arXiv subjects

Amlan Biswas

Publications and source records attributed to Amlan Biswas.

At least 19 recordsLinked to original sources

arXiv2025

Modified Kondorsky Domain Reversal in Microstructured Phase-Separated Manganites

The hole-doped manganite (La$_{1-y}$Pr$_{y}$)$_{0.67}$Ca$_{0.33}$MnO$_3$ (LPCMO) shows electronic phase separation between ferromagnetic metallic (FMM) and anti-ferromagnetic charge-ordered insulating (AFM-COI) regions. In this study, (La$_{0.5}$Pr$_{0.5}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LP5CMO) microstructures were fabricated using photolithography on thin films grown on (110) NdGaO$_3$ (NGO) substrates. We investigated the domain reversal mechanism of these microstructures through magnetotransport measurements. Our results demonstrate that, while bulk (unpatterned) films follow the standard Kondorsky model for domain reversal, the microstructures obey a modified Kondorsky model. This difference indicates that local magnetic fields from reversed domains significantly influence the coercive field in confined geometries. Although we did not observe a strong electric field effect, this study establishes that magnetotransport measurements are a feasible method for probing the competition between shape and magnetocrystalline anisotropy in manganite microstructures, which could provide an alternative path for controlling magnetic domains at low current densities.

cond-mat.str-el↗

arXiv2020

Near-field infrared nano-spectroscopy of surface phonon-polariton resonances

We present combined experimental and numerical work on light-matter interactions at nanometer length scales. We report novel numerical simulations of near-field infrared nanospectroscopy that consider, for the first time, detailed tip geometry and have no free parameters. Our results match published spectral shapes of amplitude and phase measurements even for strongly resonant surface phonon-polariton (SPhP) modes. They also verify published absolute scattering amplitudes for the first time. A novel, ultrabroadband light source enables near-field amplitude and phase acquisition into the far-infrared spectral range. This allowed us to discover a strong SPhP resonance in the polar dielectric SrTiO3 (STO) at approximately 24 micrometer wavelength of incident light.

cond-mat.mes-hall↗

arXiv2020

Dynamic percolation of ferromagnetic regions in phase separated manganites using non-uniform electric fields

Thin films of the manganite (La$_{1-y}$Pr$_y$)$_{1-x}$Ca$_x$MnO$_3$ exhibit dynamic phase coexistence with micrometer scale, fluid-like ferromagnetic metallic (FMM) regions interspersed in a charge-order insulating (COI) background. It has been previously reported that a uniform electric field realigns the fluid-like FMM regions due to a phenomenon similar to dielectrophoresis. Here we report that non-uniform electric fields have a stronger effect on the FMM regions as expected from the dielectrophoresis model. The dynamic percolation of the FMM regions is observed over a wider range of temperatures compared to the results in a uniform field. Additionally, in a non-uniform electric field, the time required for dynamic percolation along the magnetic hard axis ($t_{\mathrm{B}}$) decreased with increasing applied voltage ($V_{\mathrm{A}}$) as a power law, $V_{\mathrm{A}}^{-δ}$ with $δ\approx 5$ while $δ< 2$ for a uniform electric field. Our results in a non-uniform electric field provide strong evidence in favor of the dielectrophoresis model and a unique method for manipulating micrometer-sized ferromagnetic regions using electric fields.

cond-mat.mes-hall↗

arXiv2018

Temperature evolution of polaron dynamics and Jahn-Teller distortion modes in strongly correlated La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ manganite film

Reflectivity as a function of temperature for the La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) film has been measured across the metal-insulator phase transition. The optical properties and their temperature dependence were determined in the infrared and visible range by fits to a Drude-Lorentz model, using exact formula for the thin film optics and the measured properties of the substrate. The phonon modes were identified and verified with lattice dynamical calculations for the ideal and distorted perovskite structure of the material. The optical conductivity shows agreement with the double exchange mechanism in conjunction with the Jahn-Teller distortion term in the Hamiltonian. Low temperature metallic phase is dominated by large polaron dynamics, a key component of electron-orbital coupling in a strongly corrrelated system. Free carrier dynamics in the metallic phase is described in terms of coherent heavy polaronic motion in the DC limit with incoherent and asymmetric polaronic background in the mid-IR range. The strength and line width of Jahn-Teller modes has been discussed across the phase transition and their temperature evolution is qualitatively discussed on account of existing electron-phonon coupling. The localized Holstein polaron formation in the high temperature insulative phase is identified as optical conductivity peaks in the visible range above the critical temperature.

cond-mat.str-el↗

arXiv2015

Proximate transition temperatures amplify linear magnetoelectric coupling in strain-disordered multiferroic BiMnO3

We report a giant linear magnetoelectric coupling in strained BiMnO3 thin films in which the disorder associated with an islanded morphology gives rise to extrinsic relaxor ferroelectricity that is not present in bulk centrosymmetric ferromagnetic crystalline BiMnO3. Strain associated with the disorder is treated as a local variable which couples to the two ferroic order parameters, magnetization M and polarization P. A straightforward "gas under a piston" thermodynamic treatment explains the observed correlated temperature dependencies of the product of susceptibilities and the magnetoelectric coefficient together with the enhancement of the coupling by the proximity of the ferroic transition temperatures close to the relaxor freezing temperature. Our interpretation is based on a trilinear coupling term in the free energy of the form L(PXM) where L is a hidden antiferromagnetic order parameter, previously postulated by theory for BiMnO3. This phenomenological invariant not only preserves inversion and time reversal symmetry of the strain-induced interactions but also explains the pronounced linear magnetoelectric coupling without using the more conventional higher order biquadratic interaction proportional to (PM)^2.

cond-mat.mtrl-sci↗

arXiv2012

$a-b$ Plane Point Contact Spectroscopy measurements of optimally Cobalt doped Ba-122 iron-pnictide superconductors

Point contact spectroscopy (PCS) is a technique which can reveal the size and symmetry of a superconducting gap ($Δ$) and is especially useful for new materials such as the iron-based superconductors. PCS is usually employed in conjunction with the extended Blonder-Tinkham-Klapwijk (BTK) model which is used to extract information such as the existence of nodes in $Δ$ from PCS data obtained on unconventional superconductors. The BTK model uses a dimensionless parameter $Z$ to quantify the barrier strength across a normal metal - superconductor junction. We have used a unique feature of PCS which allows variation of $Z$ to obtain crucial information about $Δ$. We report our $Z$-dependent, $a-b$ plane PCS measurements on single crystals of the iron-based superconductor BaFe$_{2-x}$Co$_{x}$As$_{2}$. Our measurements show that BaFe$_{2-x}$Co$_{x}$As$_{2}$ ($x$=0.148) is a superconductor with two gaps which does not contain any nodes. The $Z$ dependent point contact spectra rule out a pure $d$ symmetry and the gaps at optimal doping have negligible anisotropy.

cond-mat.supr-con↗

arXiv2011

Competing `soft' dielectric phases and detailed balance in thin film manganites

Using frequency dependent complex capacitance measurements on thin films of the mixed-valence manganite (La$_{1-y}$Pr$_{y}$)$_{1-x}$Ca$_{x}$MnO$_{3}$, we identify and resolve the individual dielectric responses of two competing dielectric phases. We characterize their competition over a large temperature range, revealing they are in dynamic competition both spatially and temporally. The phase competition is shown to be governed by the thermodynamic constraints imposed by detailed balance. The consequences of the detailed balance model strongly support the notion of an `electronically soft' material in which continuous conversions between dielectric phases with comparable free energies occur on time scales that are long compared with electron-phonon scattering times.

cond-mat.mtrl-sci↗

arXiv2011

Growth of atomically smooth thin films of the electronically phase separated manganite (La$_{0.5}$Pr$_{0.5}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$

Atomically flat, epitaxial, and stoichiometric thin films of the electronically phase separated compound (La$_{0.5}$Pr$_{0.5}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ were grown on as-received and treated NdGaO$_{3}$ substrates by fine tuning of oxygen pressure during deposition. Optimal thin films with step flow growth mode show superior physical properties compared to thin films grown in off-optimal oxygen pressures, {\em viz.} the highest maximum temperature coefficient of resistance, the highest peak-resistivity temperature, and reduced coercive fields. Transport, magnetization, and x-ray diffraction measurements indicate that the oxygen pressure during growth plays a critical role in the formation of oxygen vacancies, cation vacancies, and grain boundaries.

cond-mat.mtrl-sci↗

arXiv2010

Single domain to multi-domain transition due to in-plane magnetic anisotropy in phase separated (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films

Phase separated perovskite manganites have competing phases with different crystal structures, magnetic and electronic properties. Hence, strain effects play a critical role in determining the magnetic properties of manganite thin films. Here we report the effect of anisotropic stress on the magnetic properties of the phase separated manganite (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$. Thin films of (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ grown under anisotropic in-plane stress on (110) NdGaO$_{3}$ substrates display in-plane mangetic anisotropy and single domain to multidomain transition as a function of temperature. Angle dependent magnetization measurements also show that the magnetization reversal occurs mainly through the nucleation $&$ propagation mechanism. By comparing the results with (La$_{0.4}$Pr$_{0.6}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films grown on (001) SrLaGaO$_{4}$ substrates, we have confirmed that the magnetic anisotropy is mainly due to substrate induced anisotropic stress. Our results suggest novel avenues for storing magnetic information in nanoscale magnetic media.

cond-mat.str-el↗

arXiv2010

Growth and characterization of multiferroic BiMnO$_3$ thin films

We have grown epitaxial thin films of multiferroic BiMnO$_3$ using pulsed laser deposition. The films were grown on SrTiO$_3$ (001) substrates by ablating a Bi-rich target. Using x-ray diffraction we confirmed that the films were epitaxial and the stoichiometry of the films was confirmed using Auger electron spectroscopy. The films have a ferromagnetic Curie temperature ($T_C$) of 85$\pm$5 K and a saturation magnetization of 1 $μ_B$/Mn. The electric polarization as a function of electric field ($P-E$) was measured using an interdigital capacitance geometry. The $P-E$ plot shows a clear hysteresis that confirms the multiferroic nature of the thin films.

cond-mat.mtrl-sci↗

arXiv2007

Point contact spectroscopy of Pr$_{2-x}$Ce$_x$CuO$_4$ in high magnetic fields

We have studied the density of states of the normal state of the electron-doped superconductor Pr$_{2-x}$Ce$_x$CuO$_4$ at low temperatures and in magnetic fields up to 31 Tesla using point contact spectroscopy on single crystals. Our data clearly reveal an anomalous gap in the normal state density of states. This normal state gap survives even in the highest applied field of 31 T. Our results cast doubt over whether this gap found in electron-doped cuprates is the analog of the pseudogap in hole-doped cuprates. We have suggested an alternate origin of the normal state gap, which involves the effect of disorder and electron correlations at the surface of cuprates.

cond-mat.supr-con↗

arXiv2007

Colossal magnetocapacitance and scale-invariant dielectric response in phase-separated manganites

Thin films of strongly-correlated electron materials (SCEM) are often grown epitaxially on planar substrates and typically have anisotropic properties that are usually not captured by edge-mounted four-terminal electrical measurements, which are primarily sensitive to in-plane conduction paths. Accordingly, the correlated interactions in the out-of-plane (perpendicular) direction cannot be measured but only inferred. We address this shortcoming and show here an experimental technique in which the SCEM under study, in our case a 600 Angstrom-thick (La1-yPry)0.67Ca0.33MnO3 (LPCMO) film, serves as the base electrode in a metal-insulator-metal (MIM) trilayer capacitor structure. This unconventional arrangement allows for simultaneous determination of colossal magnetoresistance (CMR) associated with dc transport parallel to the film substrate and colossal magnetocapacitance (CMC) associated with ac transport in the perpendicular direction. We distinguish two distinct strain-related direction-dependent insulator-metal (IM) transitions and use Cole-Cole plots to establish a heretofore unobserved collapse of the dielectric response onto a universal scale-invariant power-law dependence over a large range of frequency, temperature and magnetic field.

cond-mat.str-el↗

arXiv2006

Effect of strain and electric field on the electronic soft matter in manganite thin films

We have studied the effect of substrate-induced strain on the properties of the hole-doped manganite (La$_{1-y}$Pr$_{y}$)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ ($y$ = 0.4, 0.5 and 0.6) in order to distinguish between the roles played by long-range strain interactions and quenched atomic disorder in forming the micrometer-scale phase separated state. We show that a fluid phase separated (FPS) state is formed at intermediate temperatures similar to the strain-liquid state in bulk compounds, which can be converted to a metallic state by applying an external electric field. In contrast to bulk compounds, at low temperatures a strain stabilized ferromagnetic metallic (FMM) state is formed in the $y$ = 0.4 and 0.5 samples. However, in the $y$ = 0.6 sample a static phase separated (SPS) state is formed similar to the strain-glass phase in bulk compounds. Hence, we show that long-range strain interaction plays a dominant role in forming the micrometer-scale phase separated state in manganite thin films.

cond-mat.str-el↗

arXiv2003

Point contact spectroscopy of the electron-doped cuprate superconductor Pr{2-x}Ce{x}CuO4: The dependence of conductance-voltage spectra on cerium doping, barrier strength and magnetic field

We present conductance-voltage (G-V) data for point contact junctions between a normal metal and the electron doped cuprate superconductor Pr{2-x}Ce{x}CuO4 (PCCO). We observe a zero bias conductance peak (ZBCP) for the under-doped composition of this cuprate (x=0.13) which is consistent with d-wave pairing symmetry. For optimally-doped (x=0.15) and over-doped (x=0.17) PCCO, we find that the G-V characteristics indicate the presence of an order parameter without nodes. We investigate this further by obtaining point contact spectroscopy data for different barrier strengths and as a function of magnetic field.

cond-mat.supr-con↗

arXiv2002

Magnetic Field Dependence of Electronic Specific Heat in Pr_{1.85} Ce_{0.15} CuO_4

The specific heat of electron-doped Pr_{1.85} Ce_{0.15} CuO_4 single crystals is reported for the temperature range 2 - 10 K and magnetic field range 0 - 10 T. A non-linear magnetic field dependence is observed for the field range 0 - 2 T. Our data supports a model with lines of nodes in the gap function of these superconductors. Theoretical calculations of the electronic specific heat for dirty d-wave, clean d-wave, and s-wave symmetries are compared to our data.

cond-mat.supr-con↗

arXiv2001

Evidence of a d to s-wave pairing symmetry transition in the electron-doped cuprate superconductor Pr$_{2-x}$Ce$_x$CuO$_4$

We present point contact spectroscopy (PCS) data for junctions between a normal metal and the electron doped cuprate superconductor Pr$_{2-x}$Ce$_x$CuO$_4$ (PCCO). For the underdoped compositions of this cuprate ($x \sim 0.13$) we observe a peak in the conductance-voltage characteristics of the point contact junctions. The shape and magnitude of this peak suggests the presence of Andreev bound states at the surface of underdoped PCCO which is evidence for a d-wave pairing symmetry. For overdoped PCCO ($x \sim 0.17$) the PCS data does not show any evidence of Andreev bound states at the surface suggesting an s-wave pairing symmetry.

cond-mat.supr-con↗

arXiv2001

Influence of oxygen isotope exchange on the ground state of manganites

We report a study of oxygen isotope effects on low temperature specific heat, magnetization, and resistivity of La$_{1-x}$Ca$_{x}$MnO$_{3}$ and (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$. For the metallic compositions of La$_{1-x}$Ca$_{x}$MnO$_{3}$ and for charge-ordered La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ no change in low temperature specific heat has been detected with $^{16}$O -$^{18}$O exchange, while compounds of (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (0.4<y<0.6) show a significant change in low temperature properties. The low temperature specific heat indicates a presence of the charge-ordered phase even in compositions of (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ which are metallic at low temperatures. We suggest that the changes induced by the oxygen isotope exchange are caused by an increase of the charge-ordered phase in $^{18}$O samples.

cond-mat.str-el↗

arXiv2001

Gapped tunneling spectra in the normal state of Pr$_{2-x}$Ce$_x$CuO$_4$

We present tunneling data in the normal state of the electron doped cuprate superconductor Pr$_{2-x}$Ce$_x$CuO$_4$ for three different values of the doping $x$. The normal state is obtained by applying a magnetic field greater than the upper critical field, $H_{c2}$ for $T < T_c$. We observe an anomalous normal state gap near the Fermi level. From our analysis of the tunneling data we conclude that this is a feature of the normal state density of states. We discuss possible reasons for the formation of this gap and its implications for the nature of the charge carriers in the normal and the superconducting states of cuprate superconductors.

cond-mat.supr-con↗