arXiv subjects

N. Khan

Publications and source records attributed to N. Khan.

32 records · Page 2Linked to original sources

arXiv2015

Nucleon-Gold Collisions at 200 AGeV Using Tagged d+Au Interactions in PHOBOS

Forward calorimetry in the PHOBOS detector has been used to study charged hadron production in d+Au, p+Au and n+Au collisions at sqrt(s_nn) = 200 GeV. The forward proton calorimeter detectors are described and a procedure for determining collision centrality with these detectors is detailed. The deposition of energy by deuteron spectator nucleons in the forward calorimeters is used to identify p+Au and n+Au collisions in the data. A weighted combination of the yield of p+Au and n+Au is constructed to build a reference for Au+Au collisions that better matches the isospin composition of the gold nucleus. The p_T and centrality dependence of the yield of this improved reference system is found to match that of d+Au. The shape of the charged particle transverse momentum distribution is observed to extrapolate smoothly from pbar+p to central d+Au as a function of the charged particle pseudorapidity density. The asymmetry of positively- and negatively-charged hadron production in p+Au is compared to that of n+Au. No significant asymmetry is observed at mid-rapidity. These studies augment recent results from experiments at the LHC and RHIC facilities to give a more complete description of particle production in p+A and d+A collisions, essential for the understanding the medium produced in high energy nucleus-nucleus collisions.

nucl-ex↗

arXiv2015

The Q_weak Experimental Apparatus

The Jefferson Lab Q_weak experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise ${\vec{e}}$p asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 microA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Moller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8 degrees and 11.6 degrees were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cerenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q^2 = 0.025 GeV^2 was determined using dedicated low-current (~100 pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.

physics.ins-det↗

arXiv2014

Memory effects and magnetic relaxation in single-crystalline La$_{0.9}$Sr$_{0.1}$CoO$_3$

We report a detailed investigation of magnetic relaxation and memory effects in La$_{0.9}$Sr$_{0.1}$CoO$_3$ single crystal from dc magnetization measurements. The analysis of magnetic relaxation at different temperatures and magnetic fields below the freezing temperature ($T_{f}$) manifests the characteristics of spin glass phase. Below $T_{f}$, striking memory effects have been clearly observed in different experimental protocols. The memory effect in the zero-field-cooled magnetization further establishes that the glassy magnetic state arises from the co-operative spin-spin interaction but not due to the independent relaxation of metastable phase clusters. The asymmetric response with respect to negative and positive temperature change favors the hierarchical model of memory effects rather than the droplet model discussed in other works for different insulating and metallic Heisenberg spin glasses.

cond-mat.str-el↗

arXiv2014

Field-induced spin-structural transition and giant magnetostriction in Ising chain $α$-CoV$_2$O$_6$

We have investigated the temperature and magnetic field dependence of magnetization, specific heat ($C_p$), and relative sample length change ($ΔL/L_0$) for understanding the field-induced spin-structural change in quasi-one-dimensional spin chain $α$-CoV$_2$O$_6$ which undergoes antiferromagnetic (AFM) transition below $T_N$$=$15 K. Analysis of $C_p$($T$) shows that an effective $S$$=$1/2 Ising state is realized below 20 K, though the magnetic fluctuations persist well above $T_N$. $C_p$ and the coefficient of linear thermal expansion ($α$) exhibit strong $H$ dependence in the AFM state. We also observe a huge positive magnetostriction [$ΔL$($H$)/$L_0$] below 20 K which does not show any tendency of saturation up to 9 T. With increasing field, a sharp and symmetric peak emerges below $T_N$ in both $C_p$($T$) and $α$($T$) due to field-induced first order ferrimagnetic/ferromagnetic-paramagnetic transitions. The large value of magnetostriction below $T_N$ suggests strong spin-lattice coupling in $α$-CoV$_2$O$_6$.

cond-mat.str-el↗

arXiv2013

3d-4f spin interaction induced giant magnetocaloric effect in zircon-type DyCrO4 and HoCrO4 compounds

We have investigated the influence of 3d-4f spin interaction on magnetic and magnetocaloric properties of DyCrO$_4$ and HoCrO$_4$ compounds by magnetization and heat capacity measurements. Both the compounds exhibit complicated magnetic properties and huge magnetic entropy change around the ferromagnetic transition due to the strong competition between ferromagnetic and antiferromagnetic superexchange interactions. For a field change of 8 T, the maximum values of magnetic entropy change ($ΔS_{M}^{max}$), adiabatic temperature change ($ΔT_{ad}$), and refrigerant capacity (RC) reach 29 J kg$^{-1}$ K$^{-1}$, 8 K, and 583 J kg$^{-1}$, respectively for DyCrO$_4$ whereas the corresponding values for HoCrO$_4$ are 31 J kg$^{-1}$ K$^{-1}$, 12 K, and 622 J kg$^{-1}$. $ΔS_{M}^{max}$, $ΔT_{ad}$, and RC are also quite large for a moderate field change. The large values of magnetocaloric parameters suggest that the zircon-type DyCrO$_4$ and HoCrO$_4$ could be the potential magnetic refrigerant materials for liquefaction of hydrogen.

cond-mat.str-el↗

arXiv2013

REECH-ME: Regional Energy Efficient Cluster Heads based on Maximum Energy Routing Protocol for WSNs

In this paper, we propose Regional Energy Efficient Cluster Heads based on Maximum Energy (REECH-ME) Routing Protocol for Wireless Sensor Networks (WSNs) . The main purpose of this protocol is to improve the network lifetime and particularly the stability period of the network. In REECH-ME, the node with the maximum energy in a region becomes Cluster Head (CH) of that region for that particular round and the number of the cluster heads in each round remains the same. Our technique outperforms LEACH which uses probabilistic approach for the selection of CHs. We also implement the Uniform Random Distribution Model to find the packet drop to make this protocol more practical. We also calculate the confidence interval of all our results which helps us to visualize the possible deviation of our graphs from the mean value.

cs.NI↗

arXiv2013

Critical exponents and irreversibility lines of La$_{0.9}$Sr$_{0.1}$CoO$_3$ single crystal

We have studied the dynamic and static critical behavior of spin glass transition in insulating La$_{0.9}$Sr$_{0.1}$CoO$_3$ single crystal by ac susceptibility and dc magnetization measurements in the vicinity of its freezing temperature ($T_f$). The dynamic scaling analysis of the frequency dependence of ac susceptibility data yields the characteristic time constant $τ_{0}$=1.6(9)$\times10^{-12}$ s, the dynamic critical exponent $zν$=9.5(2), and a frequency dependence factor $K$=$Δ$$T_{f}/T_{f}$($Δ$$log$$f$)=0.017, indicating that the sample enters into a canonical spin-glass phase below $T_{f}$=34.8(2) K. The scaling analysis of non-linear magnetization in the vicinity of $T_{f}$ through the static scaling hypothesis yields critical exponents $β$=0.89(1) and $γ$=2.9(1), which match well with that observed for well known three-dimensional (3D) Heisenberg spin glasses. From the longitudinal component of zero-field-cooled and field-cooled magnetization measurement we have constructed the $H-T$ phase diagram which represents the field evolution of two characteristic temperatures: the upper one, $T_{w}(H)$, indicates the onset of spin freezing in a uniform external field $H$, while the lower one, $T_{s}(H)$, marks the onset of strong irreversibility of the frozen state. The low field $T_{s}(H)$ follows the critical line suggested by d'Almeida-Thouless model for canonical spin glass, whereas the $T_{w}(H)$ exhibits a reentrant behavior with a maximum in the $T_{w}(H)$ at a nonzero field above which it follows the Gabay-Toulouse (GT) critical line which is a characteristic of Heisenberg spin glass. The re-entrant behavior of the GT line resembles that predicted theoretically for $n$-component vector spin glasses in the presence of a uniaxial anisotropy field.

cond-mat.str-el↗

arXiv2012

Giant magnetocaloric effect in magnetically frustrated EuHo$_2$O$_4$ and EuDy$_2$O$_4$ compounds

We have investigated the magnetic and magnetocaloric properties of EuHo$_2$O$_4$ and EuDy$_2$O$_4$ by magnetization and heat capacity measurements down to 2 K. These compounds undergo a field-induced antiferromagnetic to ferromagnetic transition and exhibit a huge entropy change. For a field change of 0-8 T, the maximum magnetic entropy and adiabatic temperature changes are 30 (25) J kg$^{-1}$ K$^{-1}$ and 12.7 (16) K, respectively and the corresponding value of refrigerant capacity is 540 (415) J kg$^{-1}$ for EuHo$_2$O$_4$ (EuDy$_2$O$_4$). These magnetocaloric parameters also remain large down to lowest temperature measured and are even larger than that for some of the potential magnetic refrigerants reported in the same temperature range for a moderate field change. Moreover, these materials are highly insulating and exhibit no thermal and field hysteresis, fulfilling the necessary conditions for a good magnetic refrigerant in the low-temperature region.

cond-mat.str-el↗

arXiv2012

Anomalous thermal expansion of Sb$_2$Te$_3$ topological insulator

We have investigated the temperature dependence of the linear thermal expansion along the hexagonal c axis ($ΔL$), in-plane resistivity ($ρ$), and specific heat ($C_p$) of the topological insulator Sb$_2$Te$_3$ single crystal. $ΔL$ exhibits a clear anomaly in the temperature region 204-236 K. The coefficient of linear thermal expansion $α$ decreases rapidly above 204 K, passes through a deep minimum at around 225 K and then increases abruptly in the region 225-236 K. $α$ is negative in the interval 221-228 K. The temperature dependence of both $α$ and $C_p$ can be described well by the Debye model from 2 to 290 K, excluding the region around the anomaly in $α$.

cond-mat.str-el↗

arXiv2011

Griffiths phase in solid-solution of ferromagnetic manganite and cobaltite

We report the existence of Griffiths phase (GP) over a wide range of $x$ in La$_{0.6}$Sr$_{0.4}$Mn$_{1-x}$Co$_x$O$_3$, the solid solution of ferromagnetic (FM) La$_{0.6}$Sr$_{0.4}$MnO$_3$ and La$_{0.6}$Sr$_{0.4}$CoO$_3$, from magnetization measurements. In this compound, GP arises due to the quenching of randomly distributed Co-O-Mn antiferromagnetic bonds in the FM background. In contrary to divalent doped manganites, GP in the present system can exist entirely in the metallic state above $T_C$ (for $x$$<$0.10). Based on the present study, a magnetoelectronic phase diagram is drawn.

cond-mat.str-el↗

arXiv2010

Phobos results on charged particle multiplicity and pseudorapidity distributions in Au+Au, Cu+Cu, d+Au, and p+p collisions at ultra-relativistic energies

Pseudorapidity distributions of charged particles emitted in $Au+Au$, $Cu+Cu$, $d+Au$, and $p+p$ collisions over a wide energy range have been measured using the PHOBOS detector at RHIC. The centrality dependence of both the charged particle distributions and the multiplicity at midrapidity were measured. Pseudorapidity distributions of charged particles emitted with $|η|<5.4$, which account for between 95% and 99% of the total charged-particle emission associated with collision participants, are presented for different collision centralities. Both the midrapidity density, $dN_{ch}/dη$, and the total charged-particle multiplicity, $N_{ch}$, are found to factorize into a product of independent functions of collision energy, $\sqrt{s_{_{NN}}}$, and centrality given in terms of the number of nucleons participating in the collision, $N_{part}$. The total charged particle multiplicity, observed in these experiments and those at lower energies, assumes a linear dependence of $(\ln s_{_{NN}})^2$ over the full range of collision energy of $\sqrt{s_{_{NN}}}$=2.7-200 GeV.

nucl-ex↗

arXiv2008

System size, energy, centrality and pseudorapidity dependence of charged-particle density in Au+Au and Cu+Cu collisions at RHIC

Charged particle pseudorapidity distributions are presented from the PHOBOS experiment at RHIC, measured in Au+Au and Cu+Cu collisions at sqrt{s_NN}=19.6, 22.4, 62.4, 130 and 200 GeV, as a function of collision centrality. The presentation includes the recently analyzed Cu+Cu data at 22.4 GeV. The measurements were made by the same detector setup over a broad range in pseudorapidity, |eta|<5.4, allowing for a reliable systematic study of particle production as a function of energy, centrality and system size. Comparing Cu+Cu and Au+Au results, we find that the total number of produced charged particles and the overall shape (height and width) of the pseudorapidity distributions are determined by the number of nucleon participants, N_part. Detailed comparisons reveal that the matching of the shape of the Cu+Cu and Au+Au pseudorapidity distributions over the full range of eta is better for the same N_part/2A value than for the same N_part value, where A denotes the mass number. In other words, it is the geometry of the nuclear overlap zone, rather than just the number of nucleon participants that drives the detailed shape of the pseudorapidity distribution and its centrality dependence.

nucl-ex↗

arXiv2008

Non-flow correlations and elliptic flow fluctuations in Au+Au collisions at sqrt(s_NN)=200GeV

We present first results on event-by-event elliptic flow fluctuations in nucleus-nucleus collisions corrected for effects of non-flow correlations where the magnitude of non-flow correlations has been independently measured in data. Over the measured range in centrality, we see large relative fluctuations of 25-50%. The results are consistent with predictions from both color glass condensate and Glauber type initial condition calculations of the event-by-event participant eccentricity fluctuations.

nucl-ex↗

arXiv2006

Identified hadron transverse momentum spectra in Au+Au collisions at sqrt(s_(NN))=62.4 GeV

Transverse momentum spectra of pions, kaons, protons and antiprotons from Au+Au collisions at sqrt(s_(NN)) = 62.4 GeV have been measured by the PHOBOS experiment at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The identification of particles relies on three different methods: low momentum particles stopping in the first detector layers; the specific energy loss (dE/dx) in the silicon Spectrometer, and Time-of-Flight measurement. These methods cover the transverse momentum ranges 0.03-0.2, 0.2-1.0 and 0.5-3.0 GeV/c, respectively. Baryons are found to have substantially harder transverse momentum spectra than mesons. The pT region in which the proton to pion ratio reaches unity in central Au+Au collisions at sqrt(s_(NN)) = 62.4 GeV fits into a smooth trend as a function of collision energy. At low transverse momenta, the spectra exhibit a significant deviation from transverse mass scaling, and when the observed particle yields at very low pT are compared to extrapolations from higher pT, no significant excess is found. By comparing our results to Au+Au collisions at sqrt(s_(NN)) = 200 GeV, we conclude that the net proton yield at midrapidity is proportional to the number of participant nucleons in the collision.

nucl-ex↗