arXiv subjects

Pradeep Kumar

Publications and source records attributed to Pradeep Kumar.

At least 91 records · Page 5Linked to original sources

arXiv2015

High Field Ultrasound Measurements in UPt3 and the Single Energy Scale Model of Metamagnetism

We report longitudinal ultrasound velocity measurements for magnetic fields up to 33 T applied parallel to the a-axis of the heavy electron compound UPt$_{3}$. A characteristic dip in the sound velocity at the metamagnetic critical field, $H_{c}=20$ T, reported in earlier work is reproduced and shown to be independent of temperature at very low temperatures. We show that the single energy scale model (B.S. Shivaram et al., Phys. Rev. B89, 241107(R), 2014) captures the observed key features of the field dependence in the sound velocity shift, $δv_{s}$. The shift $δv_{s}$ at $H_{c}$ is found to be inversely dependent on temperature above 3\thinspace K and assumes a fixed value at low T. This saturation in $δv_{s}$ below 3 K is accounted for by level broadening of the Uranium spin states.

cond-mat.str-el↗

arXiv2015

Molecular Scale Hydrophobicity in Varying Degree of Planar Hydrophobic Nanoconfinement

We have studied the molecular scale hydrophobicity of an apolar solute, argon, confined between hydrophobic planar surfaces with different confinement widths. Specifically, we find that the hydrophobicity exhibits a non-monotonic behavior with confinement width. While hydrophobicity is usually large compared to bulk value, we find a narrow range of confinement width where the hydrophobicity displays similar values as in bulk water. Furthermore, we develop a simple model taking into account the entropic changes in nanoconfined geometry, which enables us to calculate potential of mean force between solutes as the conditions change from bulk to different degrees of planar nanoconfinement. Our results are important in understanding nanoconfinement induced stability of apolar polymers, solubility of gases, and may help design better systems for Enhanced Oil Recovery.

cond-mat.soft↗

arXiv2015

Spectroscopic Studies of Iron-based Superconductors, Multi-ferroic Oxides and Double-perovskite: Phonons, Electronic and Spin Excitations

Raman spectroscopy is a very powerful probe to study the nature of quasi-particle excitations in condensed matter physics. The work presented in this thesis is focused on two different families of novel materials, namely the iron-based superconductors (FeBS), multiferroic oxides and double perovskite. Although the properties of these two systems are quite different, some comparison can still be drawn between them. For instance, in both of these systems magnetism plays a crucial role and intricate coupling between phononic, magnetic and orbital degrees of freedom is crucial to understand their underlying physics responsible for their various exotic physical properties. Understanding the microscopic origin of quasi-particle excitations, such as phonons, magnons, orbitons, plasmons etc., and coupling between them in these complex materials, has been an intense field of research because it is believed that these excitations hold the key for explaining their rich physics. The systems studied in the thesis include (A) FeBS - (i) FeSe0.82 (ii) Ce1-zYzFeAsO1-xFx (z = 0, 0.4; x = 0.1, 0.2) (iii) Ca4Al2O5.7Fe2As2 (iv) Ca(Fe1-xCox)2As2 (x = 0.03, 0.05). (B) Multiferroic oxides - (i) AlFeO3 (ii) TbMnO3 and double perovskite (iii) La2NiMnO6.

cond-mat.mtrl-sci↗

arXiv2014

Dependence of loss rate of electrons due to elastic gas scattering on the shape of the vacuum chamber in an electron storage ring

The beam lifetime in an electron storage ring is also limited by the loss rate of the stored electrons due to the elastic coulomb scattering of electrons with the nuclei of residual gas atoms. The contribution to the beam lifetime due to this elastic scattering depends upon the shape factor which is governed by the shape of the vacuum chamber. In this paper, analytical expressions for the shape factor for a rectangular and an elliptical vacuum chamber as a function of longitudinal position along the circumference in a storage ring are derived using an approach in which the position of electrons at the focusing quadrupole is transformed to the location of defocusing quadrupole and vice versa to define the parts of the vacuum chamber, where the loss of electrons takes place at the location of quadrupoles. The expressions available in the literature are for the average shape factors. The expression of shape factor for a rectangular chamber derived in this paper are similar to the expression for average shape factor quoted in the literature, whereas a new expression for elliptical shape of vacuum chamber having no resemblance with the available expression for average shape factor is obtained. A comparative study of shape factors at each scattering location in the ring obtained from derived expressions and with the existing expressions are reported using Indus-2 lattice parameters for rectangular and elliptical shapes of the chamber. A comparison of average shape factors for these two shapes using derived as well as existing expressions is also reported. These studies indicate that the effect of the rectangular and elliptical shape of vacuum chamber on beam lifetime due to elastic coulomb scattering between electrons and nuclei of residual gas molecules is nearly same.

physics.acc-ph↗

arXiv2014

Phonon Anomalies, Orbital-Ordering and Electronic Raman Scattering in iron-pnictide Ca(Fe0.97Co0.03)2As2: Temperature-dependent Raman Study

We report inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a wide spectral range of 120-5200 cm-1 from 5K to 300K, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at Tsm ~ 160K. The mode frequencies of two first-order Raman modes B1g and Eg, both involving displacement of Fe atoms, show sharp increase below Tsm. Concomitantly, the linewidths of all the first-order Raman modes show anomalous broadening below Tsm, attributed to strong spin-phonon coupling. The high frequency modes observed between 400-1200 cm-1 are attributed to the electronic Raman scattering involving the crystal field levels of d-orbitals of Fe2+. The splitting between xz and yz d-orbital levels is shown to be ~ 25 meV which increases as temperature decreases below Tsm. A broad Raman band observed at ~ 3200 cm-1 is assigned to two-magnon excitation of the itinerant Fe 3d antiferromagnet.

cond-mat.supr-con↗

arXiv2014

Metamagnetism and the Fifth Order Susceptibility in UPt3

An enhanced susceptibility is a natural consequence of the "heavy fermion" (HF) state rendering the possibility of measurably large nonlinear susceptibilities. In recent work a universal behavior of the peaks observed in the linear (\Chi_1) and the third order (\Chi_3) susceptibility in HF metamagnets has been identified. This universality is well accounted for by a single energy scale model considering on-site correlations only. A prediction of this model is a peak in the fifth order susceptibility, \Chi_5, as well. In the first measurements on a HF metamagnet, UPt3 reported herein, we find that Chi_5 rather than attaining a peak, saturates at low temperatures and is positive. The thermodynamic implications of these towards the stability of the metamagnetic HF state are discussed.

cond-mat.str-el↗

arXiv2013

Temperature dependent magnetic, dielectric and Raman studies of partially disordered La2NiMnO6

We report a detailed magnetic, dielectric and Raman studies on partially disordered and biphasic double perovskite La2NiMnO6. DC and AC magnetic susceptibility measurements show two magnetic anomalies at TC1 ~ 270 K and TC2 ~ 240 K, which may indicate the ferromagnetic ordering of the monoclinic and rhombohedral phases, respectively. A broad peak at a lower temperature (Tsg ~ 70 K) is also observed indicating a spin-glass transition due to partial anti-site disorder of Ni2+ and Mn4+ ions. Unlike the pure monoclinic phase, the biphasic compound exhibits a broad but a clear dielectric anomaly around 270 K which is a signature of magneto-dielectric effect. Temperature-dependent Raman studies between the temperature range 12 K to 300 K in a wide spectral range from 220 cm-1 to 1530 cm-1 reveal a strong renormalization of the first as well as second-order Raman modes associated with the (Ni/Mn)O6 octahedra near TC1 implying a strong spin-phonon coupling. In addition, an anomaly is seen in the vicinity of spin-glass transition temperature in the temperature dependence of the frequency of the anti-symmetric stretching vibration of the octahedra.

cond-mat.mtrl-sci↗

arXiv2013

Raman evidence for coupling of superconducting quasi-particles with a phonon and crystal field excitation in superconductor Ce0.6Y0.4FeAsO0.8F0.2

We report inelastic light scattering experiments on superconductor Ce0.6Y0.4FeAsO0.8F0.2 from 4K to 300K covering the superconducting transition temperature Tc ~ 48.6K. A strong evidence of the superconductivity induced phonon renormalization for the A1g phonon mode near 150 cm-1 associated with the Ce/Y vibrations is observed as reflected in the anomalous red-shift and decrease in the linewidth below Tc. Invoking the coupling of this mode with the superconducting gap, the superconducting gap (2) at zero temperature is estimated to be ~ 20 meV i.e the ratio is ~ 5, suggesting Ce0.6Y0.4FeAsO0.8F0.2 to belong to the class of strong coupling superconductors. In addition, the mode near 430 cm-1 associated with Ce3+ crystal field excitation also shows anomalous increase in its linewidth below Tc suggesting strong coupling between crystal field excitation and the superconducting quasi-particles. Our observations of two high frequency modes (S9 and S10) evidence the non-degenerate nature of Fe2+ dxz/yz orbitals suggesting the electronic nematicity in these systems.

cond-mat.mtrl-sci↗

arXiv2013

Amino Acid Distributions and the Effect of Optimal Growth Temperature

We perform an exhaustive analysis of genome statistics for organisms, particularly extremophiles, growing in a wide range of physicochemical conditions. Specifically, we demonstrate how the correlation between the frequency of amino acids and their molecular weight, preserved on average, typically decreases as optimal growth temperature increases. We show how the relation between codon degeneracy and amino acid mass is enforced across these organisms. We assess the occurrence of contiguous amino acids, finding several significant short words, often containing cysteine, histidine or proline. Typically, the significance of these words is independent of growth temperature. In a novel approach, first-passage distributions are used to capture correlations between discontiguous residues. We find a nearly universal exponential background that we relate to properties of the aforementioned individual amino acid frequencies. We find this approach reliably extracts correlations that depend on growth temperature, some of which have not been previously characterized.

q-bio.GN↗

arXiv2013

The Boson peak in supercooled water

We perform extensive molecular dynamics simulations of the TIP4P/2005 model of water to investigate the origin of the Boson peak reported in experiments on supercooled water in nanoconfined pores, and in hydration water around proteins. We find that the onset of the Boson peak in supercooled bulk water coincides with the crossover to a predominantly low-density-like liquid below the Widom line $T_W$. The frequency and onset temperature of the Boson peak in our simulations of bulk water agree well with the results from experiments on nanoconfined water. Our results suggest that the Boson peak in water is not an exclusive effect of confinement. We further find that, similar to other glass-forming liquids, the vibrational modes corresponding to the Boson peak are spatially extended and are related to transverse phonons found in the parent crystal, here ice Ih.

cond-mat.soft↗

arXiv2012

Pressure and temperature dependence of growth and morphology of Escherichia coli: Experiments and Stochastic Model

We have investigated the growth of Escherichia coli E.coli, a mesophilic bacterium, as a function of pressure $P$ and temperature $T$. E.coli can grow and divide in a wide range of pressure (1-400atm) and temperature ($23-40^{\circ}$C). For $T>30^{\circ}$ C, the division time of E.coli increases exponentially with pressure and exhibit a departure from exponential behavior at pressures between 250-400 atm for all the temperatures studied in our experiments. For $T<30^{\circ}$ C, the division time shows an anomalous dependence on pressure -- first decreases with increasing pressure and then increases upon further increase of pressure. The sharp change in division time is followed by a sharp change in phenotypic transition of E. Coli at high pressures where bacterial cells switch to an elongating cell type. We propose a model that this phenotypic changes in bacteria at high pressures is an irreversible stochastic process whereas the switching probability to elongating cell type increases with increasing pressure. The model fits well the experimental data. We discuss our experimental results in the light of structural and thus functional changes in proteins and membranes.

physics.bio-ph↗

arXiv2012

Superconducting Fluctuations and Anomalous Phonon Renormalization much above superconducting transition temperature in Ca4Al2O5.7Fe2As2

Raman studies on Ca4Al2O5.7Fe2As2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature Tc ~ 28.3 K, reveal that the Raman mode at ~ 230 cm-1 shows a sharp jump in frequency by ~ 2 % and linewidth increases by ~ 175 % at To ~ 60 K. Below To, anomalous softening of the mode frequency and a large decrease by ~ 10 cm-1 in the linewidth is observed. These precursor effects at T0 (~ 2Tc) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weaked coupling between the well separated (~ 15 Å) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K to 60 K (~7%) indicates strong spin-phonon coupling in this superconductor.

cond-mat.supr-con↗

arXiv2012

Coupled Phonons, Magnetic Excitations and Ferroelectricity in AlFeO3: Raman and First-principles Studies

We determine the nature of coupled phonons and magnetic excitations in AlFeO3 using inelastic light scattering from 5 K to 315 K covering a spectral range from 100-2200 cm-1 and complementary first-principles density functional theory-based calculations. A strong spin-phonon coupling and magnetic ordering induced phonon renormalization are evident in (a) anomalous temperature dependence of many modes with frequencies below 850 cm-1, particularly near the magnetic transition temperature Tc ~ 250 K, (b) distinct changes in band positions of high frequency Raman bands between 1100-1800 cm-1, in particular a broad mode near 1250 cm-1 appears only below Tc attributed to the two-magnon Raman scattering. We also observe weak anomalies in the mode frequencies at ~ 100 K, due to a magnetically driven ferroelectric phase transition. Understanding of these experimental observations has been possible on the basis of first-principles calculations of phonons spectrum and their coupling with spins.

cond-mat.str-el↗

arXiv2011

Temperature-dependent Raman study of CeFeAsO0.9F0.1 Superconductor: Crystal field excitations, phonons and their coupling

We report temperature-dependent Raman spectra of CeFeAsO0.9F0.1 from 4 K to 300 K in spectral range of 60 to 1800 cm-1 and interpret them using estimates of phonon frequencies obtained from first-principles density functional calculations. We find evidence for a strong coupling between the phonons and crystal field excitations; in particular Ce3+ crystal field excitation at 432 cm-1 couples strongly with Eg oxygen vibration at 389 cm-1 . Below the superconducting transition temperature, the phonon mode near 280 cm-1 shows softening, signaling its coupling with the superconducting gap. The ratio of the superconducting gap to Tc thus estimated to be ~ 10 suggests CeFeAsO0.9F0.1 as a strong coupling superconductor. In addition, two high frequency modes observed at 1342 cm-1 and 1600 cm-1

cond-mat.supr-con↗

arXiv2011

Raman Evidence for Superconducting Gap and Spin-Phonon Coupling in Superconductor Ca(Fe0.95Co0.05)2As2

Inelastic light scattering studies on single crystal of electron-doped Ca(Fe0.95Co0.05)2As2 superconductor, covering the tetragonal to orthorhombic structural transition as well as magnetic transition at TSM ~ 140 K and superconducting transition temperature Tc ~ 23 K, reveal evidence for superconductivity-induced phonon renormalization; in particular the phonon mode near 260 cm-1 shows hardening below Tc, signaling its coupling with the superconducting gap. All the three Raman active phonon modes show anomalous temperature dependence between room temperature and Tc i.e phonon frequency decreases with lowering temperature. Further, frequency of one of the modes shows a sudden change in temperature dependence at TSM. Using first-principles density functional theory-based calculations, we show that the low temperature phase (Tc < T < TSM) exhibits short-ranged stripe anti-ferromagnetic ordering, and estimate the spin-phonon couplings that are responsible for these phonon anomalies.

cond-mat.supr-con↗

arXiv2011

Raman signatures of pressure induced electronic topological and structural transitions in Bi2Te3

We report Raman signatures of electronic topological transition (ETT) at 3.6 GPa and rhombohedral (α-Bi2Te3) to monoclinic (β-Bi2Te3) structural transition at ~ 8 GPa. At the onset of ETT, a new Raman mode appears near 107 cm-1 which is dispersionless with pressure. The structural transition at ~ 8 GPa is marked by a change in pressure derivative of A1g and Eg mode frequencies as well as by appearance of new modes near 115 cm-1and 135 cm-1. The mode Grüneisen parameters are determined in both the α and β-phases.

cond-mat.mtrl-sci↗

arXiv2010

Nonreciprocal Bloch Oscillations in Magneto-Optic Waveguide Arrays

We show that nonreciprocal optical Bloch-like oscillations can emerge in transversely magnetized waveguide arrays in the presence of an effective index step between the waveguides. Normal modes of the system are shown to acquire different wavenumbers in opposite propagation directions. Significant differences in phase coherence and decoherence between these normal modes are presented and discussed. Non-reciprocity is established by imposing unequal vertical refractive index gradients at the substrate/core, and core/cover interfaces in the presence of transverse magnetization.

physics.optics↗

arXiv2010

Effect of pressure on the kinetics of bulge bases in small RNAs

Using molecular dynamics simulations, we study the effect of pressure on the binding propensity of small RNAs by calculating the free energy barrier corresponding to the looped out conformations of unmatched base, which presumably acts as the binding sites for ligands. We find that the free energy associated with base looping out increases monotonically as the pressure is increased. Furthermore, we calculate the mean first passage time of conformational looping out of the base bulge using the diffusion of reaction coordinate associated with the base flipping on the underlying free energy surface. We find that the mean first passage time associated with bulge looping out increases slowly upon increasing pressures $P$ upto $2$~kbar but changes dramatically for $P>2$~kbar. Finally, we discuss our results in the light of the role of hydration shell of water around RNA.

cond-mat.soft↗