arXiv subjects

Manoranjan Kumar

Publications and source records attributed to Manoranjan Kumar.

At least 55 records · Page 3Linked to original sources

arXiv2021

Quantum phases of ferromagnetically coupled dimers on Shastry-Sutherland lattice

The ground state (gs) of antiferromagnetically coupled dimers on the Shastry-Sutherland lattice (SSL) stabilizes many exotic phases and has been extensively studied. The gs properties of ferromagnetically coupled dimers on SSL are equally important but unexplored. In this model the exchange coupling along the $x$-axis ($J_x$) and $y$-axis ($J_y$) are ferromagnetic and the diagonal exchange coupling ($J$) is antiferromagnetic. In this work we explore the quantum phase diagram of ferromagnetically coupled dimer model numerically using density matrix renormalization group (DMRG) method. We note that in $J_x$-$J_y$ parameter space this model exhibits six interesting phases:(I) stripe $(0,π)$, (II) stripe $(π,0)$, (III) perfect dimer, (IV) $X$-spiral, (V) $Y$-spiral and (VI) ferromagnetic phase. Phase boundaries of these quantum phases are determined using the correlation functions and gs energies. We also notice the correlation length in this system is less than four lattice units in most of the parameter regimes. The non-collinear behaviour in $X$-spiral and $Y$-spiral phase and the dependence of pitch angles on model parameters are also studied.

cond-mat.str-el↗

arXiv2021

Anomalous Hall effect from gapped nodal line in Co2FeGe Heusler compound

Full Heusler compounds with Cobalt as a primary element show anomalous transport properties owing to the Weyl fermions and broken time-reversal symmetry. We present here the study of anomalous Hall effect (AHE) in Co2FeGe Heusler compound. The experiment reveals anomalous Hall conductivity (AHC) 100 S/cm at room temperature with an intrinsic contribution of 78 S/cm . The analysis of anomalous Hall resistivity suggests the scattering independent intrinsic mechanism dominates the overall behaviour of anomalous Hall resistivity. The first principles calculation reveals that the Berry curvature originated by gapped nodal line near EF is the main source of AHE in Co2FeGe Heusler compound. The theoretically calculated AHC is in agreement with the experiment.

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

Fulde-Ferrel-Larkin-Ovchinnikov phase in one dimensional Fermi gas with attractive interactions and transverse spin-orbit coupling

We examine the existence and characteristics of the exotic Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase in a one-dimensional Fermi gas with attractive Hubbard interactions, in the presence of spin-orbit coupling (SOC) and Zeeman field. We show that a robust FFLO phase can be created in the presence of attractive on-site interactions and Zeeman field, and that the addition of SOC suppresses the FFLO order and enhances the pair formation. In absence of SOC, the system shows four phases: Bardeen-Cooper-Schrieffer (BCS), FFLO, multi- mode pairing and fully polarized phases by tuning the Zeeman field h, and the quantum transition between these phases is discontinuous with respect to h. In the presence of SOC, the transition from the BCS to FFLO phase becomes continuous. We present a complete phase diagram of this model both in the presence and in the absence of SOC at quarter electron filling and also explore the effect of SOC on the FFLO phase.

cond-mat.quant-gas↗

arXiv2021

Low temperature thermodynamics of the antiferromagnetic $J_1-J_2$ model: Entropy, critical points and spin gap

The antiferromagnetic $J_1-J_2$ model is a spin-1/2 chain with isotropic exchange $J_1 > 0$ between first neighbors and $J_2 = αJ_1$ between second neighbors. The model supports both gapless quantum phases with nondegenerate ground states and gapped phases with $Δ(α) > 0$ and doubly degenerate ground states. Exact thermodynamics is limited to $α= 0$, the linear Heisenberg antiferromagnet (HAF). Exact diagonalization of small systems at frustration $α$ followed by density matrix renormalization group (DMRG) calculations returns the entropy density $S(T,α,N)$ and magnetic susceptibility $χ(T,α,N)$ of progressively larger systems up to $N = 96$ or 152 spins. Convergence to the thermodynamics limit, $S(T,α)$ or $χ(T,α)$, is demonstrated down to $T/J \sim 0.01$ in the sectors $α< 1$ and $α> 1$. $S(T,α)$ yields the critical points between gapless phases with $S^\prime(0,α) > 0$ and gapped phases with $S^\prime(0,α) = 0$. The $S^\prime(T,α)$ maximum at $T^*(α)$ is obtained directly in chains with large $Δ(α)$ and by extrapolation for small gaps. A phenomenological approximation for $S(T,α)$ down to $T = 0$ indicates power-law deviations $T^{-γ(α)}$ from $\exp(-Δ(α)/T)$ with exponent $γ(α)$ that increases with $α$. The $χ(T,α)$ analysis also yields power-law deviations, but with exponent $η(α)$ that decreases with $α$. $S(T,α)$ and the spin density $ρ(T,α) = 4Tχ(T,α)$ probe the thermal and magnetic fluctuations, respectively, of strongly correlated spin states. Gapless chains have constant $S(T,α)/ρ(T,α)$ for $T < 0.10$. Remarkably, the ratio decreases (increases) with $T$ in chains with large (small) $Δ(α)$.

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↗

arXiv2021

Topological transitions to Weyl states in bulk Bi$_2$Se$_3$: Effect of hydrostatic pressure and doping

Bi$_2$Se$_3$, a layered three dimensional (3D) material, exhibits topological insulating properties due to presence of surface states and a band gap of 0.3 eV in the bulk. We study the effect hydrostatic pressure $P$ and doping with rare earth elements on the topological aspect of this material in bulk from a first principles perspective. Our study shows that under a moderate pressure of P$>$7.9 GPa, the bulk electronic properties show a transition from an insulating to a Weyl semi-metal state due to band inversion. This electronic topological transition may be correlated to a structural change from a layered van der Waals material to a 3D system observed at $P$=7.9 GPa. At large $P$ density of states have significant value at the Fermi-energy. Intercalating Gd with a small doping fraction between Bi$_2$Se$_3$ layers drives the system to a metallic anti-ferromagnetic state, with Weyl nodes below the Fermi-energy. At the Weyl nodes time reversal symmetry is broken due to finite local field induced by large magnetic moments on Gd atoms. However, substituting Bi with Gd induces anti-ferromagnetic order with an increased direct band gap. Our study provides novel approaches to tune topological transitions, particularly in capturing the elusive Weyl semimetal states, in 3D topological materials.

cond-mat.mtrl-sci↗

arXiv2020

Ground state properties and exact thermodynamics of a 2-leg anisotropic spin ladder system

We study a frustrated two-leg spin ladder with alternate isotropic Heisenberg and Ising rung exchange interactions, whereas, interactions along legs and diagonals are Ising-type. All the interactions in the ladder are anti-ferromagnetic in nature and induce frustration in the system. This model shows four interesting quantum phases: (i) stripe rung ferromagnetic (SRFM), (ii) stripe rung ferromagnetic with edge singlet (SRFM-E), (iii) anisotropic antiferromagnetic (AAFM), and (iv) stripe leg ferromagnetic (SLFM) phase. We construct a quantum phase diagram for this model and show that in stripe rung ferromagnet (SRFM), the same type of sublattice spins (either $S$ or $σ$-type spins) are aligned in the same direction. Whereas, in anisotropic antiferromagnetic phase, both $S$ and $σ$-type of spins are anti-ferromagnetically aligned with each other, two nearest $S$ spins along the rung form an anisotropic singlet bond whereas two nearest $σ$ spins form an Ising bond. In large Heisenberg rung exchange interaction limit, spins on each leg are ferromagnetically aligned, but spins on different legs are anti-ferromagnetically aligned. The thermodynamic quantities like $Cv(T)$, $χ(T)$ and $S(T)$ are also calculated using the transfer matrix method for different phase. The magnetic gap in the SRFM and the SLFM can be notice from $χ(T)$ and $Cv(T)$ curves.

cond-mat.str-el↗

arXiv2020

Bond-bond correlations, gap relations and thermodynamics of spin-$1/2$ chains with spin-Peierls transitions and bond-order-wave phases

The spin-$1/2$ chain with antiferromagnetic exchange $J_1$ and $J_2 = αJ_1$ between first and second neighbors, respectively, has both gapless and gapped ($Δ(α) > 0$) quantum phases at frustration $0 \le α\le 3/4$. The ground state instability of regular ($δ= 0$) chains to dimerization ($δ> 0$) drives a spin-Peierls transition at $T_{SP}(α)$ that varies with $α$ in these strongly correlated systems. The thermodynamic limit of correlated states is obtained by exact treatment of short chains followed by density matrix renormalization calculations of progressively longer chains. The doubly degenerate ground states of the gapped regular phase are bond order waves (BOWs) with long-range bond-bond correlations and electronic dimerization $δ_e(α)$. The $T$ dependence of $δ_e(T,α)$ is found using four-spin correlation functions and contrasted to structural dimerization $δ(T,α)$ at $T \le T_{SP}(α)$. The relation between $T_{SP}(α)$ and the $T = 0$ gap $Δ(δ(0),α)$ varies with frustration in both gapless and gapped phases. The magnetic susceptibility $χ(T,α)$ at $T > T_{SP}$ can be used to identify physical realizations of spin-Peierls systems. The $α= 1/2$ chain illustrates the characteristic BOW features of a regular chain with a large singlet-triplet gap and electronic dimerization.

cond-mat.str-el↗

arXiv2020

Fermion parity gap and exponential ground state degeneracy of the one-dimensional Fermi gas with intrinsic attractive interaction

We examine the properties of a one-dimensional (1D) Fermi gas with attractive intrinsic (Hubbard) interactions in the presence of spin-orbit coupling and Zeeman field by numerically computing the pair binding energy, excitation gap, and susceptibility to local perturbations using the density matrix renormalization group. Such a system can, in principle, be realized in a system of ultracold atoms confined in a 1D optical lattice. We note that, in the presence of spatial interfaces introduced by a smooth parabolic potential, the pair binding and excitation energy of the system decays exponentially with the system size, pointing to the existence of an exponential ground state degeneracy, and is consistent with recent works. However, the susceptibility of the ground state degeneracy of this number-conserving system to local impurities indicates that the energy gap vanishes as a power law with the system size in the presence of local perturbations. We compare this system with the more familiar system of an Ising antiferromagnet in the presence of a transverse field realized with Rydberg atoms and argue that the exponential splitting in the clean number-conserving 1D Fermi system is similar to a phase with only conventional order.

cond-mat.str-el↗

arXiv2020

Tunneling density of states in a Y junction of Tomonaga-Luttinger liquid wires: A density matrix renormalization group study

It is well known that the pristine bulk of an interacting one-dimensional system in Tomonaga-Luttinger liquid (TLL) phase shows power law suppression of quasi-particle tunneling amplitude for all values of TLL parameter $g$, in the zero energy limit. We perform a density matrix renormalization group (DMRG) study of a fully symmetric Y junction of TLL wires and observe an anomalous enhancement of the tunneling density of states (TDOS) in the vicinity of the junction for both (a) interacting bosons case and (b) interacting fermions case, when $g>1$. We also observe suppression of TDOS for $g<1$ for both bosonic and fermionic cases. We find that the TDOS enhancements follow different power laws for bosonic and fermionic cases which suggests that these represent distinct fixed points, owing to statistical correlations which play an important role at the Y junction. Analysis of static conductance for the junction indicates that the fixed point for $1<g<3$ resembles the mysterious $M$ fixed point of Y junction predicted by Oshikawa, Chamon, and Affleck [J. Stat. Mech. P02008 (2006)]. We also show that the TDOS enhancement spans over a length scale of $\propto ω^{-1}$ from the junction, for $1<g<3$.

cond-mat.str-el↗

arXiv2020

Haldane and Dimer phases in a frustrated spin chain: an exact groundstate and associated topological phase transition

A Heisenberg spin-$s$ chain with alternating ferromagnetic ($-J_1^F<0$) and antiferromagnetic ($J_1^A>0$) nearest-neighbor (NN) interactions, exhibits the Dimer and spin-$2s$ Haldane phases in the limits $J_1^F/J_1^A \rightarrow 0$ and $J_1^F/J_1^A \rightarrow \infty$ respectively. These two phases are understood to be topologically equivalent. Induction of the frustration through the next nearest-neighbor ferromagnetic interaction ($-J_2^F<0$) produces a very rich quantum phase diagram. With frustration, the whole phase diagram is divided into a ferromagnetic (FM) and a nonmagnetic (NM) phase. For $s=1/2$, the full NM phase is seen to be of Haldane-Dimer type, but for $s>1/2$, a spiral phase comes between the FM and the Haldane-Dimer phases. The study of a suitably defined string-order parameter and spin-gap at the phase boundary indicates that the Haldane-Dimer and spiral phases have different topological characters. We also find that, along the $J_2^F=\frac 12 J_1^F$ line in the NM phase, an NN dimer state is the {\it exact} groundstate, provided $J_1^A>J_C=κJ_1^F$ where $κ\le s + h$ for applied magnetic field $h$. Without magnetic field, the position of $J_C$ is on the FM-NM phase boundary when $s=1/2$, but for $s>1/2$, the location of $J_C$ is on the phase separation line between the Haldane-Dimer and spiral phases.

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↗

arXiv2020

Speed inhomogeneity accelerates the information transfer in polar flock

A collection of self-propelled particles (SPPs) shows coherent motion and exhibits a true long range ordered (LRO) state in two dimensions. Various studies show that the presence of spatial inhomogeneities can destroy the usual long-range ordering in the system. However, the effects of inhomogeneity due to the intrinsic properties of the particles are barely addressed. In this paper we consider a collection of polar SPPs moving with inhomogeneous speed (IS) on a two dimensional substrate, which can arise due to varying physical strength of the individual particle. To our surprise, the IS not only preserves the usual long-range ordering present in the homogeneous speed models but also induces faster ordering in the system. Furthermore, The response of the flock to an external perturbation is also faster, compared to Vicsek like model systems, due to the frequent update of neighbors of each SPP in the presence of the IS. Therefore, our study shows that the IS can help in faster information transfer in the moving flock.

cond-mat.soft↗

arXiv2020

Modeling the spin-Peierls transition of spin-$1/2$ chains with correlated states: $J_1-J_2$ model, CuGeO$_3$ and TTF-CuS$_4$C$_4$(CF$_3$)$_4$

The spin-Peierls transition at $T_{SP}$ of spin-$1/2$ chains with isotropic exchange interactions has previously been modeled as correlated for $T > T_{SP}$ and mean field for $T < T_{SP}$. We use correlated states throughout in the $J_1-J_2$ model with antiferromagnetic exchange $J_1$ and $J_2 = αJ_1$ between first and second neighbors, respectively, and variable frustration $0 \leq α\leq 0.50$. The thermodynamic limit is reached at high $T$ by exact diagonalization of short chains and at low $T$ by density matrix renormalization group calculations of progressively longer chains. In contrast to mean field results, correlated states of 1D models with linear spin-phonon coupling and a harmonic adiabatic lattice provide an internally consistent description in which the parameter $T_{SP}$ yields both the stiffness and the lattice dimerization $δ(T)$. The relation between $T_{SP}$ and $Δ(δ,α)$, the $T = 0$ gap induced by dimerization, depends strongly on $α$ and deviates from the BCS gap relation that holds in uncorrelated spin chains. Correlated states account quantitatively for the magnetic susceptibility of TTF-CuS$_4$C$_4$(CF$_3$)$_4$ crystals ($J_1 = 79$ K, $α= 0$, $T_{SP} = 12$ K) and CuGeO$_3$ crystals ($J_1 = 160$ K, $α= 0.35$, $T_{SP} = 14$ K). The same parameters describe the specific heat anomaly of CuGeO$_3$ and inelastic neutron scattering. Modeling the spin-Peierls transition with correlated states exploits the fact that $δ(0)$ limits the range of spin correlations at $T = 0$ while $T > 0$ limits the range at $δ= 0$.

cond-mat.str-el↗

arXiv2020

Nonquenched rotators ease flocking and memorise it

We introduce a minimal model for a two-dimensional polar flock with nonquenched rotators, and show that the rotators make the usual macroscopic long-range order of the flock more robust than the clean system. The rotators memorise the flock-information which helps in establishing the robustness. Moreover, the memory of the rotators assists in probing the moving flock. We also formulate a hydrodynamic framework for the microscopic model that makes our study comprehensive. Using linearised hydrodynamics, it is shown that the presence of such nonquenched heterogeneities increases the sound speeds of the flock. The enhanced sound speeds lead to faster convection of information and consequently the robust ordering in the system. We argue that similar nonquenched heterogeneities may be useful in monitoring and controlling large crowds.

cond-mat.stat-mech↗

arXiv2019

Quantum phase diagram of a frustrated spin-1/2 system on a Trellis Ladder

We study an isotropic Heisenberg spin-1/2 model on a trellis ladder which is composed of two $J_1-J_2$ zigzag ladders interacting through anti-ferromagnetic rung couplings $J_3$. The $J_1$ and $J_2$ are ferromagnetic zigzag spin interaction between two legs and anti-ferromagnetic interaction along each leg of a zigzag ladder. A quantum phase diagram of this model is constructed using the density matrix renormalization group (DMRG) method and linearized spin wave analysis. In small $J_2$ limit a short range stripe collinear phase is found in the presence of $J_3$, whereas, in the large $J_2/J_3$ limit non-collinear quasi-long range phase is found. The system shows a short range non-collinear state in large $J_3$ limit. The short range order phase is the dominant feature of this phase diagram. We also show that the results obtained by DMRG and linearized spin wave analysis show similar phase boundary between stripe collinear and non-collinear short range phases, and the collinear phase region shrinks with increasing $J_3$. We apply this model to understand the magnetic properties of CaV$_2$O$_5$ and also fit the experimental data of susceptibility and magnetization. The variation of magnetic specific heat capacity as function of external magnetic field is also predicted. We note that $J_3$ is a dominant interaction in this system, whereas $J_1$ and $J_2$ are approximately half of $J_3$.

cond-mat.str-el↗

arXiv2019

Spin-Peierls transition of the dimer phase of the $J_1-J_2$ model: Energy cusp and CuGeO$_3$ thermodynamics

The spin-Peierls transition is modeled in the dimer phase of the spin-$1/2$ chain with exchanges $J_1$, $J_2 = αJ_1$ between first and second neighbors. The degenerate ground state generates an energy cusp that qualitatively changes the dimerization $δ(T)$ compared to Peierls systems with nondegenerate ground states. The parameters $J_1 = 160$ K, $α= 0.35$ plus a lattice stiffness account for the magnetic susceptibility of CuGeO$_3$, its specific heat anomaly, and the $T$ dependence of the lowest gap.

cond-mat.str-el↗