer as compared to their normal counterparts. This trend manifests in the probabilities of pixel value distribution of the corresponding tissue images. Read More

We show that it is possible to change from a {\it subnatural} electromagnetically induced transparency (EIT) feature to a {\it subnatural} electromagnetically induced absorption (EIA) feature in a (degenerate) three-level $\Lambda$ system. The change is effected by turning on a second control beam counter-propagating with respect to the first beam. We observe this change in the $D_2$ line of Rb in a room-temperature vapor cell. Read More

We study limits on a primordial magnetic field arising from cosmological data, including that from big bang nucleosynthesis, cosmic microwave background polarization plane Faraday rotation limits, and large-scale structure formation. We show that the physically-relevant quantity is the value of the effective magnetic field, and limits on it are independent of how the magnetic field was generated. Read More

We demonstrate launching of laser-cooled Yb atoms in a cold atomic fountain. Atoms in a collimated thermal beam are first cooled and captured in a magneto-optic trap (MOT) operating on the strongly-allowed ${^1S}_0 \rightarrow {^1P}_1$ transition at 399~nm (blue line). They are then transferred to a MOT on the weakly-allowed ${^1S}_0 \rightarrow {^3P}_1$ transition at 556~nm (green line). Read More

We analyze theoretically the phenomenon of electromagnetically induced transparency (EIT) under conditions where the probe laser is not in the usual weak limit. We consider the effects in both three-level and four-level systems, which are either closed or open (due to losses to an external metastable level). We find that the EIT dip almost disappears in a closed three-level system but survives in an open system. Read More

We observe linewidths below the natural linewidth for a probe laser on a {\it two-level system}, when the same transition is driven by a strong control laser. We take advantage of the fact that each level is made of multiple magnetic sublevels, and use the phenomenon of electromagnetically induced transparency or absorption in multilevel systems. Optical pumping by the control laser redistributes the population so that only a few sublevels contribute to the probe absorption. Read More

Our investigations on porous Si show that on increase of pressure it undergoes crystalline phase transitions instead of pressure induced amorphization - claimed earlier, and the amorphous phase appears only on release of pressure. This amorphous phase, when subjected to higher pressures, transforms reversibly to a higher coordinated primitive hexagonal phase showing a kind of memory effect which may be the only example of its kind in the elemental solids. First principles calculations and thermodynamic arguments help understand these observations. Read More

Affiliations: 1Raman Research Institute and Carnegie Mellon University, 2Columbia University, 3Raman Research Institute

It has been proposed that primordial gas in early dark matter halos, with virial temperatures above 10^4 K, can avoid fragmentation and undergo rapid collapse, possibly resulting in a supermassive black hole (SMBH). This requires the gas to avoid cooling and to remain at temperatures near T=10^4 K. We show that this condition can be satisfied in the presence of a sufficiently strong primordial magnetic field, which heats the collapsing gas via ambipolar diffusion. Read More

A correlative study between the geomagnetic indices and the peak values of various plasma and field parameters during rising, maximum and decay phases as well as during complete solar cycle 23 have been presented. We have also presented the lag/lead analysis between the maximum of Dst and peak values of plasma and field parameters and found that peak values of lag/lead time lies in the +/-10 hr interval. Three geomagnetic storms (GMSs) and associated solar sources observed during these phases of this solar cycle have also been studied and found that GMSs are associated with large flares and halo CMEs. Read More

Kaluza-Klein gravity is revisted, with renewed interest, in a type IIB string theory on $S^1\times K3$. The irreducible curvature tensors are worked out in the, T-dual, emergent gravity in 4D to yield a non-linear U(1) gauge theory. Interestingly, the T-duality may be seen to describe an open/closed string duality at a self-dual string coupling. Read More

A covariantly constant dynamical two-form is exploited on a $D_3$-brane to obtain its gravity dual action, governing an $S^3$ deformed $AdS_5$ black hole, in a type IIB string theory on $S^1\times K3$. We invoke the Kaluza-Klein compactification to work out the open/closed string duality. Interestingly, the Reissner-Nordstrom black hole is obtained on the "non-Reimannian" braneworld. Read More

A study on North South (NS) asymmetry of different solar activity features (DSAF) such as solar proton events, solar active prominences, H alpha flare index, soft X ray flares, monthly mean sunspot area and monthly mean sunspot number were carried out from 1996 to 2008. It is found in our result that solar cycle 23 is magnetically weak compared to solar cycle 22. Study shows the Southern dominance of DSAF during the time period of study. Read More

This paper presents a statistical analysis of Soft X-ray (SXR) flares during the period January 1976 to December 2007 covering solar cycles (SCs) 21, 22, and 23. We have analysed north-south (N-S) and east-west (E-W) asymmetry of SXR at low (less than equal to 40 degree), high (greater than equal to 50 degree) and total latitudes and center meridian distances (CMDs) respectively. We have also presented the N-S and E-W asymmetry of different intensity classes (B, C, M, and X) during the period of investigation. Read More

A statistical analysis of dataset of H-alpha, soft X-ray (SXR) and Hard X-ray (HXR) flares comprising almost 318, 401 and 1920 single events respectively that occurred during the period of October-November 2003 is presented. On the basis of this dataset, statistics on temporal properties e.g. Read More

In this paper we present the results of a study of the spatial distribution and asymmetry of solar active prominences (SAP) for the period 1996-2007 (solar cycle 23). For more meaningful statistical analysis we have analysed the distribution and asymmetry of SAP in two subdivisions viz. Group1 (ADF, APR, DSF, CRN, CAP) and Group2 (AFS, ASR, BSD, BSL, DSD, SPY, LPS). Read More

Affiliations: 1Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital, India, 2Center for Research and Education in Science & Technology, Hosakote, Bangalore, India, 3Indian Institute of Astrophysics, Bangalore, India, 4Department of Physics, Kumaun University, Nainital, India
Category: Astrophysics

We present $UBVRI$ CCD photometry and optical spectra of the type Ia supernova SN 2003hx which appeared in the galaxy NGC 2076, obtained till $\sim$ 146 days after the epoch of $B$ band maximum. The supernova reached at maximum brightness in $B$ band on JD 245 2893 $\pm$ 1.0 with an apparent magnitude of 14. Read More

Using homogeneous CCD photometric data from the 105-cm Kiso Schmidt telescope covering a 50' x 50' field, we study the mass functions (MFs) of nine open clusters. The ages and Galactocentric distances of the target clusters vary from 16 - 2000 Myr and 9-10.8 kpc, respectively. Read More

Affiliations: 1ARIES, Nainital, 2RAC, Ooty, 3IGAM/Institute of Physics, Graz, 4ARIES, Nainital, 5KU, Nainital
Category: Astrophysics

The evolution of an X2.7 solar flare, that occurred in a complex $\beta\gamma\delta$ magnetic configuration region on 2003 November 3 is discussed utilizing a multi-wavelength data set. The very first signature of pre-flare coronal activity is observed in radio wavelengths as type III burst that occurred several minutes prior to the flare signature in H$\alpha$. Read More

Affiliations: 1ARIES, Nainital, India, 2ARIES, Nainital, India, 3RAC, Ooty, India, 4Department of Physics, KU, Nainital, India
Category: Astrophysics

In this paper, we have made a statistical analysis of solar H$\alpha$ flares that occurred during the period 1996 to 2005 to investigate their spatial distribution with respect to northern and southern hemispheres of the Sun. The analysis includes a total of 21608 single events. The study shows a significant N$-$S asymmetry which is persistent with the evolution of the solar cycle. Read More

We use the phenomenon of electromagnetically-induced transparency in a three-level atomic system for hyperfine spectroscopy of upper states that are not directly coupled to the ground state. The three levels form a ladder system: the probe laser couples the ground state to the lower excited state, while the control laser couples the two upper states. As the frequency of the control laser is scanned, the probe absorption shows transparency peaks whenever the control laser is resonant with a hyperfine level of the upper state. Read More

Using recent data for the photosphere-chromosphere region of the solar atmosphere the magnetic reconnection in tripolar geometry has been investigated through the procedure of Sturrock (1999). Particular attention has been given to the width of the reconnecting region, wave number of the rapidly growing tearing mode, island length scales, frequency of MHD fluctuations, tearing mode growth rate, energy dissipation rate and minimum magnetic field strength required to heat chromospheric plasma to coronal temperatures. It is found that small length scales are formed in the upper chromosphere. Read More