arXiv subjects

S. Ramasesha

Publications and source records attributed to S. Ramasesha.

At least 19 recordsLinked to original sources

arXiv2025

Electronic and magnetic ground states of {112} grain boundary in graphene in the extended Hubbard model

We study the ground state phase diagram of the extended Hubbard model in a half-filled 5/7 skewed ladder, which is topologically equivalent to a \{112\} grain boundary in graphene and related systems. Using the mean-field method, we identify various electronic and magnetic phases in the U-V plane, by calculating the site charge and spin densities. The electronic phases include partially charge-ordered metal or insulator, and fully charge-ordered insulator. The different magnetic phases of the model are non-magnet, spin density wave, spin split compensated ferrimagnet or partial antiferromagnet. Analysis of the electronic band structure reveals that the partially charge-ordered compensated ferrimagnetic phase exhibits spin polarisation, which can be quite interesting for spintronics applications. We also compute the polarisation as a function of $U$ using the Berry phase formalism and show that the system exhibits multiferroicity with coexisting compensated ferrimagnetic spin order alongside electronic polarisations.

cond-mat.str-el↗

arXiv2023

Quantum Phase Transitions in Skewed Ladder Systems

In this brief review, we introduce a new spin ladder system called skewed spin ladders and discuss the exotic quantum phases of this system. The spin ladders studied are the 5/7, 3/4 and 3/5 systems corresponding to alternately fused 5 and 7 membered rings; 3 and 4 membered rings; and 3 and 5 membered rings. These ladders show completely different behaviour as the Hamiltonian model parameter is changed. When the Hamiltonian parameter is increased the 5/7 ladder switches from an initial singlet ground state to progressively higher spin ground state and then to a reentrant singlet state before finally settling to the highest spin ground state whose spin equals the number of unit cells in the system. The 3/4 ladder goes from a singlet ground state to a high spin ground state with each unit cell contributing spin 1 to the state, as the model parameter is increased. The 3/5 ladder shows a singlet ground state for small parameters and high spin ground state for intermediate values of the parameter and for still higher parameters, a reentrant singlet ground state. They can also show interesting magnetization plateaus as illustrated by studies on a specific spin ladder.

cond-mat.str-el↗

arXiv2023

Quadrupolar Phases and Plateau States In Skewed Ladders

Two legged skewed spin-$\frac{1}{2}$ ladders are frustrated and exhibit exotic quantum phases in ground state due to strong quantum fluctuations and competing spin exchanges. Here, we study ground state properties of a spin-$\frac{1}{2}$ Heisenberg model on 3/4, 3/5 and 5/5 skewed ladders in the presence of a Zeeman magnetic field, $B$, using exact diagonalization and the density matrix renormalization group method. We note the existence of plateaus at $m =$ 1/3 and 2/3 for 3/4 skewed ladder, at $m =$ 1/4, 1/2, and 3/4 for 3/5 skewed ladder, and at $m =$ 0, 1/3, and 2/3 for 5/5 skewed ladder, where $m$ is the ratio of the observed magnetization ($M$) to the saturated magnetization (${M_\mathrm{max}}$). The plateau state is always a gapped state and the plateau width depends on the gap in the system. Surprisingly, the 3/4 and 5/5 skewed ladders show interesting quadrupolar or n-type spin nematic phases below the 1/3$^{rd}$ plateau, i.e, at very low magnetic fields. These two systems are unique as they host both a plateau and a quadrupolar phase at low magnetic fields. The linear variation of pitch angle of the spin with magnetization and behavior of binding energy of magnon pairs as function of magnetic field are also calculated in both the systems. We also study the contribution of the binding energy to two magnon condensate.

cond-mat.str-el↗

arXiv2022

Quantum phase transition in skewed ladders: an entanglement entropy and fidelity study

Entanglement entropy (EE) of a state is a measure of correlation or entanglement between two parts of a composite system and it may show appreciable change when the ground state (GS) undergoes a qualitative change in a quantum phase transition (QPT). Therefore, the EE has been extensively used to characterise the QPT in various correlated Hamiltonians. Similarly fidelity also shows sharp changes at a QPT. We characterized the QPT of frustrated antiferromagnetic Heisenberg spin-1/2 systems on 3/4, 3/5 and 5/7 skewed ladders using the EE and fidelity analysis. It is noted that all the non-magnetic to magnetic QPT boundary in these systems can be accurately determined using the EE and fidelity, and the EE exhibits a discontinuous change, whereas fidelity shows a sharp dip at the transition points. It is also noted that in case of the degenerate GS, the unsymmetrized calculations show wild fluctuations in the EE and fidelity even without actual phase transition, however, this problem is resolved by calculating the EE and the fidelity in the lowest energy state of the symmetry subspaces, to which the degenerate states belong.

cond-mat.str-el↗

arXiv2022

Magnetic structure and properties of a vanthoffite mineral Na6Mn(SO4)4

A detailed analysis of the magnetic properties of a vanthoffite type mineral Na6Mn(SO4)4 basedon dc magnetization, low temperature neutron powder diffraction and theoretical calculations is reported. The mineral crystallizes in a monoclinic system with space group P21/c, where MnO6 octahedra are linked via SO4 tetrahedra. This gives rise to super-exchange interaction between two Mn2+ ions mediated by two nonmagnetic bridging anions and leads to an antiferromagnetic ordering below 3 K. The magnetic structure derived from neutron powder diffraction at 1.7 K depicts an antiferromagnetic spin arrangement in the bc plane of the crystal. The magnetic properties are modelled by numerical calculations using exact diagonalization technique, which fits the experimental results and provides antiferromagnetic ground state of Na6Mn(SO4)4.

cond-mat.mtrl-sci↗

arXiv2021

Quantum phases of a frustrated spin-1 system: The 5/7 skewed ladder

The quantum phases in a spin-1 skewed ladder system formed by alternately fusing five- and seven-membered rings are studied numerically using the exact diagonalization technique up to 16 spins and using the density matrix renormalization group method for larger system sizes. The ladder has a fixed isotropic antiferromagnetic (AF) exchange interaction ($J_2 = 1$) between the nearest-neighbor spins along the legs and a varying isotropic AF exchange interaction ($J_1$) along the rungs. As a function of $J_1$, the system shows many interesting ground states (gs) which vary from different types of nonmagnetic and ferrimagnetic gs. The study of diverse gs properties such as spin gap, spin-spin correlations, spin density and bond order reveal that the system has four distinct phases, namely, the AF phase at small $J_1$; the ferrimagnetic phase with gs spin $S_G = n$ for $1.44 < J_1 < 4.74$ and with $S_G = 2n$ for $J_1 > 5.63$, where $n$ is the number of unit cells; and a reentrant nonmagnetic phase at $4.74 < J_1 < 5.44$. The system also shows the presence of spin current at specific $J_1$ values due to simultaneous breaking of both reflection and spin parity symmetries.

cond-mat.str-el↗

arXiv2021

Study of magnetization relaxation in molecular spin clusters using an innovative kinetic Monte Carlo method

Modeling blocking temperature in molecular magnets has been a long standing problem in the field of molecular magnetism. We investigate this problem using a kinetic Monte Carlo (kMC) approach on an assembly of 100,000 short molecular magnetic chains (SMMCs), each of six identical spins with nearest neighbour anisotropic ferromagnetic exchange interactions. Each spin is also anisotropic with an uniaxial anisotropy. The site spin on these SMMCs take values $1$, $3/2$ or $2$. Using eigenstates of these SMMCs as the states of Markov chain, we carry out a kMC simulation starting with an initial state in which all SMMCs are completely spin polarized and assembled on a one-dimensional lattice so as to experience ferromagnetic spin-dipolar interaction with each other. From these simulations we obtain the relaxation time $τ_r$ as a function of temperature and the associated blocking temperature. We study this for different exchange anisotropy, on-site anisotropy and strength of dipolar interactions. The magnetization relaxation times show non-Arrhenius behaviour for weak on-site interactions. The energy barrier to magnetization relaxation increases with increase in on-site anisotropy, exchange anisotropy and strength of spin dipolar interactions; more strongly on the last parameter. In all cases the barrier saturates at large on-site anisotropy. The barrier also increases with site spin. The large barrier observed in rare-earth single ion magnets can be attributed to large dipolar interactions due to short intermolecular distances, owing to their small size and large spin of the rare earth ion in the molecule.

cond-mat.str-el↗

arXiv2021

Quantum phases of spin-1 system on 3/4 and 3/5 skewed ladders

We study the quantum phase transitions of frustrated antiferromagnetic Heisenberg spin-1 systems on the 3/4 and 3/5 skewed two leg ladder geometries. These systems can be viewed as arising by periodically removing rung bonds from a zigzag ladder. We find that in large systems, the ground state (gs) of the 3/4 ladder switches from a singlet to a magnetic state for $J_1 \ge 1.82$; the gs spin corresponds to ferromagnetic alignment of effective $S = 2$ objects on each unit cell. The gs of antiferromagnetic exchange Heisenberg spin-1 system on a 3/5 skewed ladder is highly frustrated and has spiral spin arrangements. The amplitude of the spin density wave in the 3/5 ladder is significantly larger compared to that in the magnetic state of the 3/4 ladder. The gs of the system switches between singlet state and low spin magnetic states multiple times on tuning $J_1$ in a finite size system. The switching pattern is nonmonotonic as a function of $J_1$, and depends on the system size. It appears to be the consequence of higher $J_1$ favoring higher spin magnetic state and the finite system favoring a standing spin wave. For some specific parameter values, the magnetic gs in the 3/5 system is doubly degenerate in two different mirror symmetry subspaces. This degeneracy leads to spontaneous spin parity and mirror symmetry breaking giving rise to spin current in the gs of the system.

cond-mat.str-el↗

arXiv2020

Magnetocaloric effect in molecular spin clusters and their assemblies: model exact and Monte Carlo studies

We study the magnetocaloric effect (MCE) in spin clusters of six spins with site spins $s=1$, $3/2$ and $2$ for different exchange anisotropies. We also study MCE in an assembly of spin clusters, on a chain, a 2-D square lattice and 3-D cubic lattice with spin-dipolar interactions using a Monte Carlo method in spin-1 systems. We compute the magnetic Grüneisen parameter $Γ_H$, and study its dependence on exchange anisotropy and spin-dipolar interaction. With increase of exchange anisotropy the maxima in $Γ_H$, shifts to higher magnetic fields and becomes a sharp singularity. The first maximum in $Γ_H$ shifts to lower fields as we increase spin-dipolar interaction. The behaviour of $Γ_H$ on applied magnetic field is also reflected in the magnetic entropy of the systems. The first maximum in $Γ_H$ also shifts to lower magnetic field strength as the magnitude of the site spin increases. We also show the dependence of $Γ_H$ on dimensionality of the lattice for a fixed lattice constant.

cond-mat.str-el↗

arXiv2020

Magnetization plateaus of spin-$\mathbf{\frac{1}{2}}$ system on a 5/7 skewed ladder

Magnetization plateaus are some of the most striking manifestations of frustration in low-dimensional spin systems. We present numerical studies of magnetization plateaus in the fascinating spin-1/2 skewed ladder system obtained by alternately fusing five- and seven-membered rings. This system exhibits three significant plateaus at $m = 1/4$, $1/2$ and $3/4$, consistent with the Oshikawa-Yamanaka-Affleck condition. Our numerical as well as perturbative analysis shows that the ground state can be approximated by three weakly coupled singlet dimers and two free spins, in the absence of a magnetic field. With increasing applied magnetic field, the dimers progressively become triplets with large energy gaps to excited states, giving rise to stable magnetization plateaus. Finite-temperature studies show that $m=1/4$ and $1/2$ plateaus are robust and survive thermal fluctuations while the $m=3/4$ plateau shrinks rapidly due to thermal noise. The cusps at the ends of a plateau follow the algebraic square-root dependence on $B$.

cond-mat.str-el↗

arXiv2019

The Correlated Electronic States of a few Polycyclic Aromatic Hydrocarbons: A Computational Study

In recent years Polycyclic Aromatic Hydrocarbons (PAHs) have been studied for their electronic properties as they are viewed as nanodots of graphene. They have also been of interest as functional molecules for applications such as light emitting diodes and solar cells. Since last few years varying structural and chemical properties corresponding to the size and geometry of these molecules have been studied both theoretically and experimentally. In this paper, we carry out a systematic study of the electronic states of several PAHs using the Pariser-Parr-Pople model which incorporates long-range electron correlations. In all the molecules studied by us, we find that the 2A state is below the 1B state and hence none of them will be fluorescent in the gaseous phase. The singlet-triplet gap is more than one-half of the singlet-singlet gap in all cases and hence none of these PAHs can be candidates for improved solar cell efficiencies in a singlet fission. We discuss in detail the properties of the electronic states which include bond orders and spin densities (in triplets) of these systems.

cond-mat.str-el↗

arXiv2019

Correlated Electronic Properties of a Graphene Nanoflake: Coronene

We report studies of the correlated excited states of coronene and substituted coronene within the Pariser-Parr-Pople (PPP) correlated $π$-electron model employing symmetry adapted density matrix renormalization group technique. These polynuclear aromatic hydrocarbons can be considered as graphene nanoflakes. We review their electronic structures utilizing a new symmetry adaptation scheme that exploits electron-hole symmetry, spin-inversion symmetry and end-to-end interchange symmetry. Study of the electronic structures sheds light on the electron correlation effects in these finite-size graphene analogues, which diminishes on going from one-dimensional to higher-dimensional systems, yet is significant within these finite graphene derivatives.

physics.chem-ph↗

arXiv2018

A model Exact Study of the properties of low-lying electronic states of Perylene and Substituted Perylenes

There is a resurgence of interest in the electronic structure of perylene for its applications in molecular devices such as organic photovoltaics and organic light emitting diodes. In this study, we have obtained the low-lying singlet states of perylene by exactly solving the Parisar-Parr-Pople model Hamiltonian of this system with 20 sites and 20 electrons, in the VB basis where dimensionality is $\sim$ 5.92 billion. The triplet states of perylene are obtained using a DMRG scheme with symmetry adaptation. The one and two photon states are very close in energy $\sim$ 3.2 eV while the lowest triplet state is slightly below 1.6 eV indicating that perylene is a good candidate for singlet fission. To explore the tunability of the electronic states, we have studied donor-acceptor substituted perylenes. The two donors and two acceptors are substituted symmetrically either at the four bay sites or four peri sites. In all the bay substitution and one peri substitution at moderate D/A strength, the optical gap is lowered to about 2.8 eV. These molecules can be used as blue emitters. We have also reported bond orders in all the cases and perylene as well as substituted perylenes can be viewed as two weakly coupled naphthalene in the singlet states but in triplets these bonds tend to be comparable to other bonds in strength. The charge densities in substituted perylenes are mostly localized around the substitution sites in the ground state. The positive spin densities in triplets are concentrated around the peri and bay sites with the remaining sites having small spin densities of either sign.

cond-mat.str-el↗

arXiv2018

Modeling Molecular Magnets with Large Exchange and On-Site Anisotropies

Spins in molecular magnets can experience both anisotropic exchange interactions and on-site magnetic anisotropy. In this paper we study the effect of exchange anisotropy on the molecular magnetic anisotropy both with and without on-site anisotropy. When both the anisotropies are small, we find that the axial anisotropy parameter $D_M$ in the effective spin Hamiltonian is the sum of the individual contributions due to exchange and on-site anisotropies. We find that even for axial anisotropy of about $15\%$, the low energy spectrum does not correspond to a single parent spin manifold but has intruders states arising from other parent spin. In this case, the low energy spectrum can not be described by an effective Hamiltonian spanning the parent spin space. We study the magnetic susceptibility, specific heat as a function of temperature and magnetization as a function of applied field to characterize the system in this limit. We find that there is synergy between the two anisotropies, particularly for large systems with higher site spins.

cond-mat.mtrl-sci↗

arXiv2017

Exact Wave Packet Dynamics of Singlet Fission in Unsubstituted and Substituted Polyene Chains within Long-Range Interacting Models

Singlet fission (SF) is a potential pathway for significant enhancement of efficiency in organic solar cells (OSC). In this paper, we study singlet fission in a pair of polyene molecules in two different stacking arrangements employing exact many-body wave packet dynamics. In the non-interacting model, the SF yield is absent. The individual molecules are treated within Hubbard and Pariser-Parr-Pople (PPP) models and the interaction between them involves transfer terms, intersite electron repulsions and site-charge--bond-charge repulsion terms. Initial wave packet is constructed from excited singlet state of one molecule and ground state of the other. Time development of this wave packet under the influence of intermolecular interactions is followed within the Schrödinger picture by an efficient predictor-corrector scheme. In unsubstituted Hubbard and PPP chains, $2{}^1A$ excited singlet state leads to significant SF yield while the $1{}^1B$ state gives negligible fission yield. On substitution by donor-acceptor groups of moderate strength, the lowest excited state will have sufficient $2{}^1A$ character and hence results in significant SF yield. Because of rapid internal conversion, the nature of the lowest excited singlet will determine the SF contribution to OSC efficiency. Furthermore, we find the fission yield depends considerably on the stacking arrangement of the polyene molecules.

physics.chem-ph↗

arXiv2017

Quantum phases of frustrated 2-leg spin-1/2 ladders with skewed rungs

The quantum phases of 2-leg spin-1/2 ladders with skewed rungs are obtained using exact diagonalization of systems with up to 26 spins and by density matrix renormalization group calculations to 500 spins. The ladders have isotropic antiferromagnetic (AF) exchange $J_2 > 0$ between first neighbors in the legs, variable isotropic AF exchange $J_1$ between some first neighbors in different legs, and an unpaired spin per odd-membered ring when $J_1 \gg J_2$. Ladders with skewed rungs and variable $J_1$ have frustrated AF interactions leading to multiple quantum phases: AF at small $J_1$, either F or AF at large $J_1$, as well as bond-order-wave phases or reentrant AF (singlet) phases at intermediate $J_1$.

cond-mat.str-el↗

arXiv2017

Modeling Magnetic Anisotropy of Single Chain Magnets in $|d/J| \geq 1$ Regime

Single molecule magnets (SMMs) with single-ion anisotropies $\mathbf d$, comparable to exchange interactions J, between spins have recently been synthesized. In this paper, we provide theoretical insights into the magnetism of such systems. We study spin chains with site spins, s=1, 3/2 and 2 and on-site anisotropy $\mathbf d$ comparable to the exchange constants between the spins. We find that large $\mathbf d$ leads to crossing of the states with different $M_S$ values in the same spin manifold of the $\mathbf d = 0$ limit. For very large $\mathbf d$'s we also find that the $M_S$ states of the higher energy spin states descend below the $M_S$ states of the ground state spin manifold. Total spin in this limit is no longer conserved and describing the molecular anisotropy by the constants $D_M$ and $E_M$ is not possible. However, the total spin of the low-lying large $M_S$ states is very nearly an integer and using this spin value it is possible to construct an effective spin Hamiltonian and compute the molecular magnetic anisotropy constants $D_M$ and $E_M$. We report effect of finite sizes, rotations of site anisotropies and chain dimerization on the effective anisotropy of the spin chains.

cond-mat.str-el↗

arXiv2016

Density matrix renormalization group (DMRG) for cyclic and centrosymmetric linear chains

The density matrix renormalization group (DMRG) method generates the low-energy states of linear systems of $N$ sites with a few degrees of freedom at each site by starting with a small system and adding sites step by step while keeping constant the dimension of the truncated Hilbert space. DMRG algorithms are adapted to open chains with inversion symmetry at the central site, to cyclic chains and to weakly coupled chains. Physical properties rather than energy accuracy is the motivation. The algorithms are applied to the edge states of linear Heisenberg antiferromagnets with spin $S \ge 1$ and to the quantum phases of a frustrated spin-1/2 chain with exchange between first and second neighbors. The algorithms are found to be accurate for extended Hubbard and related 1D models with charge and spin degrees of freedom.

cond-mat.str-el↗