arXiv subjects

Sesha Vempati

Publications and source records attributed to Sesha Vempati.

9 recordsLinked to original sources

arXiv2025

Electron Hydrodynamics in Graphene : Experimental and Theoretical Status

The present work comprehensively reviews electron hydrodynamics in graphene, highlighting both experimental observations and theoretical developments. Key experimental signatures such as negative vicinity resistance, Poiseuille flow, and significant violation of the Wiedemann-Franz (WF) law have been discussed, with special emphasis on Lorenz ratio measurements. In the theoretical direction, recent efforts have focused on developing hydrodynamic frameworks for calculating the thermodynamic and transport coefficients of electrons in graphene. The present work has briefly addressed the theoretical framework adopted by our group.

cond-mat.mes-hall↗

arXiv2025

On the Wiedemann-Franz law violation in Graphene and quark-gluon plasma systems

A comparative study of the thermodynamic and transport properties of the ultra-relativistic quark-gluon plasma produced in heavy ion collisions with the "quasi-relativistic" massless electron-hole plasma in graphene sample has been performed. We observe that the enthalpy per net charge carriers emerges as a useful physical quantity determining the transport variables in hydrodynamic domain. Lorenz ratio is defined as thermal to electrical conductivity ratio, normalized by temperature and Lorenz number $L_{0}=\frac{π^{2}}{3}\left(\frac{k_{B}}{e}\right)^{2}$. The validity of the Wiedemann-Franz law can be checked by evaluating the Lorenz ratio, which is expected to be unity. We investigate the validity of the Wiedemann-Franz law by examining whether the Lorenz ratio equals unity or deviates from it. Our findings indicate that, within the fluid-based framework, the Lorenz ratio consistently leads to a violation of the Wiedemann-Franz law. This is attributed to the proportional relation between Lorenz ratio and enthalpy per net charge carriers in the fluid. Based on the experimental observation, graphene and quark-gluon plasma, both systems at a low net carrier density, violate the Wiedemann-Franz law due to their fluidic nature. However, graphene at a relatively high net carrier density obeys the Wiedemann-Franz law, followed by metals with high Fermi energy or electron density. It indicates a fluid to the non-fluid transition of the graphene system from low to high carrier density. In this regard, the fluid or non-fluid aspect of quark-gluon plasma at high density is yet to be explored by future facilities like Compressed Baryonic Matter and Nuclotron-based Ion Collider fAcility experiments.

cond-mat.str-el↗

arXiv2024

Dynamics of Hot QCD Matter 2024 -- Bulk Properties

The second Hot QCD Matter 2024 conference at IIT Mandi focused on various ongoing topics in high-energy heavy-ion collisions, encompassing theoretical and experimental perspectives. This proceedings volume includes 19 contributions that collectively explore diverse aspects of the bulk properties of hot QCD matter. The topics encompass the dynamics of electromagnetic fields, transport properties, hadronic matter, spin hydrodynamics, and the role of conserved charges in high-energy environments. These studies significantly enhance our understanding of the complex dynamics of hot QCD matter, the quark-gluon plasma (QGP) formed in high-energy nuclear collisions. Advances in theoretical frameworks, including hydrodynamics, spin dynamics, and fluctuation studies, aim to improve theoretical calculations and refine our knowledge of the thermodynamic properties of strongly interacting matter. Experimental efforts, such as those conducted by the ALICE and STAR collaborations, play a vital role in validating these theoretical predictions and deepening our insight into the QCD phase diagram, collectivity in small systems, and the early-stage behavior of strongly interacting matter. Combining theoretical models with experimental observations offers a comprehensive understanding of the extreme conditions encountered in relativistic heavy-ion and proton-proton collisions.

nucl-th↗

arXiv2024

Right On Time: Ultrafast Charge Separation Before Hybrid Exciton Formation

Organic/inorganic hybrid systems offer great potential for novel solar cell design combining the tunability of organic chromophore absorption properties with high charge carrier mobilities of inorganic semiconductors. However, often such material combinations do not show the expected performance: while ZnO, for example, basically exhibits all necessary properties for a successful application in light-harvesting, it was clearly outpaced by TiO$_2$ in terms of charge separation efficiency. The origin of this deficiency has long been debated. This study employs femtosecond time-resolved photoelectron spectroscopy and many-body ab initio calculations to identify and quantify all elementary steps leading to the suppression of charge separation at an exemplary organic/ZnO interface. We demonstrate that charge separation indeed occurs efficiently on ultrafast (350 fs) timescales, but that electrons are recaptured at the interface on a 100 ps timescale and subsequently trapped in a strongly bound (0.7 eV) hybrid exciton state with a lifetime exceeding 5 $μ$s. Thus, initially successful charge separation is followed by delayed electron capture at the interface, leading to apparently low charge separation efficiencies. This finding provides a sufficiently large timeframe for counter-measures in device design to successfully implement specifically ZnO and, moreover, invites material scientists to revisit charge separation in various kinds of previously discarded hybrid systems.

cond-mat.mtrl-sci↗

arXiv2021

Ultrafast generation and decay of a surface metal

Band bending at semiconductor surfaces induced by chemical doping or electric fields can create metallic surfaces with properties not found in the bulk, such as high electron mobility, magnetism or superconductivity. Optical generation of such metallic surfaces via BB on ultrafast timescales would facilitate a drastic manipulation of the conduction, magnetic and optical properties of semiconductors for high-speed electronics. Here, we demonstrate the ultrafast generation of a metal at the (10-10) surface of ZnO upon photoexcitation. Compared to hitherto known ultrafast photoinduced semiconductor-to-metal transitions that occur in the bulk of inorganic semiconductors, the metallization of the ZnO surface is launched by 3-4 orders of magnitude lower photon fluxes. Using time- and angle-resolved photoelectron spectroscopy, we show that the phase transition is caused by photoinduced downward surface band bending due to photodepletion of donor-type deep surface defects. At low photon flux, surface-confined excitons are formed. Above a critical exciton density, a Mott transition occurs, leading to a partially filled metallic band below the equilibrium Fermi energy. This process is in analogy to chemical doping of semiconductor surfaces. The discovered mechanism is not material-specific and presents a general route for controlling metallicity confined to semiconductor interfaces on ultrafast timescales.

cond-mat.mtrl-sci↗

arXiv2020

Distinguishing strain, charge and molecular orbital induced effects on the electronic structure: graphene/ammonia system

Molecular adsorption at the surface of a 2D material poses numerous questions regarding the modification to the band structure and interfacial states, which of course deserve full attention. In line with this, first-principle density functional theory is employed on the graphene/ammonia system. We identify the effects on the band structure due to strain, charge transfer and presence of molecular orbitals (MOs) of NH3 for six adsorption configurations. The induced strain upon ammonia-adsorption opens the bandgap (Eg) of graphene due to the breaking of translational symmetry and shifts the equilibrium Fermi energy (EF). The Eg and EF values and charge density distribution are dependent on the adsorption configuration, where the MO structure of NH3 plays a crucial role. The presence of MOs of N or H -originated pushes the unoccupied states of graphene towards EF. NH3 forms an interfacial occupied state originating from N2p below the EF within 1.6 - 2.2 eV for all configurations. These findings enhance the fundamental understanding of the graphene/NH3 system.

cond-mat.mtrl-sci↗

arXiv2019

Photoexcited organic molecules $en~route$ to highly efficient autoionization

The conversion of optical and electrical energy in novel materials is key to modern optoelectronic and light-harvesting applications. Here, we investigate the equilibration dynamics of photoexcited 2,7-bis(biphenyl-4-yl)-2,7-ditertbutyl-9,9-spirobiuorene (SP6) molecules adsorbed on ZnO(10-10) using femtosecond time-resolved two-photon photoelectron (2PPE) and optical spectroscopy. We find that, after initial ultrafast relaxation on fs and ps timescales, an optically dark state is populated, likely the SP6 triplet (T) state, that undergoes Dexter-type energy transfer ($r_{\mathrm{Dex}} = 1.3~\mathrm{nm}$) and exhibits a long decay time of 0.1 s. Because of this long lifetime a photostationary state with average T-T distances below 2 nm is established at excitation densities in the $10^{20}~\mathrm{cm}^{-2}~\mathrm{s}^{-1}$ range. This large density enables decay by T-T annihilation (TTA) mediating autoionization despite an extremely low TTA rate of $k_{\mathrm{TTA}} = 4.5~10^{-26}~\mathrm{m}^3~\mathrm{s}^{-1}$. The large external quantum efficiency of the autoionization process (up to 15 %) and photocurrent densities in the \mathrm{mA}~\mathrm{cm}^{-2}$ range offer great potential for light-harvesting applications.

cond-mat.mtrl-sci↗

arXiv2019

Revealing the Competing Contributions of Charge Carriers, Excitons, and Defects to the Non-Equilibrium Optical Properties of ZnO

Due to its wide band gap and high carrier mobility, ZnO is an attractive material for light-harvesting and optoelectronic applications. Its functional efficiency, however, is strongly affected by defect-related in-gap states that open up extrinsic decay channels and modify relaxation timescales. As a consequence, almost every ZnO sample behaves differently, leading to irreproducible or even contradicting observations. Here, a complementary set of time-resolved spectroscopies is applied to two ZnO samples of different defect density to disentangle the competing contributions of charge carriers, excitons, and defects to the non-equilibrium dynamics after photoexcitation: Time-resolved photoluminescence, excited state transmission, and electronic sum frequency generation. Remarkably, defects affect the transient optical properties of ZnO across more than eight orders of magnitude in time, starting with photodepletion of normally occupied defect states on femtosecond timescales, followed by the competition of free exciton emission and exciton trapping at defect sites within picoseconds, photoluminescence of defect-bound and free excitons on nanosecond timescales, and deeply trapped holes with microsecond lifetimes. These findings do not only provide the first comprehensive picture of charge and exciton relaxation pathways in ZnO, but also uncover the microscopic origin of previous conflicting observations in this challenging material and thereby offer means of overcoming its difficulties.

cond-mat.mtrl-sci↗

arXiv2014

Comment on 'Semiconductor nanocrystals: structure, properties, and band gap engineering' [Acc. Chem. Res. Vol 43 (2) pp 190, (2010)]

Surface traps and associated emission in quantum dots (QDs) sought a lot of research attention because of the fundamental interests apart from their influence on the emission characteristics. In ref [Acc. Chem. Res. Vol 43 (2) pp 190, 2010] the hole traps (h-traps) are depicted close to the conduction band (CB) for CdSe QDs while discussing the emission mechanism. However, notably electron traps (e-traps) are close to CB and h-traps are supposedly close to the valance band, especially in anion rich CdSe QDs. Although such emission (so called deep-trap) is a well known phenomenon, the energetic locations of these traps and the associated discrepancy require further attention. Hence the distinction between e, h-traps and their energetic location within the band gap is addressed in a general context, which is essentially a revisit to the surface states. Finally this general description is put to the context of surface states of CdSe QDs.

cond-mat.mtrl-sci↗