arXiv subjects

P. Mandal

Publications and source records attributed to P. Mandal.

At least 37 records · Page 2Linked to original sources

arXiv2015

Magnetic field induced drastic violation of Wiedemann-Franz law in Dirac semimetal Cd$_{3}$As$_{2}$

The journey through the nontrivial band topology beyond the conventional band structure has resulted in the recent discovery of three-dimensional Dirac semimetal phase in Na$_{3}$Bi and Cd$_{3}$As$_{2}$. The bulk state of which is semi-metallic obeying linear energy dispersion, while the surface state is topology protected Fermi arc. Due to the unique band topology, they show different exotic electronic properties of both fundamental and technological interest. From electrical and thermal transport measurements, we have demonstrated a remarkable violation of Wiedemann-Franz law (WFL) under application of magmatic field in Cd$_{3}$As$_{2}$ and the violation becomes more and more drastic with increasing magnetic field strength. Whereas the validity of WFL is the key feature of Landau Fermi-liquid theory in metal, the notion of quasiparticles is the building block to this theory. This implies that the fundamental concept of Landau quasiparticle no longer holds in Cd$_{3}$As$_{2}$ in presence of magnetic field. The continuous break down of Landau quasiparticle framework with field introduces a concept of field induced quantum critical point (QCP) as in the case of heavy fermion compounds YbRh$_{2}$Si$_{2}$, Sr$_{3}$Ru$_{2}$O$_{7}$, etc.

cond-mat.str-el↗

arXiv2015

Field induced metamagnetic transitions in quasi-one-dimensional Ising spin chain CoV$_{2}$O$_{6}$

We have investigated the temperature and magnetic field dependence of magnetization ($M$), specific heat ($C_p$), and relative sample length change ($ΔL/L_0$) for understanding the field-induced metamagnetic transitions in quasi-one-dimensional monoclinic ($α$ phase) and triclinic ($γ$ phase) CoV$_2$O$_6$ spin chains. With the application of external magnetic field, a sharp peak emerges in $C_p$($T$) below $T_N$$=$15 K in $α$ phase and a hump-like feature appears above $T_N$$=$6.5 K in $γ$ phase due to the field-induced ferrimagnetic and ferromagnetic transitions, respectively. Strong field dependence of linear thermal expansion and large positive magnetostriction have been observed in $α$ phase. Though magnetization data indicate strong spin-orbit coupling for both phases, other measurements show that this effect is very weak for the $γ$ phase.

cond-mat.str-el↗

arXiv2015

Spin dynamics and frequency dependence of magnetic damping study in soft ferromagnetic FeTaC film with a stripe domain structure

Perpendicular magnetic anisotropy (PMA) and low magnetic damping are the key factors for the free layer magnetization switching by spin transfer torque technique in magnetic tunnel junction devices. The magnetization precessional dynamics in soft ferromagnetic FeTaC thin film with a stripe domain structure was explored in broad band frequency range by employing micro-strip ferromagnetic resonance technique. The polar angular variation of resonance field and linewidth at different frequencies have been analyzed numerically using Landau-Lifshitz-Gilbert equation by taking into account the total free energy density of the film. The numerically estimated parameters Landé $g$-factor, PMA constant, and effective magnetization are found to be 2.1, 2$\times10^{5}$ erg/cm$^{3}$ and 7145 Oe, respectively. The frequency dependence of Gilbert damping parameter ($α$) is evaluated by considering both intrinsic and extrinsic effects into the total linewidth analysis. The value of $α$ is found to be 0.006 at 10 GHz and it increases with decreasing precessional frequency.

cond-mat.mtrl-sci↗

arXiv2015

Probing the Fermi Surface of 3D Dirac Semimetal Cd$_{3}$As$_{2}$ through de Haas-van Alphen Technique

We have observed Shubnikov-de Haas and de Haas-van Alphen effect in the single crystals of three dimensional Dirac semimetal Cd$_{3}$As$_{2}$ upto 50 K, traceable at field as low as 2 T and 1 T, respectively. The values of Fermi wave vector, Fermi velocity, and effective cyclotron mass of charge carrier, calculated from both the techniques, are close to each other and match well with the earlier reports. However, the de Haas-van Alphen effect clearly reflects the existence of two different Fermi surface cross-sections along certain direction and a non-trivial Berry's phase which is the signature of 3D Dirac Fermion in Cd$_{3}$As$_{2}$.

cond-mat.str-el↗

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

Space charge and collective oscillation of ion cloud in a linear Paul trap

The presence of charged particles in an ion trap modifies the harmonic trapping potential in which they are trapped, leading to observed shifts in secular frequency as well as appearance of collective oscillation. In a linear trap geometry, both of these effects have been observed under different trapping conditions using narrow non-linear resonance and external excitation. The observations have been modeled with minimal fitting parameter showing good agreement with results obtained. The space charge in our experiment plays an important role in terms of criticality of the onset of collective oscillation.

physics.atom-ph↗

arXiv2013

Temperature-dependent structural property and power factor of n type thermoelectric Bi0.90Sb0.10 and Bi0.86Sb0.14 alloys

Thermal variation of structural property, linear thermal expansion coefficient, resistivity, thermopower and power factor of polycrystalline Bi1-xSbx (x=0.10, 0.14) samples are reported. Temperature-dependent powder diffraction experiments indicate that samples do not undergo any structural phase transition. Rietveld refinement technique has been used to perform detailed structural analysis. Temperature dependence of thermal expansion coefficient is found to be stronger for Bi0.90Sb0.10. Also, power factor for direct band gap Bi0.90Sb0.10 is higher as compared to that for indirect band gap Bi0.86Sb0.14. Role of electron-electron and electron-phonon scattering on resistivity, thermopower and power factor have been discussed.

cond-mat.mtrl-sci↗

arXiv2013

Large magnetocapacitance in electronic ferroelectric manganite systems

We have observed a sizable positive magnetocapacitance ($\sim$$5-90\%$) in perovskite Pr$_{0.55}$Ca$_{0.45}$MnO$_3$ and bilayer Pr(Sr$_{0.1}$Ca$_{0.9}$)$_2$Mn$_2$O$_7$ system under 5T magnetic field across 20-100 K below the magnetic transition point T$_N$. The magnetodielectric effect, on the other hand, exhibits a crossover: (a) from positive to negative for the perovskite system and (b) from negative to positive for the bilayer system over the same temperature range. The bilayer Pr(Sr$_{0.1}$Ca$_{0.9}$)$_2$Mn$_2$O$_7$ system exhibits a sizable anisotropy as well. We have also noticed the influence of magnetic field on the dielectric relaxation characteristics of these systems. These systems belong to a class of improper ferroelectrics and are expected to exhibit charge/orbital order driven ferroelectric polarization below the transition point T$_{CO}$. Large magnetocapacitance in these systems shows typical multiferroic behavior even though the ferroelectric polarization is small in comparison to that of other ferroelectrics.

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

Dynamics of ion cloud in a linear Paul trap

A linear ion trap setup has been developed for studying the dynamics of trapped ion cloud and thereby realizing possible systematics of a high precision measurement on a single ion within it. The dynamics of molecular nitrogen ion cloud has been investigated to extract the characteristics of the trap setup. The stability of trap operation has been studied with observation of narrow nonlinear resonances pointing out the region of instabilities within the broad stability region. The secular frequency has been measured and the motional spectra of trapped ion oscillation have been obtained by using electric dipole excitation. It is applied to study the space charge effect and the axial coupling in the radial plane.

physics.atom-ph↗

arXiv2013

Vortex dynamics and second magnetization peak in PrFeAsO$_{0.60}$F$_{0.12}$ superconductor

We have studied the vortex dynamics in the PrFeAsO$_{0.60}$F$_{0.12}$ superconducting sample by dc magnetization and dynamic magnetization-relaxation rate $(Q)$ measurements. The field dependence of the superconducting irreversible magnetization $M_s$ reveals a second magnetization peak or fishtail effect. The large value of $Q$ is an indication of moderate vortex motion and relatively weak pinning energy. Data analysis based on the generalized inversion scheme suggests that the vortex dynamics can be described by the collective pinning model. The temperature dependence of the critical current is consistent with the pinning due to the spatial variation in the mean free path near a lattice defect ($δl$ pinning). The temperature and field dependence of $Q$ indicates a crossover from elastic to plastic vortex creep with increasing temperature and magnetic field. Finally, we have constructed the vortex phase diagram based on the present data.

cond-mat.supr-con↗

arXiv2013

Scaling of NonOhmic Conduction in Strongly Correlated Systems

A new scaling formalism is used to analyze nonlinear I-V data in the vicinity of metal-insulator transitions (MIT) in five manganite systems. An exponent, called the nonlinearity exponent, and an onset field for nonlinearity, both characteristic of the system under study, are obtained from the analysis. The onset field is found to have an anomalously low value corroborating the theoretically predicted electronically soft phases. The scaling functions above and below the MIT of a polycrystalline sample are found to be the same but with different exponents which are attributed to the distribution of the MIT temperatures. The applicability of the scaling in manganites underlines the universal response of the disordered systems to electric field.

cond-mat.dis-nn↗

arXiv2013

Electric field driven destabilization of the insulating state in nominally pure LaMnO3

We report an electric field driven destabilization of the insulating state in nominally pure LaMnO3 single crystal with a moderate field which leads to a resistive state transition below 300 K. The transition is between the insulating state in LaMnO3 and a high resistance bad metallic state that has a temperature independent resistivity. The transition occurs at a threshold field (Eth) which shows a steep enhancement on cooling. While at lower temperatures the transition is sharp and involves large change in resistance but it softens on heating and eventually absent above 280K. When the Mn4+ content is increased by Sr substitution up to x=0.1, the observed transition though observable in certain temperature range, softens considerably. The observation has been explained as bias driven percolation type transition between two coexisting phases, where the majority phase is a charge and orbitally ordered polaronic insulating phase and the minority phase is a bad metallic phase. The mobile fraction f of the bad metallic phase deduced from the experimental data follows an activated kinetics with the activation energy nearly equal to 200 meV and the prefactor fo is a strong function of the field that leads to a rapid enhancement of f on application of field leading to the resistive state transition. We suggest likely scenarios for such co-existing phases in nominally pure LaMnO3 that can lead to the bias driven percolation type transition.

cond-mat.str-el↗

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

Near dispersion-less surface plasmon polariton resonances at a metal-dielectric interface

Omni-directional light coupling to surface plasmon polariton (SPP) modes to make use of plasmon mediated near-field enhancement is challenging. We report possibility of near dispersion-less modes in structures with unpatterned metal-dielectric interfaces having 2-D dielectric patterns on top. We show that the position and dispersion of the excited modes can be controlled by the excitation geometry and the 2-D pattern. The anti-crossings resulting from the in-plane coupling of different SPP modes are also shown.

cond-mat.mtrl-sci↗

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↗