arXiv subjects

Priya Mahadevan

Publications and source records attributed to Priya Mahadevan.

At least 19 recordsLinked to original sources

arXiv2026

Ferroelastic exciton splitting in hybrid perovskite nanowalls

Hybrid metal-halide perovskites are soft semiconductors in which electronic excitations are strongly influenced by lattice distortions and structural phase transitions. An important open question is whether ferroelastic symmetry breaking merely broadens optical resonances or instead modifies excitonic states through exciton-lattice coupling. Here, we address this question using highly aligned MAPbI3 nanowalls fabricated by glancing-angle deposition, enabling symmetry-selective coupling between ferroelastic texture, structural anisotropy, and a well-defined optical axis. Combining temperature-dependent photoluminescence, X-ray diffraction and polarization-resolved ultrafast transient absorption spectroscopy, we observe a polarization-selective excitonic splitting in the orthorhombic phase at 5 K, characterized by orthogonal optical selection rules and a 45 meV energy separation. Near 160 K, where orthorhombic and tetragonal phases coexist, a lower-energy lattice-coupled excitation emerges 58 meV below the centre of the anisotropically split excitonic structure, consistent with coupling between excitonic and lattice-dressed states. At higher temperatures, these excitations progressively acquire lattice-dressed character accompanied by reduced optical anisotropy. A symmetry-guided effective Hamiltonian captures the evolution from anisotropically split excitons to coupled excitonic and lattice-dressed states across the structural transition. Our results show that ferroelastic texture and phase coexistence can modify exciton-lattice coupling, providing a route to symmetry-selective optical responses in soft polar semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Spin orientation -- a subtle interplay between strain and multipole Coulomb interactions

We address the technologically important issue of the spin orientation on a correlated magnetic surface and how to manipulate it. We consider a prototypical strongly correlated system, NiO, and show that a single particle approach with anisotropic hoppings, or even a many-electron model with a scalar Hubbard $U$ and Hund's $J$ fails to explain the strain driven spin reorientation transition (SRT). We set up a model treating both anisotropic single particle effects and orbital-dependent, full multipole electron-electron interaction effects at the same footing. Within this model, predictive power to explain the observed SRT is regained and the results indicate the novel possibility of using an electric field to control SRT in magnetic films grown on piezoelectric substrates.

cond-mat.str-el↗

arXiv2025

Direction-Dependent Conduction Polarity in Altermagnetic CrSb

CrSb has recently gained immense attention as an altermagnetic candidate. This work reports on the experimental observation of direction-dependent conduction polarity (DDCP) in altermagnetic CrSb through Hall and Seebeck thermopower measurements. Conduction is dominated by holes along the c-axis and by electrons in the ab-plane of the hexagonal crystal of CrSb. Density functional theory (DFT) calculations indicate that DDCP in CrSb arises from a multicarrier mechanism, where electrons and holes living in distinct bands dominate conduction along different crystallographic directions. Furthermore, DFT predicts that DDCP exists within a narrow energy window near the Fermi level and is sensitive to small doping levels. This prediction is experimentally validated by the loss of DDCP in hole-doped Cr$_{0.98}$V$_{0.02}$Sb. These findings highlight the potential for tunable electronic behavior in CrSb, offering promising avenues for applications in devices that require both p-type and n-type functionalities within a single material.

cond-mat.mtrl-sci↗

arXiv2024

Origin of multiple Lifshitz transitions in the Weyl semi-metal RhSi

It is known from density functional theory (DFT) calculations that RhSi has a multifold degenerate Dirac point at the Fermi energy, with the dominant states in the low-energy region displaying mostly Rh $d$ character. Using DFT+U, we calculate the band structure by considering an effective local interaction on the Rh $d$ states, with a realistic effective Hubbard $U_\textrm{eff}=2.5$ eV derived from a constrained random-phase approximation calculation, and find the emergence of a double hump structure close to the Fermi energy.By further deriving a low-energy tight-binding model from our first-principles results, we show that the double hump is a direct consequence of a competition between the Rh $d$-Rh $d$ and Rh $d$-Si $p$ interactions, which differ in their momentum dependence. As a consequence, through an artificial tuning of the energy level of the Si $p$ orbitals this hump structure can be suppressed due to the effectively reduced Rh $d$ -Si $p$ interaction.This peculiar low-energy electronic structure additionally results in that a small hole/electron doping ($\sim$ 0.1 $\%$) can tune the Fermi surface topology, going from closed to open Fermi surfaces, which has dramatic consequences for the thermal transport.

cond-mat.str-el↗

arXiv2023

The role of stacking on the electronic structure of MoSe2 at small twist angles

We consider two high symmetry stackings AA and AB and examine the changes induced in the electronic structure by considering small angles of rotation of 3.48degrees from both these stackings. In both cases we largely recover the low energy electronic structure of the untwisted limit. We additionally find flat bands emerging above the dispersing bands. Surprisingly, while the rotation from the AA end leads to one flat band above the highest occupied band at {\Gamma}, one finds two flat bands emerging for small rotations from the AB end. Examining the real space localization of the flat bands allows us to discuss the origin of the flat bands in terms of quantum well states and qualitatively understand the dependence of the number of flat bands found on the twist angle.

cond-mat.mtrl-sci↗

arXiv2023

Detection of nontrivial topology driven by charge density wave in a semi-Dirac metal

The presence of electron correlations in a system with topological order can lead to exotic ground states. Considering single crystals of LaAgSb2 which has a square net crystal structure, one finds multiple charge density wave transitions (CDW) as the temperature is lowered. We find large planar Hall (PHE) signals in the CDW phase, which are still finite in the high temperature phase though they change sign. Optimising the structure within first-principles calculations, one finds an unusual chiral metallic phase. This is because as the temperature is lowered, the electrons on the Ag atoms get more localized, leading to stronger repulsions between electrons associated with atoms on different layers. This leads to successive layers sliding with respect to each other, thereby stabilising a chiral structure in which inversion symmetry is also broken. The large Berry curvature associated with the low temperature structure explains the low temperature PHE. At high temperature the PHE arises from the changes induced in the tilted Dirac cone in a magnetic field. Our work represents a route towards detecting and understanding the mechanism in a correlation driven topological transition through electron transport measurements, complemented by ab-initio electronic structure calculations.

cond-mat.mtrl-sci↗

arXiv2022

Hole doping in a negative charge transfer insulator

$RE$NiO$_3$ is a negative charge transfer energy system and exhibits a temperature-driven metal-insulator transition (MIT), which is also accompanied by a bond disproportionation (BD) transition. In order to explore how hole doping affects the BD transition, we have investigated the electronic structure of single-crystalline thin films of Nd$_{1-x}$Ca$_x$NiO$_3$ by synchrotron based experiments and {\it ab-initio} calculations. For a small value of $x$, we find that the doped holes are localized on one or more Ni sites around the dopant Ca$^{2+}$ ions, while the BD state for the rest of the lattice remains intact. The effective charge transfer energy ($\Delta$) increases with Ca concentration and the formation of BD phase is not favored above a critical $x$, suppressing the insulating phase. Our present study firmly demonstrates that the appearance of BD mode is essential for the MIT and settles a long-standing debate about the role of structural distortions for the MIT of the $RE$NiO$_3$ series.

cond-mat.str-el↗

arXiv2022

Microscopic Origin Of Room Temperature Ferromagnetism in a Double Perovskite Sr$_2$FeReO$_6$: a first principle and model Hamiltonian study

The puzzling observation of room temperature ferromagnetism in double perovskites (A$_2$BB$'$O$_6$), despite having the magnetic lattice of B-ions diluted by non-magnetic B$'$-ions, have been examined for Sr$_2$FeReO$_6$. {\it Ab-initio} spin spiral electronic structure calculations along various high symmetry directions in reciprocal space are used to determine the exchange interactions entering an extended Heisenberg model, which is then solved classically using Monte Carlo simulations to determine the ferromagnetic transition temperature T$_c$. We find that one must consider onsite Coulomb interactions at the nonmagnetic Re sites ($U$) in order to obtain a T$_c$ close to the experimental value. Analysis of the $ab$-$initio$ electronic structure as well as an appropriate model Hamiltonian trace the origin of enhancement in T$_c$ with $U$ to the enhanced exchange splitting that is introduced at these sites. This in turn destabilizes the antiferromagnetic exchange channels, thereby enhancing the T$_c$. The role of occupancy at the non-magnetic sites is examined by contrasting with the case of Sr$_2$FeMoO$_6$.

cond-mat.str-el↗

arXiv2021

Diffusion Monte Carlo Study on Relative Stabilities of Boron Nitride Polymorphs

Although Boron nitride (BN) is a well-known compound widely used for engineering and scientific purposes, the phase stability of its polymorphs, one of its most fundamental properties, is still under debate. The ab initio determination of the ground state of the BN polymorphs, such as hexagonal and zinc-blende, is difficult because of the elusive Van der Waals interaction, which plays a decisive role in some of the polymorphs, making quantitative prediction highly challenging. Hence, despite multiple theoretical studies, there has been no consensus on the ground state yet, primarily due to contradicting reports. In this study, we apply a state-of-the-art ab initio framework - fixed-node diffusion Monte Carlo (FNDMC), to four well known BN polymorphs, namely hexagonal, rhombohedral, wurtzite, and zinc-blende BNs. Our FNDMC calculations show that hBN is thermodynamically the most stable among the four polymorphs at 0 K as well as at 300K. This result agrees with the experimental data of Corrigan~{\it et al.} and Fukunaga. The conclusions are consistent with those obtained using other high-level methods, such as coupled cluster. We demonstrate that the FNDMC is a powerful method to address polymorphs that exhibit bonds of various forms. It also provides valuable information, like reliable reference energies, when reliable experimental data are missing or difficult to access. Our findings should promote the application of FNDMC for other van der Waals materials.

cond-mat.mtrl-sci↗

arXiv2021

High Responsivity Gate Tunable UV-Visible Broadband Phototransistor Based on Graphene-WS2 Mixed Dimensional (2D-0D) Heterostructure

Recent progress in the synthesis of highly stable, eco-friendly, cost-effective transition metal-dichalcogenides (TMDC) quantum dots (QDs) with their broadband absorption spectrum and wavelength selectivity features have led to their increasing use in broadband photodetectors. With the solution based processing, we demonstrate a super large (~ 0.75 mm^2), UV-Vis broadband (365-633 nm), phototransistor made of WS_2 QDs decorated CVD graphene as active channel with extraordinary stability and durability in ambient condition (without any degradation of photocurrent till 4 months after fabrication). Here, colloidal 0D WS_2-QDs are used as the photo absorbing material and graphene acts as the conducting channel. A high photoresponsivity (3.1 x 10^2 A/W), higher detectivity (2.2 x 10^12 Jones) and low noise equivalent power (4 x 10^{-14} W/Hz^0.5) are obtained at a low bias voltage (V_{ds} = 1V) at an illumination of 365 nm with an optical power as low as 0.8 \mu W/cm^2, which can further be tuned by modulating the gate bias. While comparing the photocurrent between two different morphologies of WS_2 (QDs and 2D nanosheets), a significant enhancement of photocurrent is observed in case of QDs based device. Ab initio density functional theory based calculations further support our observation, revealing the role of quantum confinement for the enhanced photo response. Our work reveals a strategy towards making a scalable, cost-effective, highly performing hybrid two-dimensional (2D/0D) photo detector with graphene-WS_2 QDs, paving the way towards the next generation optoelectronic applications.

physics.app-ph↗

arXiv2021

Probing Metal-Molecule Contact at the Atomic Scale via Conductance Jump

Understanding the formation of metal-molecule contact at the microscopic level is the key towards controlling and manipulating atomic scale devices. Employing two isomers of bipyridine, $4, 4^\prime$ bipyridine and $2, 2^\prime$ bipyridine between gold electrodes, here, we investigate the formation of metal-molecule bond by studying charge transport through single molecular junctions using a mechanically controlled break junction technique at room temperature. While both molecules form molecular junctions during the breaking process, closing traces show the formation of molecular junctions unambiguously for $4, 4^\prime$ bipyridine via a conductance jump from the tunneling regime, referred as `jump to molecular contact', being absent for $2, 2^\prime$ bipyridine. Through statistical analysis of the data, along with, molecular dynamics and first-principles calculations, we establish that contact formation is strongly connected with the molecular structure of the electrodes as well as how the junction is broken during breaking process, providing important insights for using a single-molecule in an electronic device.

cond-mat.mes-hall↗

arXiv2020

Signatures of a spin-1/2 cooperative paramagnet in the diluted triangular lattice of Y$_2$CuTiO$_6$

We present a combination of thermodynamic and dynamic experimental signatures of a disorder driven dynamic cooperative paramagnet in a 50% site diluted triangular lattice spin-1/2 system, Y$_2$CuTiO$_6$. Magnetic ordering and spin freezing are absent down to 50 mK, far below the Curie Weiss scale of ~-134 K. We observe scaling collapses of the magnetic field- and temperature-dependent magnetic heat capacity and magnetisation data, respectively, in conformity with expectations from the random singlet physics. Our experiments establish the suppression of any freezing scale, if at all present, by more than three orders of magnitude, opening a plethora of interesting possibilities such as disorder-stabilized long range quantum entangled ground states.

cond-mat.mtrl-sci↗

arXiv2019

Understanding the ferromagnetic insulating state in Cr doped VO$_2$

Experimentally Cr doping in the rutile phase of VO$_2$ is found to stabilize a charge ordered ferromagnetic insulating state in the doping range of 10\% to 20\%. In this work, we investigated its origin at 12.5\% Cr doping using a combination of ab-initio electronic structure calculations as well as microscopic modeling. Our calculations are found to reproduce the ferromagnetic insulating state as well as a charge ordering at the V and Cr sites. The mapping of the ab-initio band structure onto a tight-binding Hamiltonian allows one to calculate the energy gain from different exchange pathways. This gain is quantified in this work for the first time and the role of charge ordering in stabilizing a ferromagnetic insulating state is understood.

cond-mat.mtrl-sci↗

arXiv2019

Anomalous electron transport in epitaxial NdNiO$_3$ films

The origin of simultaneous electronic, structural and magnetic transitions in bulk rare-earth nickelates ($RE$NiO$_3$) remains puzzling with multiple conflicting reports on the nature of these entangled phase transitions. Heterostructure engineering of these materials offers unique opportunity to decouple metal-insulator transition (MIT) from the magnetic transition. However, the evolution of underlying electronic properties across these decoupled transitions remains largely unexplored. In order to address this, we have measured Hall effect on a series of epitaxial NdNiO$_3$ films, spanning a variety of electronic and magnetic phases. We find that the MIT results in only partially gapped Fermi surface, whereas full insulating phase forms below the magnetic transition. In addition, we also find a systematic reduction of the Hall coefficient ($R_H$) in the metallic phase of these films with epitaxial strain and also a surprising transition to negative value at large compressive strain. Partially gapped weakly insulating, paramagnetic phase is reminiscence of pseudogap behavior of high $T_c$ cuprates. The precursor metallic phase, which undergoes transition to insulating phase is a non-Fermi liquid with the temperature exponent ($n$) of resistivity of 1, whereas the exponent increases to 4/3 in the non-insulating samples. Such nickelate phase diagram with sign-reversal of $R_H$, pseudo-gap phase and non Fermi liquid behavior are intriguingly similar to high $T_c$ cuprates, giving important guideline to engineer unconventional superconductivity in oxide heterostructure.

cond-mat.str-el↗

arXiv2017

Structural distortions in monolayers of binary semiconductors

We examine the structural properties of free standing II-VI and III-V semiconductors at the monolayer limit within first principle density functional theory calculations. A non-polar buckled structure was found to be favoured over a polar buckled structure. While an obvious reason for this may be traced to the contribution from dipole dipole interactions present in the polar structure which would destabilize it with respect to the nonpolar structure, Coulomb interactions between electrons on the cations and anions are found to be the reason for the nonpolar structure to be favoured. A route to tune the Coulomb interaction between the electrons on the cations and anions is through biaxial tensile strain. This allows for a planar graphitic phase in CdS to be stabilized at just 2\% tensile strain. Strain also shifts the valence band maximum from the $\Gamma$ point to the K point opening up opportunities for exploring spin-valley physics in these materials.

cond-mat.mes-hall↗

arXiv2017

Layer dependent electronic structure changes in transition metal dichalcogenides- The microscopic origin

We have examined the electronic structure evolution in transition metal dichalcogenides MX$_2$, where M=Mo,W and X=S,Se and Te. These are generally referred to as van der Waals heterostructures on the one hand, yet one has band gap changes as large as 0.6 eV with thickness in some instances. This does not seem to be consistent with a description where the dominant interactions are van der Waals interactions. Mapping onto a tight binding model allows us to quantify the electronic structure changes which are found to be dictated solely by interlayer hopping interactions. Different environments that an atom encounters could change the Madelung potential and therefore the onsite energies. This could happen while going from monolayer to bilayer as well as in cases where the stackings are different from what is found in 2H structures. These effects are quantitatively found to be negligible, enabling us to quantify the thickness dependent electronic structure changes as arising from interlayer interactions alone.

cond-mat.mes-hall↗

arXiv2017

The driving force for charge ordering in rare earth nickelates

We show that charge ordering (more precisely, two-sublattice bond disproportionation) in the rare earth nickelate perovskites is intimately related to a negative charge transfer energy. By adding an additional potential on the Ni d states we are able to vary the charge tranfer energy and compute relaxed structures within an ab-initio framework. We show that the difference in Ni-O bond lengths and the value of the ordered state magnetic moment correlate with the charge transfer energy and that the transition to the bond-disproportionated state occurs when the effective charge transfer energy becomes negative.

cond-mat.str-el↗

arXiv2016

Driving force for martensitic transformation in Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$

The martensitic transformation in Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$ alloys has been investigated within ab-initio density functional theory. The experimental trend of a martensitic transition happening beyond $x$ = 0.36 is captured within these calculations. The microscopic considerations leading to this are traced to increased Ni-Mn hybridization which results from the Ni atom experiencing a resultant force along a lattice parameter and moving towards the Mn atoms above a critical concentration. The presence of the lone pair electrons on Sn forces the movement of Ni atoms away from Sn. While band Jahn Teller effects have been associated with this transition, we show quantitatively that atleast in this class of compounds they have a minor role.

cond-mat.mtrl-sci↗