arXiv subjects

Sahinur Reja

Publications and source records attributed to Sahinur Reja.

16 recordsLinked to original sources

arXiv2025

Competing magnetic and topological orders in the spin-1 Kitaev-Heisenberg chain with single-ion anisotropy

We investigate the ground-state phase diagram of the spin-1 Kitaev--Heisenberg chain in the presence of uniaxial single-ion anisotropy (SIA) $D_z$ by density-matrix renormalization group (DMRG) calculations. By combining energy-curvature diagnostics on periodic $N=24$ clusters with a refined characterization based on order parameters and correlation functions for open chains up to $N=144$, we establish a comprehensive phase diagram in the $\phi$--$D_z$ plane. We identify four magnetically ordered phases -- FM-$z$, FM-$xy$, N\'eel-$z$, and a two-sublattice collinear LLRR2 state -- as well as magnetically disordered/critical regimes including N\'eel-$xy$, LLRR1, and two Kitaev spin-liquid (KSL) regions. A topological Haldane phase also emerges near the Heisenberg limit. Our results provide evidence that both AFM- and FM-KSL regimes acquire finite parameter widths in the spin-1 model, while the Haldane phase is fragile against Kitaev-type anisotropy, particularly for $D_z<0$. Increasing (decreasing) $D_z$ suppresses (enhances) magnetic order and expands (shrinks) the KSL and other magnetically disordered sectors. Also, at $D_z=0$, we identify an exactly solvable point at $\phi=\tan^{-1}(-2)$, which enforces a first-order transition between N\'eel-$z$ and LLRR2. We further contrast these findings with the spin-$1/2$ KH chain and with the spin-1 honeycomb KH model, highlighting the distinct roles of dimensionality and SIA in Kitaev-type magnets.

cond-mat.str-el↗

arXiv2025

Exotic coupled spin-charge states in decorated honeycomb magnets: A hybrid-Monte Carlo study

We uncover four exotic coupled spin-charge ground states in the strong coupling limit of the Kondo lattice model at various electronic fillings on a frustrated decorated honeycomb lattice, where each regular honeycomb sublattice point is occupied by three-site triangular units. We employ a hybrid Markov Chain Monte Carlo (hMCMC) simulation method which combines classical MCMC for localized spins and exact diagonalization of the electronic Hamiltonian. Two of the spin-charge ground states, respectively consists of three-site and six-site ferromagnetic (FM) clusters arranged in anti-FM and $120^{\circ}$ Yafet-Kittel (YK) phase which we label as S-AF (super-antiferromagnet) and S-YK (super-YK) respectively. Two even more interesting coupled spin-charge states, respectively accommodate FM dimers and trimers (as three-site line segment), which we label as FM-D and FM-T. In both cases, the anti-FM aligned dimers and trimers in respective phases, are arranged in stripes along one of three lattice directions: the spontaneously symmetry broken phases giving rise to non-trivial macroscopic degeneracy. These underlying magnetic textures (except S-YK state) restrict electrons in fragmented small regions (e.g, triangular units, two-site dimers, three-site line segments respectively in S-AF, FM-D and FM-T), resulting in flat bands by opening large gaps in electronic density of states, which in turn stabilize these coupled spin-charge states: a "band effect". These exotic spin-charge ground states could be relevant to electron-doped spin-systems resulting from various metal-organic frameworks (MOFs), which have attracted significant attention to condensed matter physics

cond-mat.str-el↗

arXiv2024

Topological Phases in Half-Integer Higher Spin $J_1$-$J_2$ Heisenberg Chains

We study the ground state properties of antiferromagnetic $J_1$-$J_2$ chains with half-integer spins ranging from $S=\frac{3}{2}$ to $S=\frac{11}{2}$ using the density-matrix renormalization group method. We map out the ground-state phase diagrams as a function of $\frac{J_2}{J_1}$ containing topological phases with alternating $\frac{2S-1}{2}$ and $\frac{2S+1}{2}$ valence bonds. We identify these topological phases and their boundaries by calculating the string order parameter, the dimer order parameter, and the spin gap for those high-$S$ systems in thermodynamic limit (finite size scaling). We find that these topological regions narrow down inversely with $S$ and converge to a single point at $\frac{J_2}{J_1}=\frac{1}{4}$ in the classical limit -- a critical threshold between commensurate and incommensurate orders. In addition, we extend the discussion of the Majumder-Ghosh state, previously noted only for $S=\frac{1}{2}$, and speculate its possible presence as a ground state in half-integer high spin systems over a substantial range of $\frac{J_2}{J_1}$ values.

cond-mat.str-el↗

arXiv2020

Skyrmion crystal phases in antiferromagnetic itinerant triangular magnets

Very often the skyrmions form a triangular crystal in chiral magnets. Here we study the effect of itinerant electrons on the structure of skyrmion crystal (SkX) on triangular lattice using Kondo lattice model in the large coupling limit and treating the localized spins as classical vectors. To simulate the system, we employ hybrid Markov Chain Monte Carlo method (hMCMC) which includes electron diagonalization in each MCMC update for classical spins. We present the low temperature results for $12\times 12$ system at electron density $n=1/3$ which show a sudden jump in skyrmion number when we increase the hopping strength of the itinerant electrons. We find that this high skyrmion number SkX phase is stabilized by combined effects: lowering of density of states at electron filling $n=1/3$ and also pushing the bottom energy states further down. We show that these results hold for larger system using travelling cluster variation of hMCMC. We expect that itinerant triangular magnets might exhibit the possible transition between low density to high density SkX phases by applying external pressure.

cond-mat.str-el↗

arXiv2019

Critical spin-$\frac{1}{2}$ tetramer compound CuInVO$_5$: Exploring the vicinity of two multimerized singlet states

Using the density-matrix renormalization group technique, we study a one-dimensional spin-$\frac{1}{2}$ Heisenberg chain consisting of coupled tetramers as an effective spin model for copper vanadate CuInVO$_5$. We obtain the ground-state phase diagram as a function of intra-tetramer and inter-tetramer exchange interactions, exhibiting two multimerized valence-bond-solid (VBS) phases : one is characterized by the formation of tetramer-singlet units; the other by the formation of dimer-singlet pairs. We show that the finite spin gaps in both the VBS phases smoothly vanish at the phase boundary: a second order phase transition defining a quantum critical point (QCP). The phase boundary is also captured by the fact that the central charge is unity at the phase boundary and zero otherwise in the thermodynamic limit. We further demonstrate that the experimental magnetization curve (which starts increasing with zero or tiny field) can be reasonably explained only by assuming the exchange parameters of CuInVO$_5$ to be very close to the phase boundary. Thus, we argue that CuInVO$_5$ may be a first example material which at ambient pressure stands near a QCP between two VBS phases. By varying the balance of exchange interactions with pressure, a transition from Néel to either of the VBS phases could be observed.

cond-mat.str-el↗

arXiv2019

Triplet superconductivity in coupled odd-gon rings

Shedding light on the nature of spin-triplet superconductivity has been a long-standing quest of solid-state physics since the discovery of superfluidity in liquid $^3$He. Nevertheless, the mechanism of spin-triplet pairing is much less understood than that of spin-singlet pairing explained by the Bardeen-Cooper-Schrieffer theory or even observed in high-temperature superconductors. Here we propose a versatile mechanism for spin-triplet superconductivity, which is mediated through a melting of macroscopic spin polarization in weakly coupled odd-gon-unit system (e.g., triangular unit, pentagon unit, etc). We demonstrate the application of this mechanism by considering a new class of quasi-one-dimensional superconductors A$_2$Cr$_3$As$_3$ (A=K, Rb, and Cs). Furthermore, we derive a simple effective Hamiltonian to easily illustrate the adaptability of the mechanism to general coupled odd-gon-unit systems. We thus argue that materials consisting of odd-numbered geometric units would be a prospect of spin-triplet superconductivity.

cond-mat.str-el↗

arXiv2018

Spin Stiffness and Domain Walls in Dirac-Electron Mediated Magnets

Spin interactions of magnetic impurities mediated by conduction electrons is one of the most interesting and potentially useful routes to ferromagnetism in condensed matter. In recent years such systems have received renewed attention due to the advent of materials in which Dirac electrons are the mediating particles, with prominent examples being graphene and topological insulator surfaces. In this paper, we demonstrate that such systems can host a remarkable variety of behaviors, in many cases controlled only by the density of electrons in the system. Uniquely characteristic of these systems is an emergent long-range form of the spin stiffnes when the Fermi energy resides at a Dirac point, becoming truly long-range as the magnetization density becomes very small. It is demonstrated that this leads to screened Coulomb-like interactions among domain walls, via a subtle mechanism in which the topology of the Dirac electrons plays a key role: the combination of attraction due to bound in-gap states that the topology necessitates, and repulsion due to scattering phase shifts, yields logarithmic interactions over a range of length scales. We present detailed results for domain walls in a particularly rich system, the (111) surface of a model topological crystalline insulator. This hosts two-fold and six- fold degenerate groundstates, with either short-range or emergent long-range interactions among the spins. In the latter case we demonstrate in detail the presence of in-gap states associated with domain walls, and argue that this stabilizes a pseudogap regime at finite temperature. Thus the topological nature of these systems, through its impact on domain wall excitations, leads to unique behaviors distinguishing them markedly from their non-topological analogs.

cond-mat.str-el↗

arXiv2018

Phase diagram of Hubbard-Holstein model on 4-leg tube system at quarter-filling

We derive an effective electronic Hamiltonian for square lattice Hubbard-Holstein model (HHM) in the strong electron-electron (e-e) and electron-phonon (e-ph) coupling regime and under non-adiabatic conditions ($t/ω_0 \leq 1$), $t$ and $ω_0$ being the electron hopping and phonon frequency respectively. Using Density Matrix Renormalization Group method, we simulate this effective electronic model on $4-$Leg cylinder system at quarter-filling and present a phase diagram in $g-U$ plane where $g$ and $U$ are being the e-ph coupling constant and Hubbard on-site interaction respectively. For larger $g$, we find cluster of spins i.e. phase separation (PS) gives way to a charge density wave (CDW) phase made of NN singlets which abruptly goes to another CDW phase as we increase $U$. But for smaller $g$, we find a metallic phase sandwiched between PS and singlet CDW phase. This phase is characterized by vanishing charge gap but finite spin gap -- suggesting a singlet superconducting phase.

cond-mat.str-el↗

arXiv2017

Surface Magnetism in Topological Crystalline Insulators

We study topological crystalline insulators doped with magnetic impurities, in which ferromagnetism at the surface lowers the electronic energy by spontaneous breaking of a crystalline symmetry. The number of energetically equivalent ground states is sensitive to the crystalline symmetry of the surface, as well as the precise density of electrons at the surface. We show that for a SnTe model in the topological state, magnetic states can have twofold symmetry, sixfold symmetry, or eightfold degenerate minima. We compute spin stiffnesses within the model to demonstrate the stability of ferromagnetic states, and consider their ramifications for thermal disordering. Possible experimental consequences of the surface magnetism are discussed.

cond-mat.str-el↗

arXiv2017

From Frustrated to Unfrustrated: Coupling two triangular-lattice itinerant Quantum Magnets

Motivated by systems that can be seen as composed of two frustrated sublattices combined into a less frustrated total lattice, we study the double-exchange model with nearest-neighbor (NN) and next--nearest-neighbor (NNN) couplings on the honeycomb lattice. When adding NN hopping and its resulting double exchange to the antiferromagnetic (AFM) Heisenberg coupling, the resulting phase diagram is quite different from that of purely Heisenberg-like magnetic models and strongly depends on electron filling. For half filling, patterns of AFM dimers dominate, where the effective electronic bands remain graphene-like with Dirac cones in all phases, from the FM to the $120^\circ$ limit. When the density of states at the Fermi level is sizable, we find non-coplanar incommensurate states as well as a small-vortex phase. Finally, a non-coplanar commensurate pattern realizes a Chern insulator at quarter filling. In the case of both NN and NNN hopping, the noncoplanar spin pattern inducing Chern insulators in triangular lattices is found to be quite stable under coupling into a honeycomb system. The resulting total phases are topologically nontrivial and either a Chern insulator with $C=2$ or a magnetic topological crystalline insulator protected by a combination or mirror-reflection and time-reversal symmetries arise.

cond-mat.str-el↗

arXiv2015

Strongly enhanced superconductivity in coupled t-J segments

The $t\!\!-\!\!J$ Hamiltonian is one of the cornerstones in the theoretical study of strongly correlated copper-oxide based materials. Using the density matrix renormalization group method we calculate the phase diagram of the one-dimensional (1D) $t\!\!-\!\!J$ chain in the presence of a periodic hopping modulation, as a prototype of coupled-segment models. While in the uniform 1D $t\!\!-\!\!J$ model near half-filling superconducting (SC) state dominates only at unphysically large values of the exchange coupling constant $J/t>3$, we show that a small hopping and exchange modulation very strongly reduces the critical coupling to be as low as $J/t\sim1/3$ -- well within the physical regime. The phase diagram as a function of the electron filling also exhibits metallic, insulating line phases and regions of phase separation. We suggest that a SC state is easily stabilized if $t\!\!-\!\!J$ segments creating local spin-singlet pairing are coupled to each other -- another example is ladder system.

cond-mat.supr-con↗

arXiv2015

Electronic route to stabilize nanoscale spin textures in itinerant frustrated magnets

We unveil novel spin textures in an itinerant fermion model on a frustrated triangular lattice in the limit of low electronic density. Using hybrid Monte Carlo simulations on finite clusters we identify two type of nanoscale spin textures in the background of 120 degree order : (i) a planar ferromagnetic cluster, and (ii) and a non-coplanar cluster with spins oriented perpendicular to the 120 degree plane. Both these textures lead to localization of the electronic wavefunctions and are in-turn stabilized by the concomitant charge modulations. The non-coplanar spin texture is accompanied by an unusual scalar chirality pattern. A well defined electric charge and magnetic moment associated with these textures allow for their easy manipulation by external electric and magnetic fields - a desirable feature for data storage. We identify a localization-delocalization behavior for electronic wavefunctions which is unique to frustrated magnets, and propose a general framework for stabilizing similar spin textures in spin-charge coupled systems.

cond-mat.str-el↗

arXiv2014

Coupled spin-charge order in frustrated itinerant triangular magnets

We uncover four new spin-charge ordered ground states in the strong coupling limit of the Kondo lattice model on triangular geometry. Two of the states at one-third electronic filling ($n=1/3$) consist of decorated ferromagnetic chains coupled antiferromagnetically with the neighboring chains. The third magnetic ground state is noncollinear, consisting of antiferromagnetic chains separated by a pair of canted ferromagnetic chains. An even more unusual magnetic ground state, a variant of the $120^{\circ}$ Yafet-Kittel phase, is discovered at $n=2/3$. These magnetic orders are stabilized by opening a gap in the electronic spectrum: a "band effect". All the phases support modulations in the electronic charge density due to the presence of magnetically inequivalent sites. In particular, the charge ordering pattern found at $n=2/3$ is observed in various triangular lattice systems, such as, 2H-AgNiO$_2$, 3R-AgNiO$_2$ and Na$_x$CoO$_2$.

cond-mat.str-el↗

arXiv2014

Polariton condensation with saturable molecules dressed by vibrational modes

Polaritons, mixed light-matter quasiparticles, undergo a transition to a condensed, macroscopically coherent state at low temperatures or high densities. Recent experiments show that coupling light to organic molecules inside a microcavity allows condensation at room temperature. The molecules act as saturable absorbers with transitions dressed by molecular vibrational modes. Motivated by this we calculate the phase diagram and spectrum of a modified Tavis-Cummings model, describing vibrationally dressed two-level systems, coupled to a cavity mode. Coupling to vibrational modes can induce re-entrance, i.e. a normal-condensed-normal sequence with decreasing temperature and can drive the transition first order.

cond-mat.quant-gas↗

arXiv2012

Correlated singlet phase in the one-dimensional Hubbard-Holstein model

We show that a nearest-neighbor singlet phase results (from an effective Hamiltonian) for the one-dimensional Hubbard-Holstein model in the regime of strong electron-electron and electron-phonon interactions and under non-adiabatic conditions ($t/ω_0 \leq 1$). By mapping the system of nearest-neighbor singlets at a filling $N_p/N$ onto a hard-core-boson (HCB) $t$-$V$ model at a filling $N_p/(N-N_p)$, we demonstrate explicitly that superfluidity and charge-density-wave (CDW) occur mutually exclusively with the diagonal long range order manifesting itself only at one-third filling. Furthermore, we also show that the Bose-Einstein condensate (BEC) occupation number $n_0$ for the singlet phase, similar to the $n_0$ for a HCB tight binding model, scales as $\sqrt N$; however, the coefficient of $\sqrt N$ in the $n_0$ for the interacting singlet phase is numerically demonstrated to be smaller.

cond-mat.str-el↗

arXiv2011

Phase diagram of one-dimensional Hubbard-Holstein model at quarter-filling

We derive an effective Hamiltonian for the one-dimensional Hubbard-Holstein model, valid in a regime of both strong electron-electron (e-e) and electron-phonon (e-ph) interactions and in the non-adiabatic limit ($t/ω_0 \leq 1$), by using a non-perturbative approach. We obtain the phase diagram at quarter-filling by employing a modified Lanczos method and studying various density-density correlations. The spin-spin AF (antiferromagnetic) interactions and nearest-neighbor repulsion, resulting from the e-e and the e-ph interactions respectively, are the dominant terms (compared to hopping) and compete to determine the various correlated phases. As e-e interaction $(U/t)$ is increased, the system transits from an AF cluster to a correlated singlet phase through a discontinuous transition at all strong e-ph couplings $2 \leq g \leq 3$ considered. At higher values of $U/t$ and moderately strong e-ph interactions ($2 \leq g \leq 2.6$), the singlets break up to form an AF order and then to a paramagnetic order all in a single sublattice; whereas at larger values of $g$ ($> 2.6$), the system jumps directly to the spin disordered charge-density-wave (CDW) phase.

cond-mat.str-el↗