arXiv subjects

A. Taraphder

Publications and source records attributed to A. Taraphder.

At least 91 records · Page 5Linked to original sources

arXiv2013

Oxygen-vacancy clustering and pseudogap behaviour at LaAlO3/SrTiO3 interface

The 2DEG at the LaAlO3/SrTiO3 interface promises to add a new dimension to emerging electronic devices due to its high tunability. Defects in the form of Oxygen vacancies in titanate surfaces and interfaces, on the other hand, play a key role in the emergence of novel phases. Based on an effective model, we study the influence of Oxygen vacancies on superconductivity and ferromagnetism at LaAlO3/SrTiO3 interface. Using the Bogoliubov-de Gennes formulation in conjunction with Monte-Carlo simulation, we find a clustering of the Oxygen vacancies at the interface that favours formation of coexisting ferromagnetic puddles spatially separated from superconductivity. We also find a carrier freeze-out at low temperatures, observed experimentally in wide-variety of samples. Sufficiently large amount of Oxygen vacancy leads to pseudo-gap like behaviour in the superconducting state.

cond-mat.supr-con↗

arXiv2012

Preformed Excitons, Orbital Selectivity, and Charge-Density-Wave Order in 1T-TiSe_2

Traditional routes to Charge-Density-Wave in transition metal dichalcogenides, relying on Fermi surface nesting or Jahn-Teller instabilities have recently been brought into question. While this calls for exploration of alternative views, paucity of theoretical guidance sustains lively controversy on the origin of, and interplay between CDW and superconductive orders in transition metal dichalcogenides. Here, we explore a preformed excitonic liquid route, heavily supplemented by modern correlated electronic structure calculations, to an excitonic-CDW order in 1T-TiSe$_{2}$. We show that orbital-selective dynamical localisation arising from preformed excitonic liquid correlations is somewhat reminiscent to states proposed for $d$- and $f$-band quantum criticality at the border of magnetism. Excellent quantitative explication of a wide range of spectral and transport responses in both normal and CDW phases provides strong support for our scenario, and suggests that soft excitonic liquid fluctuations mediate superconductivity in a broad class of transition metal dichalcogenides on the border of CDW. This brings the transition metal dichalcogenides closer to the bad actors in $d$- and $f$-band systems, where anomalously soft fluctuations of electronic origin are believed to mediate unconventional superconductivity on the border of magnetism.

cond-mat.str-el↗

arXiv2011

Unusual ferromagnetism in nanoparticles of doped oxides and manganites

The observation of unusually large ferromagnetism in the nanoparticles of doped oxides and enhanced ferromagnetic tendencies in manganite nanoparticles have been in focus recently. For the transition metal-doped oxide nanoparticles a phenomenological `charge transfer ferromagnetism' model is recently proposed by Coey et al. From a microscopic calculation with charge transfer between the defect band and mixed valent dopants, acting as reservoir, we show how the unusually high ferromagnetic response develops. The puzzle of nanosize-induced ferromagnetic tendencies in manganites is also addressed within the same framework where lattice imperfections and uncompensated charges at the surface of the nanoparticle are shown to reorganize the surface electronic structures with enhanced double exchange.

cond-mat.str-el↗

arXiv2011

Orbital order in NaTiO$_2$ : A first principles study

The debate over the orbital order in layered triangular lattice system NaTiO$_2$ has been rekindled by the recent experiments of McQueen, et al. \cite{cava} on NaVO$_2$ ({\em Phys. Rev. Lett.} {\bf 101}, 166402 (2008)). In view of this, the nature of orbital ordering, in both high and low temperature states, is studied using an ab-initio electronic structure calculation. The orbital order observed in our calculations in the low temperature structure of NaTiO$_2$ is consistent with the predictions of McQueen, et. al. An LDA plus dynamical mean-field calculation shows considerable transfer of spectral weight from the Fermi level but no metal- insulator transition, confirming the poor metallic behaviour observed in transport measurements.

cond-mat.str-el↗

arXiv2011

Preformed excitonic liquid route to a charge density wave in 2H-TaSe$_2$

Recent experiments on 2H-TaSe$_2$ contradict the long-held view of the charge density wave arising from a nested band structure. An intrinsically strong coupling view, involving a charge density wave state arising as a Bose condensation of preformed excitons emerges as an attractive, albeit scantily investigated alternative. Using local density approximation plus multiorbital dynamic mean-field theory, we show that this scenario agrees with a variety of normal state data for 2H-TaSe$_{2}$. Based thereupon, the ordered states in a subset of dichalcogenides should be viewed as instabilities of a correlated, preformed excitonic liquid.

cond-mat.str-el↗

arXiv2011

Field dependent dynamics in the metallic regime of the half-filled Hubbard model

A systematic study of the effect of magnetic field (h) on Hubbard model has been carried out at half filling within dynamical mean field theory. In agreement with previous studies, we find a zero temperature itinerant metamagnetic transition, reflected in the discontinuous changes in magnetization as well as in the hysteresis, from a paramagnetic (PM) metallic state to a polarized quasi-ferromagnetic (QFM) state, at intermediate and large interaction strength ($U$). The jump in magnetization vanishes smoothly with decreasing interaction strength, and at a critical $U$, the transition becomes continuous. The region of `coexistence' of the PM and QFM solutions in the field-$U$ plane obtained in this study agrees quantitatively with recent numerical renormalization group calculations, thus providing an important benchmark. We highlight the changes in dynamics and quasiparticle weight across this transition. The effective mass increases sharply as the transition is approached, exhibiting a cusp-like singularity at the critical field,and decreases with field monotonically beyond the transition. We conjecture that the first order metamagnetic transition is a result of the competition between Kondo screening, that tries to quench the local moments, and Zeeman coupling, which induces polarization and hence promotes local moment formation. A comparison of our theoretical results with experiments on $^{3}He$ indicate that, a theory of $^{3}He$ based on the half-filled Hubbard model places it in a regime of intermediate interaction strength.

cond-mat.str-el↗

arXiv2010

An extended Falicov-Kimball model on a triangular lattice

The combined effect of frustration and correlation in electrons is a matter of considerable interest of late. In this context a Falicov-Kimball model on a triangular lattice with two localized states, relevant for certain correlated systems, is considered. Making use of the local symmetries of the model, our numerical study reveals a number of orbital ordered ground states, tuned by the small changes in parameters while quantum fluctuations between the localized and extended states produce homogeneous mixed valence. The inversion symmetry of the Hamiltonian is broken by most of these ordered states leading to orbitally driven ferroelectricity. We demonstrate that there is no spontaneous symmetry breaking when the ground state is inhomogeneous. The study could be relevant for frustrated systems like $GdI_2$, $NaTiO_2$ (in its low temperature C2/m phase) where two Mott localized states couple to a conduction band.

cond-mat.str-el↗

arXiv2010

On the question of metallicity and ferromagnetism in charge ordered insulating Nd0.5Sr0.5MnO3 nanoparticles

The fascinating phenomenon of destabilization of charge/orbital order in Nd0.5Sr0.5MnO3 with the reduction of grain size is critically investigated. Based on our magnetic and transport experiments followed by a theoretical analysis, we analyze various possible mechanisms and try to delineate a universal scenario behind this phenomenon. We, revisit this issue and discuss the overwhelming evidence from experiments on nano and bulk manganites as well as the absence of correlation between size reduction and pressure effects on manganites. We propose a phenomenological understanding on the basis of enhanced surface disorder to explain the appearance of weak ferromagnetism and metallicity in nanoparticles of Nd0.5Sr0.5MnO3. We also provide supportive evidence from an ab-initio electron structure calculation and a recent numerical simulation and argue that the mechanism is universal in all nanosize charge ordered manganites.

cond-mat.str-el↗

arXiv2010

Ground state phase diagram of a spinless, extended Falicov-Kimball model on the triangular lattice

Correlated systems with hexagonal layered structures have come to fore with renewed interest in Cobaltates, transition-metal dichalcogenides and GdI2. While superconductivity, unusual metal and possible exotic states (prevented from long range order by strong local fluctuations) appear to come from frustration and correlation working in tandem in such systems, they freeze at lower temperature to crystalline states. The underlying effective Hamiltonian in some of these systems is believed to be the Falicov-Kimball model and therefore, a thorough study of the ground state of this model and its extended version on a non-bipartite lattice is important. Using a Monte Carlo search algorithm, we identify a large number of different possible ground states with charge order as well as valence and metal-insulator transitions. Such competing states, close in energy, give rise to the complex charge order and other broken symmetry structures as well as phase segregations observed in the ground state of these systems.

cond-mat.str-el↗

arXiv2009

Towards A Possible Charge-Kondo effect in Optical Lattices

The Kondo effect underpins a large body of recent developments in the physics of $d$- and $f$-band compounds. Although its {\it charge} analog is a rarity in solids, the recent observations of the charge Kondo effect and the consequent rise in superconducting T$_c$ encourage a search for other accessible systems. Motivated by the possibility of wilfully tuning the {\it sign} of the inter-electronic interaction in optical lattices, we study conditions for the elusive {\it charge Kondo liquid} (CKL) state to manifest. We propose that a combination of Feshbach resonances and sequentially controlled laser pulses may produce the CKL. We show that the CKL is {\it never} a stable ground state, appearing only when the ordered ground states are destabilized. Finally, we discuss interesting analogies with nuclear matter.

cond-mat.str-el↗

arXiv2008

GdI_2: A New Ferromagnetic Excitonic Solid?

The two-dimensional, colossal magnetoresistive system GdI_2 develops an unusual metallic state below its ferromagnetic transition and becomes insulating at low temperatures. It is argued that this geometrically frustrated, correlated poor metal is a possible candidate for a ferromagnetic excitonic liquid. The renormalized Fermi surface supports a further breaking of symmetry to a charge ordered, excitonic solid ground state at lower temperatures via order by disorder mechanism. Several experimental predictions are made to investigate this unique orbitally correlated ground state.

cond-mat.str-el↗

arXiv2007

Effect of substrate-induced strain on transport and magnetic properties of epitaxial La_0.66Sr_0.33MnO_3 thin films

We have studied electrical transport and magnetic properties of epitaxial La_0.66 Sr_0.33 Mn O_3 thin films grown on different substrates having different lattice strains induced by different film-substrate lattice-mismatch. Both electrical transport and magnetic properties exhibit strong correlation with Jahn-Teller strain. Our study shows a sufficiently large Jahn-Teller strain giving rise to distinct semi-conducting/insulating state in LSMO thin films even below the respective para-ferromagnetic Curie temperatutes Tc, which is in contradiction with the established phase diagram of Sr-doped manganites. We have presented a microscopic model for the analysis of our data instead of the usual expansion around the undistorted state generally in use in the literature. The model incorporates the two relevant e_g orbitals (and their ordering thereof) and the effect of both Jahn-Teller and bulk strain on the transition temperature via double exchange.

cond-mat.mtrl-sci↗

arXiv2007

Pairing in disordered s-wave superconductors and the effect of their coupling

Inhomogeneity is introduced through random local interactions (Ui) in an attractive Hubbard model on a square lattice and studied using mean-field Bogoliubov-de Gennes formalism. Superconductivity is found to get suppressed by the random Ui contrary to the results of a bimodal distribution of Ui. The proximity effect of superconductivity is found to be strong, all sites develop non-zero pairing amplitude. The gap in the density of states is always non-zero and does not vanish even for strong disorder. When two such superconductors are coupled via a channel, the effect of one on the other is negligible. The length and width of the connector, do not seem to have any noticeable effect on the superconductivity in either systems. The superconducting blocks behave as independent entity and the introduction of the channel have no effect on them.

cond-mat.supr-con↗

arXiv2006

A study of long range order in certain two-dimensional frustrated lattices

We have studied the Heisenberg antiferromagnets on two-dimensional frustrated lattices, triangular and kagome lattices using linear spin-wave theory. A collinear ground state ordering is possible if one of the three bonds in each triangular plaquette of the lattice becomes weaker or frustrated. We study spiral order in the Heisenberg model along with Dzyaloshinskii-Moriya (DM) interaction and in the presence of a magnetic field. The quantum corrections to the ground state energy and sublattice magnetization are calculated analytically in the case of triangular lattice with nearesr-neighbour interaction. The corrections depend on the DM interaction strength and the magnetic field. We find that the DM interaction stabilizes the long-range order, reducing the effect of quantum fluctuations. Similar conclusions are reached for the kagome lattice. We work out the linear spin-wave theory at first with only nearest-neighbour (nn) terms for the kagome lattice. We find that the nn interaction is not sufficient to remove the effects of low energy fluctuations. The flat branch in the excitation spectrum becomes dispersive on addition of furthet neighbour interactions. The ground state energy and the excitation spectrum have been obtained for various cases.

cond-mat.str-el↗

arXiv2006

Magnetic properties of doped GdI2

Motivated by the recent experimental studies on layered ferromagnetic metallic system GdI2 and its doped variant GdI2Hx we develop a model to understand their ground state magnetic phase diagram. Based on first principle electronic structure calculations we write down a phenomenological model and solve it under certain approximations to obtain the ground state energy. In the process we work out the phase diagram of the correlated double exchange model on a triangular lattice for the specific band structure at hand.

cond-mat.other↗

arXiv2005

Charge order and phase segregation in overdoped bilayer manganites

There have been recent reports of charge ordering around $x=0.5$ in the bilayer manganites. At $x=0.5$, there appears to be a coexistence region of layered A-type antiferromagnetc and charge order. There are also reports of orbital order in this region without any Jahn-Teller effect. Based on physical grounds, this region is investigated from a model that incorporates the two $e_g$ orbitals at each Mn site and a near-neighbour Coulomb repulsion. It is shown that there indeed is both charge and orbital order close to the half-doped region coincident with a layered magnetic structure. Although the orbital order is known to drive the magnetic order, the layered magnetic structure is also favoured in this system by the lack of coherent transport across the planes and the reduced dimensionality of the lattice. The anisotropic hopping across the $e_g$ orbitals and the underlying layered structure largely determine the orbital arrangements in this region, while the charge order is primarily due to the long range interactions.

cond-mat.str-el↗

arXiv2004

Orbital ordering and magnetic structures in $\LMFO$ and $\LWFO$ double perovskites

We analyzed the possible magnetic and orbital orderings of double perovskites, using a simple extension of the double exchange model well suited for these compounds. Orbital ordering is favored by the on site repulsion at the Fe ions. We obtain a rich phase diagram, including ferri- and antiferromagnetic phases, which can, in turn, be metallic or insulating, depending on the existence of orbital order.

cond-mat.mtrl-sci↗

arXiv2003

Magnetic and orbital order in overdoped bilayer manganites

The magnetic and orbital orders for the bilayer manganites in the doping region $0.5 < x <1.0$ have been investigated from a model that incorporates the two $e_g$ orbitals at each Mn site, the inter-orbital Coulomb interaction and lattice distortions. The usual double exchange operates via the $e_g$ orbitals. It is shown that such a model reproduces much of the phase diagram recently obtained for the bilayer systems in this range of doping. The C-type phase with ($π,0,π$) spin order seen by Ling et al. appears as a natural consequence of the layered geometry and is stabilised by the static distortions of the system. The orbital order is shown to drive the magnetic order while the anisotropic hopping across the $e_g$ orbitals, layered nature of the underlying structure and associated static distortions largely determine the orbital arrangements.

cond-mat.str-el↗