arXiv subjects

A. Banerjee

Publications and source records attributed to A. Banerjee.

At least 181 records · Page 10Linked to original sources

arXiv2007

Changeover from Glassy ferromagnetism of the orbital domain state to long range ferromagnetic ordering in La{_{0.9}}Sr_{0.1}MnO{_3}

An attempt is made to resolve the controversy related to the low temperature phase (ground state) of the low-doped ferromagnetic (FM)- insulator(I) manganite through bulk magnetic measurements on La${_{0.9}}$Sr$_{0.1}$MnO${_3}$ sample. It is shown that the FM phase, formed out of well defined transition in the low-doped system, becomes inhomogeneous with decrease in temperature. This inhomogeniety is considered to be an outcome of the formation of orbital domain state of e$_g$-electrons having hole rich (metallic) walls separating the hole deficient (insulating) regions. The resulting complexity brings in metastability and glassy behaviour within the FM phase at low temperature, however, with no resemblance to spin glass, cluster glass or reentrant phases. It shows ageing effect without memory but magnetic relaxation shows signatures of inter-cluster interaction. The energy landscape picture of this glassy phase is described in terms of hierarchical model. Further, it is shown that this inhomogeneity disappear in La${_{0.9}}$Sr$_{0.1}$MnO$_{3.08}$ where, the orbital domain state is destroyed by self doping resulting in reduction of Mn$^{3+}$ and hence e$_g$-electrons. The ferromagnetic phase of the non-stoichiometric sample, does not show glassy behaviour. It neither follows 'hierarchical model' nor 'droplet model' generally used to explain glassy or inhomogeneous systems. Its magnetic response can be explained simply from the domain wall dynamics of otherwise homogeneous ferromagnet.

cond-mat.str-el↗

arXiv2007

Anomalous First Order Transition in $Nd{_{0.5}}Sr{_{0.5}}MnO{_{3}}$: An interplay between kinetic arrest and thermodynamic transitions

A detailed investigation of the first order antiferromagnetic insulator (AFI) to ferromagnetic metal (FMM) transition in $Nd{_{0.5}}Sr{_{0.5}}MnO{_{3}}$ is carried out by resistivity and magnetization measurements. These studies reveal several anomalous features of thermomagnetic irreversibility across the first order transition. We show that these anomalous features can not be explained in terms of supercooling effect alone and H-T diagram based on isothermal MH or RH measurement alone do not reflect true nature of the first order transition in this compound. Our investigations reveal glass-like arrest of kinetics at low temperature which plays a dominant role in the anomalous thermomagnetic irreversibility observed in this system. The interplay between kinetic arrest and supercooling is investigated by following novel paths in the H-T space. It is shown that coexisting FMM and AFI phases can be tuned in a number of ways at low temperature. These measurements also show that kinetic arrest temperature and supercooling temperature are anti-correlated i.e. regions which are arrested at low temperature have higher supercooling temperature and vice versa.

cond-mat.str-el↗

arXiv2007

Two-phase coexistence is tunable and is not the equilibrium state in half-doped manganites

We discuss our very interesting experimental observation that the low-temperature two-phase coexistence in half-doped manganites is multi-valued (at any field) in that we can tune the coexisting antiferromagnetic-insulating(AF-I) and the ferromagnetic-metallic(FM-M) phase-fractions by following different paths in (H,T) space. We have shown experimentally that the phase-fraction, in this two-phase coexistence, can take continuous infinity of values. All but one of these are metastable, and two-phase coexistence is not an equilibrium state.

cond-mat.str-el↗

arXiv2006

Coexisting tuneable fractions of glassy and equilibrium long-range-order phases in manganites

Antiferromagnetic-insulating(AF-I) and the ferromagnetic-metallic(FM-M) phases coexist in various half-doped manganites over a range of temperature and magnetic field, and this is often believed to be an essential ingredient to their colossal magnetoresistence. We present magnetization and resistivity measurements on Pr(0.5)Ca(0.5)Mn(0.975)Al(0.025)O(3) and Pr(0.5)Sr(0.5)MnO(3) showing that the fraction of the two coexisting phases at low-temperature in any specified measuring field H, can be continuously controlled by following designed protocols traversing field-temperature space; for both materials the FM-M fraction rises under similar cooling paths. Constant-field temperature variations however show that the former sample undergoes a 1st order transition from AF-I to FM-M with decreasing T, while the latter undergoes the reverse transition. We suggest that the observed path-dependent phase-separated states result from the low-T equilibrium phase coexisting with supercooled glass-like high temperature phase, where the low-T equilibrium phases are actually homogeneous FM-M and AF-I phases respectively for the two materials.

cond-mat.str-el↗

arXiv2006

Ferromagnetic ground state of the robust charge-ordered manganite Pr(0.5)Ca(0.5MnO(3)obtained by minimal Al-substitution

We show that minimal disturbance to the robust charge ordered Pr(0.5)Ca(0.5)MnO(3) by 2.5% Al substitution on Mn-site drives the system towards ferromagnetic ground state. The history-dependent coexisting phases observed are explained as an outcome of a hindered first order transition with glass like arrest of kinetics resulting in irreversibility. Consistent with a simple phase diagram having ferromagnetic ground state, it is experimentally shown that these coexisting phases are far from the equilibrium.

cond-mat.str-el↗

arXiv2006

Direct evidence of random field effect on magnetic ordering of La_(0.5)Gd_(0.2)Sr_(0.3)MnO_(3) manganite system

We report direct experimental evidence of the effect of quenched random field on magnetic ordering of La_(0.5)Gd_(0.2)Sr_(0.3)MnO_(3) manganites system. We demonstrate magnetic measurements providing serious evidence to support that quenched random field of Gd divides the system in finite size clusters which undergo cluster-glass like freezing. The size or concentration of these clusters is found to be closely related to the effect of random field, which in turn is a function of applied field.

cond-mat.str-el↗

arXiv2006

PT-symmetric quartic anharmonic oscillator and position-dependent mass in a perturbative approach

To lowest order of perturbation theory we show that an equivalence can be established between a $\cal PT$-symmetric generalized quartic anharmonic oscillator model and a Hermitian position-dependent mass Hamiltonian $h$. An important feature of $h$ is that it reveals a domain of couplings where the quartic potential could be attractive, vanishing or repulsive. We also determine the associated physical quantities.

quant-ph↗

arXiv2006

Relating supercooling and glass-like arrest of kinetics for phase separated systems: studies on doped CeFe$_2$ and (La,Pr,Ca)MnO$_3$

Coexisting ferromagnetic and antiferromagnetic phases over a range of temperature as well as magnetic field have been reported in many materials of current interest, showing disorder-broadened 1st order transitions. Anomalous history effects observed in magnetization and resistivity are being explained invoking the concepts of kinetic arrest akin to glass transitions. From magnetization measurements traversing novel paths in field-temperature space, we obtain the intriguing result that the regions of the sample which can be supercooled to lower temperatures undergo kinetic-arrest at higher temperatures, and vice versa. Our results are for two diverse systems viz. the inter-metallic doped CeFe$_2$ which has an antiferromagnetic ground state, and the oxide La-Pr-Ca-Mn-O which has a ferromagnetic ground state, indicating the possible universality of this effect of disorder on the widely encountered phenomenon of glass-like arrest of kinetics.

cond-mat.str-el↗

arXiv2006

Correlating supercooling limit and glass-like arrest of kinetics for disorder-broadened 1st order transitions: relevance to phase separation

Coexisting ferromagnetic (FM) and antiferromagnetic (AFM) phases over a range of temperature (T) and magnetic field (H), have been reported in many materials. The 1st order FM-AFM transition is completed over a broad T (or H) range; this is ascribed to a landscape of free-energy density in systems with frozen disorder. Kinetic arrest, akin to glass transition, has been reported in doped CeFe$_2$, La-Pr-Ca-Mn-O, Gd$_5$Ge$_4$, etc., such that the coexisting fraction is frozen. The de-arrest, or melting, of this glass-like arrested state is seen to occur over a range of temperatures, implying a landscape (of T$_K$ or T$_g$). We argue that measuring magnetization along various specific paths in H-T space can help infer whether the landscapes of free-energy and for kinetic arrest are correlated. This will help to determine whether disorder affects glass formation, and the underlying 1st order transition, in contrasting (or similar) ways.

cond-mat.str-el↗

arXiv2005

Hamiltonians with position-dependent mass, deformations and supersymmetry

A new method for generating exactly solvable Schrödinger equations with a position-dependent mass is proposed. It is based on a relation with some deformed Schrödinger equations, which can be dealt with by using a supersymmetric quantum mechanical approach combined with a deformed shape-invariance condition. The solvability of the latter is shown to impose the form of both the deformed superpotential and the position-dependent mass. The conditions for the existence of bound states are determined. A lot of examples are provided and the corresponding bound-state spectrum and wavefunctions are reviewed.

quant-ph↗

arXiv2005

Finite size effects and the effect of disorder on electronic phase separation in Pr_(0.5)Ca_(0.5)Mn_(1-x)Al_(x)O_(3)

We report a bulk magnetisation study of the low temperature magnetic phases in the Mn site substituted half doped manganite series Pr_(0.5)Ca_(0.5)Mn_(1-x)Al_(x)O_(3). Our results indicate the formation of self organised structures in this series of compounds which is driven by electronic phase separation and stabilised by the presence of finite quenched disorder, thus providing a basis for understanding the anomalously large supression of the CE phase in the presence of disorder. Striking magnetic memory effects similar to that seen in spin glasses and interacting superparamagnets are also observed. Increasing disorder is seen to make electronic phase separation weaker owing to the destabilisation of the CE state which results in an enhanced mobility of the charge carriers.

cond-mat.str-el↗

arXiv2005

Accelerating Universe from Extra Spatial Dimension

We present a simple higher dimensional FRW type of model where the acceleration is apparently caused by the presence of the extra dimensions. Assuming an ansatz in the form of the deceleration parameter we get a class of solutions some of which shows the desirable feature of dimensional reduction as well as reasonably good physical properties of matter. Interestingly we do not have to invoke an extraneous scalar field or a cosmological constant to account for this acceleration. One argues that the terms containing the higher dimensional metric coefficients produces an extra negative pressure that apparently drives the inflation of the 4D space with an accelerating phase. It is further found that in line with the physical requirements our model admits of a decelerating phase in the early era along with an accelerating phase at present.Further the models asymptotically mimic a steady state type of universe although it starts from a big type of singularity. Correspondence to Wesson's induced matter theory is also briefly discussed and in line with it it is argued that the terms containing the higher dimensional metric coefficients apparently creates a negative pressure which drives the inflation of the 3-space with an accelerating phase.

gr-qc↗

arXiv2005

Field induced first order antiferromagnetic to ferromagnetic transition in Al-doped CeFe$_2$: a calorimetric investigation

Field variation of heat capacity at fixed temperatures is investigated to identify the origin of the field induced first order phase transition in polycrystalline Ce(Fe$_{0.094}$Al$_{0.04}$)$_2$ sample. The heat capacity at 4.5K and 3.5K shows hysteresis in different field cycles and the virgin curve stays outside the envelope curve. This is analogous to the magnetization and magneto-resistance behavior observed in this system, where the amount of hysteresis, and the magnitude of zero-field irreversibility are attributed to the degree of supercooling/superheating, and the extent of kinetic arrest of the reverse transition from ferro- to antifrromagnetic state in field reducing cycle respectively. However, contrary to the magnetization and magneto-resistance, in heat capacity both these features have decreased with reducing temperature signifying the importance of structural contribution.

cond-mat.str-el↗

arXiv2005

Deformed shape invariance and exactly solvable Hamiltonians with position-dependent effective mass

Known shape-invariant potentials for the constant-mass Schrodinger equation are taken as effective potentials in a position-dependent effective mass (PDEM) one. The corresponding shape-invariance condition turns out to be deformed. Its solvability imposes the form of both the deformed superpotential and the PDEM. A lot of new exactly solvable potentials associated with a PDEM background are generated in this way. A novel and important condition restricting the existence of bound states whenever the PDEM vanishes at an end point of the interval is identified. In some cases, the bound-state spectrum results from a smooth deformation of that of the conventional shape-invariant potential used in the construction. In others, one observes a generation or suppression of bound states, depending on the mass-parameter values. The corresponding wavefunctions are given in terms of some deformed classical orthogonal polynomials.

quant-ph↗

arXiv2004

CPT-conserving Hamiltonians and their nonlinear supersymmetrization using differential charge-operators C

A brief overview is given of recent developments and fresh ideas at the intersection of PT and/or CPT-symmetric quantum mechanics with supersymmetric quantum mechanics (SUSY QM). We study the consequences of the assumption that the "charge" operator C is represented in a differential-operator form. Besides the freedom allowed by the Hermiticity constraint for the operator CP, encouraging results are obtained in the second-order case. The integrability of intertwining relations proves to match the closure of nonlinear SUSY algebra. In an illustration, our CPT-symmetric SUSY QM leads to non-Hermitian polynomial oscillators with real spectrum which turn out to be PT-asymmetric.

hep-th↗

arXiv2004

Formation of finite antiferromagnetic clusters and the effect of electronic phase separation in Pr{_0.5}Ca{_0.5}Mn{_0.975}Al{_0.025}O{_3}

We report the first experimental evidence of a magnetic phase arising due to the thermal blocking of antiferromagnetic clusters in the weakened charge and orbital ordered system Pr{_0.5}Ca{_0.5}Mn{_0.975}Al{_0.025}O{_3}. The third order susceptibility (χ_3) is used to differentiate this transition from a spin or cluster glass like freezing mechanism. These clusters are found to be mesoscopic and robust to electronic phase separation which only enriches the antiphase domain walls with holes at the cost of the bulk, without changing the size of these clusters. This implies that Al substitution provides sufficient disorder to quench the length scales of the striped phases.

cond-mat.str-el↗

arXiv2003

Dilution of 2D antiferromagnetism by Mn site substitution in La{_1}Sr{_2}Mn{_{2-x}}Al{_x}O{_7}

We report the effect of Al substitution on the Mn site of the bilayered half doped manganite La{_1}Sr{_2}Mn{_2}O{_7}. This substitution dilutes the magnetically active Mn-O-Mn network without introducing an appreciable distortion in the lattice and ionic considerations leads to a predominant reduction of Mn{^{4+}} with increasing Al. The rate of fall of the long range antiferromagnetic transition temperature as a function of substitution is seen to match well with established quasi 2D Heisenberg systems indicating that the nature of magnetic interactions in this quasi 2-D system is of the short range Heisenberg type. The magnetic contribution of the specific heat estimated using Fisher's relation could be fitted with a function incorporating the presence of a gapped Fermi surface appropriate for this type of systems. The resistivity was seen to increase as a function of substitution due to the weakening of the double exchange within the ferromagnetic layers of these A type of antiferromagnets and justifiably, in the paramagnetic region the data could be fitted to Mott's equation for the variable range hopping of polarons in 2 dimensions.

cond-mat.str-el↗

arXiv2003

Effect of Al substitution on the charge ordered Pr{_0.5}Ca{_0.5}MnO{_3}: Structure, Magnetism and Transport

We report the effect of Mn site substitution of Al on the structural, magnetic and transport properties of charge ordered Pr{_0.5}Ca{_0.5}MnO{_3}. This substitution introduces impurities in the Mn-O-Mn network, causes the Mn{^3+}/Mn{^4+} ratio to deviate away from unity and is seen to predominantly replace Mn{^4+}. The strength of charge ordering is supressed with increasing Al doping, as is seen from ac susceptibility, the magnetocaloric effect and dc resistivity. The observation of traces of charge ordering over an extended range of impurity doping of 10% besides being indicative of the robust nature of charge ordering in Pr{_0.5}Ca{_0.5}MnO{_3}, also underscores the relatively insignificant effect that Al substitution has on the lattice distortions and the importance of non-magnetic substitutions. For all the samples, the conductivity in the temperature range above the charge ordering transition is observed to be due to the adiabatic hopping of small polarons, whereas below the charge ordering transition, Mott's variable range hopping mechanism is seen to be valid.

cond-mat↗