arXiv subjects

Bhavtosh Bansal

Publications and source records attributed to Bhavtosh Bansal.

At least 19 recordsLinked to original sources

arXiv2026

Development of a high-field pulsed magnet and optical fiber coupled cryostat system for magneto-photoluminescence measurements

This paper presents the development of a high field pulsed magnet system combined with an optical fiber-coupled low temperature cryostat for magneto-photoluminescence measurements. A novel aspect of our system is the use of electrolyte capacitors in a 75 kJ capacitor bank to drive the pulsed magnet, enabling the generation of high magnetic fields at a relatively low charging voltage (400 V). The zylon reinforced wire wound magnet coil achieves a maximum field strength of 35 tesla with a magnetic field rise time of 10 ms. The integrated cryostat was developed to meet the specific requirements of magneto-photoluminescence experiments using a 4 K helium closed-cycle cryocooler to provide a stable low-temperature environment down to 5 K.

physics.ins-det↗

arXiv2025

Evidence of Athermal Metastable Phase in a Halide Perovskite: Optically Tracked Thermal-Breach Memory

Halide perovskite materials have been extensively studied in the last decade because of their impressive optoelectronic properties. However, their one characteristic that is uncommon for semiconductors is that many undergo thermally induced structural phase transitions. The transition is hysteretic, with the hysteresis window marking the boundary of the metastable phase. We have discovered that in methylammonium lead iodide, this hysteretic metastable phase is athermal, meaning it shows almost no temporal phase evolution under isothermal conditions. We also show that a large number of distinguishable metastable states can be prepared following different thermal pathways. Furthermore, under a reversible thermal perturbation, the states in the metastable phase either show return-point memory or undergo a systematic nonrecoverable phase evolution, depending on the thermal history and the sign of the temperature perturbation. Since the phase fraction can be probed with extreme sensitivity via luminescence, we have an optically retrievable memory that reliably records any breach in temperature stability. Such thermal-breach memory in athermal martensites, of which there are numerous examples, may be useful for tagging packages requiring strict temperature control during transportation or preservation.

cond-mat.mtrl-sci↗

arXiv2024

Exceptionally Slow, Long Range, and Non-Gaussian Critical Fluctuations Dominate the Charge Density Wave Transition

$(TaSe_4)_2I$ is a well-studied quasi-one-dimensional compound long-known to have a charge-density wave (CDW) transition around 263 K. We argue that the critical fluctuations of the pinned CDW order parameter near the transition can be inferred from the resistance noise on account of their coupling to the dissipative normal carriers. Remarkably, the critical fluctuations of the CDW order parameter are slow enough to survive the thermodynamic limit and dominate the low-frequency resistance noise. The noise variance and relaxation time show rapid growth (critical opalescence and critical slowing down) within a temperature window of $ \varepsilon \approx \pm 0.1$, where $\varepsilon$ is the reduced temperature. This is very wide but consistent with the Ginzburg criterion. We further show that this resistance noise can be quantitatively used to extract the associated critical exponents. Below $|\varepsilon | \lesssim 0.02$, we observe a crossover from mean-field to a fluctuation-dominated regime with the critical exponents taking anomalously low values. The distribution of fluctuations in the critical transition region is skewed and strongly non-Gaussian. This non-Gaussianity is interpreted as the breakdown of the validity of the central limit theorem as the diverging coherence volume becomes comparable to the macroscopic sample size. The large magnitude critical fluctuations observed over an extended temperature range, as well as the crossover from the mean-field to the fluctuation-dominated regime highlight the role of the quasi-one dimensional character in controlling the phase transition.

cond-mat.str-el↗

arXiv2023

Dynamic hysteresis at a noisy saddle-node shows power-law scaling but nonuniversal exponent

Dynamic hysteresis, viz., delay in switching of a bistable system on account of the finite sweep rate of the drive has been extensively studied in dynamical and thermodynamic systems. Dynamic hysteresis results from slowing of the response around a saddle-node bifurcation. As a consequence, the hysteresis area increases with the sweep rate. Mean-field theory, relevant for noise-free situations, predicts power law scaling with the area scaling exponent of 2/3. We have experimentally investigated the dynamic hysteresis for a thermally-driven metal-insulator transition in a high quality NdNiO$_3$ thin film and found the scaling exponent to be about 1/3, far less than the mean field value. To understand this, we have numerically studied Langevin dynamics of the order parameter and found that noise, which can be thought to parallel finite temperature effects, influences the character of dynamic hysteresis by systematically lowering the dynamical exponent to as small as 0.2. The power law scaling character, on the other hand, is unaffected in the range of chosen parameters. This work rationalizes the ubiquitous power law scaling of the dynamic hysteresis as well as the wide variation in the scaling exponent between 0.66 and 0.2 observed in different systems over the last 30 years.

cond-mat.stat-mech↗

arXiv2023

Critical Slowing Down at the Abrupt Mott Transition: When the First-Order Phase Transition Becomes Zeroth-Order and Looks Like Second-Order

We report that the thermally-induced Mott transition in vanadium sesquioxide shows critical-slowing-down and enhanced variance ('critical opalescence') of the order parameter fluctuations measured through low-frequency resistance-noise spectroscopy. Coupled with the observed increase of also the phase-ordering time, these features suggest that the strong abrupt transition is controlled by a critical-like singularity in the hysteretic metastable phase. The singularity is identified with the spinodal point and is a likely consequence of the strain-induced long-range interaction.

cond-mat.stat-mech↗

arXiv2022

Carrier thermalization and zero-point bandgap renormalization in halide perovskites from the Urbach tails of the emission spectrum

We develop techniques to study the temperature dependent localization, thermalization, and the effects of phonon scattering on the excitons in halide perovskites from the analysis of the emission spectra. The excitonic Urbach edge, when inferred from the low energy tails of the temperature dependent luminescence spectra, is shown to be sensitive to the electron distribution and thermalization. A method to observe the Urbach focus is devised for halide perovskites where the temperature dependence of the excitonic gap is anomalous. The value of the zero-point bandgap renormalization is inferred to be about 33 meV. This small value of the bandgap renormalization rules out the formation of small polarons and points to weak electron-phonon coupling. The experiments are performed on the nanosheets of the archetypal halide perovskite, CsPbBr$_3$.

cond-mat.mtrl-sci↗

arXiv2021

Absolute calibration of the latent heat of transition using differential thermal analysis

We describe a simple and accurate differential thermal analysis set up to measure the latent heat of solid state materials undergoing abrupt phase transitions in the temperature range from 77 K to above room temperature. We report a numerical technique for the absolute calibration of the latent heat of the transition, without the need of a reference sample. The technique is applied to three different samples -- vanadium sesquioxide undergoing the Mott transition, bismuth barium ruthenate undergoing a magnetoelastic transition, and an intermetallic Heusler compound. In each case, the inferred latent heat value agrees with the literature value to within its error margins. To further demonstrate the importance of absolute calibration, we show that the changes in the latent heat of the Mott transition in vanadium sesquioxide (V$_2$O$_3$) stays constant to within 2% even as the depth of supersaturation changes by about 10 K, in non-equilibrium dynamic hysteresis measurements. We also apply this technique for the measurement of the temperature-dependent specific heat.

cond-mat.stat-mech↗

arXiv2020

Dissipation-induced symmetry breaking: Emphanitic transitions in lead- and tin-containing chalcogenides and halide perovskites

Lead and tin-based chalcogenide semiconductors like PbTe or SnSe have long been known to exhibit an unusually low thermal conductivity that makes them very attractive thermoelectric materials. An apparently unrelated fact is that the excitonic bandgap in these materials increases with temperature, whereas for most semiconductors one observes the opposite trend. These two anomalous features are also seen in a very different class of photovoltaic materials, namely the halide-perovskites such as CsPbBr3. It has been previously proposed that emphanisis, a local symmetry-breaking phenomenon, is the one common origin of these unusual features. Discovered a decade ago, emphanisis is the name given to the observed displacement of the lead or the tin ions from their cubic symmetry ground state to a locally distorted phase at high temperature. This phenomenon has been puzzling because it is unusual for the high-temperature state to be of a lower symmetry than the degenerate ground state. Motivated by the celebrated vibration-inversion resonance of the ammonia molecule, we propose a quantum tunneling-based model for emphanisis where decoherence is responsible for the local symmetry breaking with increasing temperature. From the analytic expression of the temperature dependence of the tunnel splitting (which serves as an order parameter), we provide three-parameter fitting formulae which capture the observed temperature dependence of the ionic displacements as well as the anomalous increase of the excitonic bandgap in all the relevant materials.

cond-mat.mtrl-sci↗

arXiv2019

Experimental determination of the bare energy gap of GaAs without the zero-point renormalization

The energy gap of simple band insulators like GaAs is a strong function of temperature due to the electron-phonon interactions. Interestingly, the perturbation from zero-point phonons is also predicted to cause significant (a few percent) renormalization of the energy gap at absolute zero temperature but its value has been difficult to estimate both theoretically and, of course, experimentally. Given the experimental evidence [Bhattacharya, et al., Phys. Rev. Lett. 114, 047402 (2015)] that strongly supports that the exponential broadening (Urbach tail) of the excitonic absorption edge at low temperatures is the manifestation of this zero temperature electron-phonon scattering, we argue that the location of the Urbach focus is the zero temperature unrenormalized gap. Experiments on GaAs yield the zero temperature bare energy gap to be 1.581 eV and thus the renormalization is estimated to be 66 meV.

cond-mat.mtrl-sci↗

arXiv2018

How pump-probe differential reflectivity at negative delay yields the perturbed free-induction-decay: Theory of the experiment and its verification

While time-resolved pump-probe differential reflectivity and transmitivity measurements are routinely used to monitor the population relaxation dynamics on the subpicosecond time scale, it is also known that the signal in the negative delay can yield direct signatures of the perturbed-free-induction-decay of polarization. Yet this technique, especially in reflection geometry, has never been popular because the experiment is conceptually not very intuitive. Coherent dynamics is therefore usually studied using the more complex four-wave-mixing experiments. Here we derive from first principles the simplest possible but mathematically complete framework for the negative delay signal in both the time and the spectral domains. The calculation involving the optical Bloch equations to describe the induced polarization and the Ewald-Oseen idea to calculate the reflected signal as a consequence of the free oscillations of perturbed dipoles, also explicitly includes the process of lock-in detection of a double-chopped signal after it has passed through a monochromator. The theory is compared with experiments on high quality GaAs quantum well sample. The dephasing time inferred experimentally at 4 K compares remarkably well with the inverse of the absorption linewidth of the continuous-wave photoluminescence excitation spectrum. Spectrally resolved signal at negative delay calculated from our theoretical expression nicely reproduces the coherent spectral oscillations, although exact fitting of the experimental spectra with the theoretical expression is difficult. This is on account of additional resonances present in the sample corresponding to lower energy bound states.

cond-mat.mes-hall↗

arXiv2018

Distinguishing quantum dot-like localized states from quantum well-like extended states across the exciton emission line in a quantum well

We have closely examined the emission spectrum at the heavy-hole exciton resonance in a high-quality GaAs multi-quantum well (MQW) sample using picosecond excitation-correlation photoluminescence (ECPL) spectroscopy. Dynamics of the ECPL signal at low and high energy sides of the excitonic photoluminescence (PL) peak shows complementary behavior. The ECPL signal is positive (negative) below (above) the PL peak and it changes sign within a narrow band of energy lying between excitonic absorption and emission peaks. The energy at which this sign change takes place is interpreted as the excitonic mobility edge as it separates localized excitons in quantum dot-like states from mobile excitons in quantum well-like states.

cond-mat.mes-hall↗

arXiv2018

Kinetic Spinodal Instabilities in the Mott Transition in V2O3: Evidence from Hysteresis Scaling and Dissipative Phase Ordering

We present the first systematic observation of scaling of thermal hysteresis with the temperature scanning rate around an abrupt thermodynamic transition in correlated electron systems. We show that the depth of supercooling and superheating in vanadium sesquioxide (V2O3) shifts with the temperature quench rates. The dynamic scaling exponent is close to the mean field prediction of 2/3. These observations, combined with the purely dissipative continuous ordering seen in "quench-and-hold" experiments, indicate departures from classical nucleation theory toward a barrier-free phase ordering associated with critical dynamics. Observation of critical-like features and scaling in a thermally induced abrupt phase transition suggests that the presence of a spinodal-like instability is not just an artifact of the mean field theories but can also exist in the transformation kinetics of real systems, surviving fluctuations.

cond-mat.stat-mech↗

arXiv2013

Scattering of Carriers by Charged Dislocations in Semiconductors

The scattering of carriers by charged dislocations in semiconductors is studied within the framework of the linearized Boltzmann transport theory with an emphasis on examining consequences of the extreme anisotropy of the scattering potential. A new closed-form approximate expression for the carrier mobility valid for all temperatures is proposed. The ratios of quantum and transport scattering times are evaluated after averaging over the anisotropy in the relaxation time. The value of the Hall scattering factor computed for charged dislocation scattering indicates that there may be a factor of two error in the experimental mobility estimates using the Hall data. An expression for the resistivity tensor when the dislocations are tilted with respect to the plane of transport is derived. Finally an expression for the isotropic relaxation time is derived when the dislocations are located within the sample with a uniform angular distribution.

cond-mat.dis-nn↗

arXiv2012

On conversion of luminescence into absorption and the van Roosbroeck-Shockley relation

The problem of conversion of experimentally measured luminescence spectrum into the absorption cross section is revisited. The common practice of using the van Roosbroeck-Shockley (or Kubo-Martin-Schwinger or Kennard-Stepanov) relation in this context is incorrect because luminescence from semiconductors is essentially all due to the spontaneous emission component of the recombination of carriers distributed far-from-equilibrium. A simple, physically consistent, and practical prescription for converting the luminescence spectra into absorption is presented and its relation to the so-called nonequilibrium generalization of the van Roosbroeck-Shockley relationship is discussed.

cond-mat.mes-hall↗

arXiv2009

Extended excitons and compact helium-like biexcitons in type-II quantum dots

We have used magneto-photoluminescence measurements to establish that InP/GaAs quantum dots have a type-II (staggered) band alignment. The average excitonic Bohr radius and the binding energy are estimated to be 15nm and 1.5 meV respectively. When compared to bulk InP, the excitonic binding is weaker due to the repulsive (type-II) potential at the hetero-interface. The measurements are extended to over almost six orders of magnitude of laser excitation powers and to magnetic fields of up to 50 tesla. It is shown that the excitation power can be used to tune the average hole occupancy of the quantum dots, and hence the strength of the electron-hole binding. The diamagnetic shift coefficient is observed to drastically reduce as the quantum dot ensemble makes a gradual transition from a regime where the emission is from (hydrogen-like) two-particle excitonic states to a regime where the emission from (helium-like) four-particle biexcitonic states also become significant.

cond-mat.mes-hall↗

arXiv2008

Photoluminescence from localized states in disordered indium nitride

Photoluminescence spectra from disordered InN were studied in very high magnetic fields. The samples had Gaussian spectra with low temperature emission peaks at 0.82 and 0.98eV respectively. The average spatial extent of the excitonic wave functions, inferred from the diamagnetic shift, is only 2-3nm. This shows that the recombination is from an ensemble of highly localized states within a landscape of a smooth (classical) disorder potential of strength of the order of 10meV. The anomalies in the temperature dependence of the photoluminescence peak and linewidth give further support to the picture of trapped photoexcited carriers.

cond-mat.mtrl-sci↗

arXiv2007

Excitonic Mott transition in type-II quantum dots

Photoluminescence spectra measured on a type-II GaSb/GaAs quantum dot ensemble at high excitation power indicate a Mott transition from the low density state comprising of spatially-indirect excitons to a high density electron-plasma state. Under the influence of a very high magnetic field, the electron-plasma that is formed at high excitation powers is `frozen-out' into a state of optically inactive magneto-excitons.

cond-mat.other↗

arXiv2007

Alloying induced degradation of the absorption edge of InAs_{x}Sb_{1-x}

InAs_{x}Sb_{1-x} alloys show a strong bowing in the energy gap, the energy gap of the alloy can be less than the gap of the two parent compounds. We demonstrate that a consequence of this alloying is a systematic degradation in the sharpness of the absorption edge. The alloy disorder induced band-tail (Urbach tail) characteristics are quantitatively studied for InAs_{0.05}Sb_{0.95}.

cond-mat.mtrl-sci↗