arXiv subjects

Anjana Dogra

Publications and source records attributed to Anjana Dogra.

9 recordsLinked to original sources

arXiv2024

Novel Emergent Phases in a Two-Dimensional Superconductor

In this letter, we report our observation of an extraordinarily rich phase diagram of a LaScO$_3$/SrTiO$_3$ heterostructure. Close to the superconducting transition temperature, the system hosts a superconducting critical point of the Infinite-randomness type characterized by an effective dynamical exponent $νz$ that diverges logarithmically. At lower temperatures, we find the emergence of a magnetic field-tuned metallic phase that co-exists with a quantum Griffiths phase (QGP). Our study reveals a previously unobserved phenomenon in 2D superconductors -- an unanticipated suppression of the QGP below a crossover temperature in this system. This concealment is accompanied by the destruction of the superconducting quantum critical point signaled by a power-law divergence (in temperature) of the effective dynamical exponent. These observations are entirely at odds with the predictions of the infinite-randomness scenario and challenge the very concept of a vanishing energy scale associated with a quantum critical point. We develop and discuss possible scenarios like smearing of the phase transition that could plausibly explain our observations. Our findings challenge the notion that QGP is the ultimate ground state in two-dimensional superconductors.

cond-mat.supr-con↗

arXiv2019

Effect of spin-orbit interaction on the vortex dynamics in LaAlO$_3$/SrTiO$_3$ interfaces near the superconducting transition

Controlling spin-orbit interaction and its effect on superconductivity has been a long-standing problem in two-dimensional inversion symmetry broken superconductors. An open challenge is to understand the role of various energy scales in shaping the complex phase diagram in these systems. From a combined experimental and theoretical study of resistance fluctuations and its higher order statistics, we propose a phase diagram for the superconducting phase in the magnetic-field--spin orbit interaction energy plane for the quasi-two dimensional electron gas at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. The relative variance of resistance fluctuations increases by few orders of magnitude below the spin-orbit field B$_{SO}$ and a non-Gaussian component to the fluctuations arises for fields below the upper critical field B$_{C2}$. Theoretical calculations show that the non-Gaussian noise predominantly arises due to percolative nature of the superconducting transition. We quantify the strength and the relative importance of the spin-orbit interaction energy, Zeeman energy and the pairing potential. Our work highlights the important role played by the interplay between these energy scales in framing the fascinating phases seen in two-dimensional inversion-symmetry-broken superconductors.

cond-mat.mes-hall↗

arXiv2018

Effect of multiband transport on charge carrier density fluctuations at the LaAlO$_3$/SrTiO$_3$ interface

Multiband transport in superconductors is interesting both from an academic as well as an application point of view. It has been postulated that interband scattering can significantly affect the carrier dynamics in these materials. In this article we present a detailed study of the electrical transport properties of the high-mobility two-dimensional electron gas residing at the interface of LaAlO$_3$/SrTiO$_3$, a prototypical multi-band superconductor. We show, through careful measurements of the gate dependence of the magnetoresistance and resistance fluctuations at ultra-low temperatures, that transport in the superconducting regime of this system has contributions from two bands which host carriers of very different characters. We identify a gate-voltage tunable Lifshitz transition in the system and show that the resistance fluctuations have strikingly different features on either side of it. At low carrier densities, resistance noise is dominated by number-density fluctuations arising from trapping-detrapping of charge carriers from defects in the underlying SrTiO$_3$ substrate, characteristic of a single-band semiconductor. Above the Lifshitz transition, the noise presumably originates from inter-band scattering. Our work highlights the importance of inter-band scattering processes in determining the transport properties of low-dimensional systems and projects resistance fluctuation spectroscopy as a viable technique for probing the charge carrier dynamics across a Lifshitz transition.

cond-mat.mes-hall↗

arXiv2017

Observation of transient superconductivity at the LaAlO$_3$/SrTiO$_3$ interface

We report the observation of a magnetic field assisted transient superconducting state in the two dimensional electron gas existing at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. This metastable state depends critically on the density of charge carriers in the system. It appears concomitantly with a Lifshitz transition as a consequence of the interplay between ferromagnetism and superconductivity and the finite relaxation time of the in-plane magnetization. Our results clearly demonstrate the inherently metastable nature of the superconducting state competing with a magnetic order in these systems. The co-existence of superconductivity and ferromagnetism in the conducting electronic layer formed at the interface of insulating oxides has thrown up several intriguing and as yet unanswered questions. An open question in this field is the energetics of the interplay between these two competing orders and the present observation goes a long way in understanding the underlying mechanism.

cond-mat.str-el↗

arXiv2016

Correlated Non-Gaussian phase fluctuations in LaAlO$_3$/SrTiO$_3$ heterointerface

We probe the existence of large correlated non-Gaussian phase fluctuations in the vicinity of the superconducting phase transition in the conducting layer residing at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. The non-Gaussian fluctuations appear between the Berezinskii-Kosterlitz-Thouless transition temperature $T_{BKT}$ and the mean field transition temperature $T_C$. Subsequent theoretical analysis reveals that non-Gaussianity arises predominantly due to the percolative transition of a Josephson coupled network of superconductors. Our results confirm that the superconductivity in this system is confined to two-dimensions. Our study of the non-Gaussian resistance fluctuation spectrum provides a novel means to explore the BKT-transition in two-dimensional inhomogeneous superconductors.

cond-mat.supr-con↗

arXiv2014

Enhanced spin-orbit coupling and charge carrier density suppression in LaAl1-xCrxO3/SrTiO3 heterointerfaces

We report a gradual suppression of the two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface on substitution of chromium at the Al sites. The sheet carrier density at the interface (ns) drops monotonically from 2.2x10(14) cm-2 to 2.5x10(13) cm-2 on replacing nearly 60 percent of Al sites by Cr and the sheet resistance (Rs) exceeds the quantum limit for localization (h/2e2) in the concentrating range 40 to 60 percent of Cr.

cond-mat.str-el↗

arXiv2014

Chemical potential shift and gap-state formation in SrTiO$_{3-δ}$ revealed by photoemission spectroscopy

In this study, we report on investigations of the electronic structure of SrTiO$_3$ annealed at temperature ranging between 550 and 840$^\circ$C in an ultrahigh vacuum. Annealing induced oxygen vacancies (O$_{vac}$) impart considerable changes in the electronic structure of SrTiO$_3$. Using core-level photoemission spectroscopy, we have studied the chemical potential shift ($Δμ$) as a function of annealing temperature. The result shows that the chemical potential monotonously increases with electron doping in SrTiO$_{3-δ}$. The monotonous increase of the chemical potential rules out the existence of electronic phase separation in the sample. Using valence band photoemission, we have demonstrated the formation of a low density of states at the near Fermi level electronic spectrum of SrTiO$_{3-δ}$. The gap-states were observed by spectral weight transfer over a large energy scale of the stoichiometric band gap of SrTiO$_3$ system leading finally to an insulator - metal transition. We have interpreted our results from the point of structural distortions induced by oxygen vacancies.

cond-mat.mtrl-sci↗

arXiv2009

Dielectric Anomalies in a New Manganocuprate, Gd3Ba2Mn2Cu2O12

Dielectric response has been studied for a new manganocuprate, Gd3Ba2Mn2Cu2O12 (Gd3222) as a function of temperature (100 - 300 K) and frequency (75 kHz to 1 MHz). The dielectric constant (e) exhibits a two step increase (two peaks) in e(T) with increasing temperature from 100 to 300 K. The first peak is seen around 150 K and the second one around 210 K (both for 75 kHz). Increasing frequency shifts both the peaks to higher temperature side. The behavior of dielectric constant (e) and dielectric loss (tan_delta) matches with glassy behavior observed in many dipolar molecules.

cond-mat.mtrl-sci↗

arXiv2004

Effect of Co-Ga paired substitution on superconductivity in YBa2Cu3O7-d

The effect of Co-Ga paired substitution on the superconducting properties of YBa2Cu3O7-d (Y-123) has been investigated by X-ray diffraction, ac susceptibility,dc resistivity and oxygen content measurements. We report in this paper the results of our studies on the paired substitution of a magnetic and non-magnetic ion at Cu site in Y-123, while keeping the total dopant concentration fixed. The simultaneous substitution of Co and Ga at Cu site shows variation in the transition temperature (Tc), oxygen content and hole concentration as a function of change in the balance of magnetic (Co) and non-magnetic (Ga) concentration. Orthorhombicity (D), given as (b-a)/a, also varies as a function of increasing dopant concentration. The variation in Tc due to Co-Ga substitution is discussed in the light of dopant valency and hole filling mechanism.

cond-mat.supr-con↗