arXiv subjects

Sudhakar Yarlagadda

Publications and source records attributed to Sudhakar Yarlagadda.

At least 19 recordsLinked to original sources

arXiv2021

Experimental and theoretical demonstration of negative magnetization induced by particle size reduction in nano-form Gd$_{1-x}$Ca$_x$MnO$_3$

We report a pervasive phenomenon of gradual emergence of negative magnetization in typical $R_{1-x}D_x$MnO$_3$ orthorhombic perovskite manganites in nano form (with $R^{3+}$ being a trivalent high-magnetic-moment ion from the heavier rare-earth elements Gd, Tb, Dy and $D^{2+}$ refers to divalent alkaline elements Sr, Ca, etc.), while the bulk form manifests no negative magnetization. Extensive magnetization studies have been carried out in Gd$_{1-x}$Ca$_x$MnO$_3$ manganites around half doping. We demonstrate experimentally that particle size reduction in nano-form manganites enhances the propensity of the system to exhibit negative magnetization and provide a theoretical explanation for the phenomenon. To test the universality of our findings, we have extended the measurement to Dy$_{0.6}$Ca$_{0.4}$MnO$_3$ and obtained similar results.

cond-mat.str-el↗

arXiv2019

Mixed-order transition and tricritical point associated with checkerboard supersolidity in the two-dimensional $t_2-V_1$ model

We use Quantum Monte Carlo method employing stochastic-series-expansion technique to study the ground state properties of the $t_2-V_1$ model on a square lattice. We find that, away from half-fillings, the minimal combination of nearest-neighbor repulsion $V_1$ and next-nearest-neighbor hopping $t_2$ may give rise to checkerboard supersolidity. The nature of the quantum phase transition, where the superfluid changes to a checkerboard supersolid, depends on the relative strength of $V_1/t_2$ and the average site occupancy. Interestingly, the model exhibits a mixed-order transition near half filling; at a higher (lower) filling, tricriticality is witnessed followed by a second-order transition at densities even further away from half filling. Close to half filling, the model displays the extreme Thouless effect and transits from a superfluid to a checkerboard solid.

cond-mat.str-el↗

arXiv2018

Study of the ferromagnetic-insulator phase in manganites

Understanding the coexistence of ferromagnetism and insulating behavior in manganites is an unsolved problem. We propose a localized-band model involving effective intermediate-range electron-electron (electron-hole) repulsion (attraction) generated by cooperative electron-phonon interaction. Double exchange mechanism, involving holes virtually hopping to nearest neighbors and back, produces magnetic polarons in an antiferromagnetic environment; when these magnetic polarons coalesce and percolate the system, we get a ferromagnetic insulator. Ferromagnetism gets more pronounced when the holes (doping) increases or when the ratio hopping/polaronic-energy dominates over superexchange-coupling/hopping.

cond-mat.str-el↗

arXiv2017

Study of long-range orders of hard-core bosons coupled to cooperative normal modes in two-dimensional lattices

Understanding the microscopic mechanism of coexisting long-range orders (such as lattice supersolidity) in strongly correlated systems is a subject of immense interest. We study the possible manifestations of long-range orders, including lattice-supersolid phases with differently broken symmetry, in a two-dimensional square lattice system of hard-core bosons (HCBs) coupled to archetypal cooperative/coherent normal-mode distortions such as those in perovskites. At strong HCB-phonon coupling, using a duality transformation to map the strong-coupling problem to a weak-coupling one, we obtain an effective Hamiltonian involving nearest-neighbor, next-nearest-neighbor, and next-to-next-nearest-neighbor hoppings and repulsions. Using stochastic series expansion quantum Monte Carlo, we construct the phase diagram of the system. As coupling strength is increased, we find that the system undergoes a first-order quantum phase transition from a superfluid to a checkerboard solid at half filling and from a superfluid to a diagonal striped solid [with crystalline ordering wavevector $\vec{Q}=(2π/3,2π/3)$ or $(2π/3,4π/3)$] at one-third filling without showing any evidence of supersolidity. On tuning the system away from these commensurate fillings, checkerboard supersolid is generated near half filling whereas a rare diagonal striped supersolid is realized near one-third filling. Interestingly, there is an asymmetry in the extent of supersolidity about one-third filling. Within our framework, we also provide an explanation for the observed checkerboard and stripe formations in ${\rm La}_{2-x}{\rm Sr}_x{\rm NiO_4}$ at $x=1/2$ and $x=1/3$.

cond-mat.str-el↗

arXiv2017

Giant magnetoelectric effect in pure manganite-manganite heterostructures

Obtaining strong magnetoelectric couplings in bulk materials and heterostructures is an ongoing challenge. We demonstrate that manganite heterostructures of the form ${\rm (Insulator)/(LaMnO_3)_n/(CaMnO_3)_n/(Insulator)}$ show strong multiferroicity in magnetic manganites where ferroelectric polarization is realized by charges leaking from ${\rm LaMnO_3}$ to ${\rm CaMnO_3}$ due to repulsion. Here, an effective nearest-neighbor electron-electron (electron-hole) repulsion (attraction) is generated by cooperative electron-phonon interaction. Double exchange, when a particle virtually hops to its unoccupied neighboring site and back, produces magnetic polarons that polarize antiferromagnetic regions. Thus a striking giant magnetoelectric effect ensues when an external electrical field enhances the electron leakage across the interface.

cond-mat.str-el↗

arXiv2016

Giant magnetoelectric effect in pure manganite-manganite heterostructures

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arXiv2016

Correlation between battery material performance and cooperative electron-phonon interaction in LiCo$_y$Mn$_{2-y}$O$_{4}$

Understanding the basic physics related to archetypal lithium battery material (such as LiCo$_y$Mn$_{2-y}$O$_{4}$) is of considerable interest and is expected to aid designing of cathodes of high capacity. The relation between electrochemical performance, activated-transport parameters, thermal expansion, and cooperativity of electron-phonon-interaction distortions in LiCo$_y$Mn$_{2-y}$O$_{4}$ is investigated. The first order cooperative-normal-mode transition, detected through coefficient of thermal expansion, is found to disappear at a critical doping ($y \sim 0.16$); interestingly, for $y \gtrsim 0.16$ the resistivity does not change much with doping and the electrochemical capacity becomes constant over repeated cycling. The critical doping $y \sim 0.16$ results in breakdown of the network of cooperative/coherent normal-mode distortions; this leads to vanishing of the first-order transition, establishment of hopping channels with lower resistance, and enhancing lithiation and delithiation of the battery, thereby minimizing electrochemical capacity fading.

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arXiv2016

Charge and orbital order due to cooperative Jahn-Teller effect in manganite chains

We derive an effective Hamiltonian that takes into account the quantum nature of phonons and models cooperative Jahn-Teller effect in the adiabatic regime and at strong electron-phonon coupling in one dimension. Our approach involves mapping a strong-coupling problem to a weak-coupling one by using a duality transformation. Subsequently, a sixth-order perturbation theory is employed in the polaronic frame of reference where the small parameter is inversely (directly) proportional to the coupling (adiabaticity). We study charge and orbital order in ferromagnetic manganite chains and address the pronounced electron-hole asymmetry in the observed phase diagram. In particular, at strong coupling, we offer an explanation for the observed density dependence of the wavevector of charge modulation, i.e., wavevector is proportional to (independent of) electron density on the electron-doped (hole-doped) side of the phase diagram of manganites. We also provide a picture for the charge and orbital order at special fillings $ \frac{1}{2}$, $\frac{1}{3}$, $\frac{1}{4}$, and $ \frac{1}{5}$; while focusing on the ordering controversy at fillings $\frac{1}{3}$ and $\frac{1}{4}$, we find that Wigner-crystal arrangement is preferred over bi-stripe order.

cond-mat.str-el↗

arXiv2015

Coexistence of long-range orders in a Bose-Holstein model

Exploring supersolidity in naturally occurring and artificially designed systems has been and will continue to be an area of immense interest. Here, we study how superfluid and charge-density-wave (CDW) states cooperate or compete in a minimal model for hard-core-bosons (HCBs) coupled locally to optical phonons: a two-dimensional Bose-Holstein model. Our study is restricted to the parameter regimes of strong HCB-phonon coupling and non-adiabaticity. We use Quantum Monte Carlo simulation (involving stochastic-series-expansion technique) to study phase transitions and to investigate whether we have homogeneous or phase-separated coexistence. The effective Hamiltonian involves, besides a nearest-neighbor hopping and a nearest-neighbor repulsion, sizeable double-hopping terms (obtained from second-order perturbation). At densities not far from half-filling, in the parameter regime where the double-hopping terms are non-negligible (negligible) compared to the nearest-neighbor hopping, we get checkerboard-supersolidity (phase separation) with CDW being characterized by ordering wavevector $\vec{Q}=(π,π)$.

cond-mat.str-el↗

arXiv2015

A treatment of cooperative Jahn-Teller effect in interacting chains

Studying the nature and consequences of electron-phonon interaction in manganites is an area of intense ongoing research. Here, in an attempt to model charge and orbital ordering in manganites displaying C-type antiferromaganetism, we study cooperative Jahn-Teller effect in two-band one-dimensional chains in the regimes of both strong and weak electron-phonon couplings. These chains exhibit orbital ferromagnetism with only $d_{z^2}$ orbitals being occupied. At strong coupling and in the antiadiabatic regime, using a controlled analytic nonperturbative treatment that accounts for the quantum nature of the phonons, we derive the effective polaronic Hamiltonians for a single chain as well as for interacting identically-long chains. Due to cooperative effects, these effective Hamiltonians manifest a dominant next-nearest-neighbor hopping compared to the usual nearest-neighbor hopping and a significantly enhanced nearest-neighbor repulsion. For densities up to half filling, upon tuning electron-phonon coupling, interacting-chain [single-chain] Jahn-Teller systems undergo quantum phase transition from a charge disordered state to a conducting charge-density-wave state characterized by a wavevector $\vec{k} = (π,π)$ [$k = π$]. On the other hand, up to half filling, a weak coupling analysis reveals a transition from a disordered state to an insulating charge-density-wave state with a wavevector that depends linearly on the density; the ordering is analyzed within a Peierls instability framework involving the dynamic noninteracting susceptibility at nesting wavevector and phonon frequency. Our analysis provides an opportunity to identify the regime of electron-phonon coupling in manganites through experimentally determining the charge-ordering wavevector.

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arXiv2014

Polaron dynamics and decoherence in an interacting two-spin system coupled to optical phonon environment

We study two anisotropically interacting spins coupled to optical phonons; we restrict our analysis to the regime of strong coupling to the environment, to the antiadiabatic region, and to the subspace with zero value for $S^z_T$ (the z-component of the total spin). In the case where each spin is coupled to a different phonon bath, we assume that the system and the environment are initially uncorrelated (and form a simply separable state) in the polaronic frame of reference. By analyzing the {polaron} dynamics through non-Markovian quantum master equation, we find that the system manifests a small amount of decoherence that decreases both with increasing non-adiabaticity and with enhancing strength of coupling; whereas, under the Markovian approximation, the polaronic system exhibits a decoherence free behavior. For the situation where both spins are coupled to the same phonon bath, we also show that the system is decoherence free in the subspace where $S^z_T$ is fixed. To suppress decoherence through quantum control, we employ a train of pi pulses and demonstrate that unitary evolution of the system can be retained. We propose realization of a weakly decohering charge qubit from an electron in an oxide-based (tunnel-coupled) double quantum dot system.

cond-mat.str-el↗

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↗

arXiv2012

A study of cooperative breathing-mode in molecular chains

Using a controlled analytic non-perturbative treatment, that accounts for the quantum nature of the phonons, we derive a model that generically describes cooperative breathing-mode at strong electron-phonon interaction in one-band one-dimensional systems. The effective model involves a {\em next-nearest-neighbor} hopping (that dominates over the nearest-neighbor hopping at strong coupling) and a nearest-neighbor repulsion that is significantly enhanced due to incompatibility of neighboring dilations/compressions. At non-half filling, upon tuning the electron-phonon coupling, the system undergoes a period-doubling second-order quantum phase transition from a Luttinger liquid to a {\em conducting commensurate} charge-density-wave state: a phenomenon absent in both the Holstein model and the t-V model. Using fidelity to study the nature of the quantum phase transition, we find that the fidelity susceptibility shows a superextensive power law divergence as well as a remarkable scaling behavior: both together establish a second-order transition.

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arXiv2011

Study of maximal bipartite entanglement and robustness in resonating-valence-bond states

We study maximal bipartite entanglement in valence-bond states and show that the average bipartite entanglement $E_v^2$, between a sub-system of two spins and the rest of the system, can be maximized through a homogenized superposition of the valence-bond states. Our derived maximal $E_v^2$ rapidly increases with system size and saturates at its maximum allowed value. We also demonstrate that our maximal $E^2_v$ states are ground states of an infinite range Heisenberg model (IRHM) and represent a new class of resonating-valence-bond (RVB) states. The entangled RVB states produced from our IRHM are robust against interaction of spins with both local and global phonons and represent a new class of decoherence free states.

quant-ph↗

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.

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arXiv2009

Supersolidity in a Bose-Holstein model

We derive an effective d-dimensional Hamiltonian for a system of hard-core-bosons coupled to optical phonons in a lattice. At non-half-fillings, a superfluid-supersolid transition occurs at intermediate boson-phonon couplings, while at strong-couplings the system phase separates. We demonstrate explicitly that the presence of next-nearest-neighbor hopping and nearest-neighbor repulsion leads to supersolidity. Thus we present a microscopic mechanism for the homogeneous coexistence of charge-density-wave and superfluid orders.

cond-mat.str-el↗

arXiv2009

Many-player entangled state solutions in game theory problems

We propose a non-classical multi-player entangled state which eliminates the need for communication, yet can solve problems (that require coordination) better than classical approaches. For the entangled state, we propose a slater determinant of all allowed states of a filled band in a condensed matter system -- the integer quantum Hall state at filling factor 1. Such a state gives the best solution (i.e., best Nash equilibrium) for some classical stochastic problems where classical solutions are far from ideal.

quant-ph↗

arXiv2008

Quantum phase transition in Bose-Holstein model in two dimensions

We derive an effective d-dimensional Hamiltonian for a system of hard-core-bosons coupled to optical phonons in a lattice. Away from half-filling, we show that the presence of next-nearest-neighbor hopping in the effective Hamiltonian leads to a superfluid-to-supersolid transition at intermediate boson-phonon (b-p) couplings, while at strong-couplings the system phase separates. However, at half-filling and at a critical b-p coupling (as in the xxz-model), the system undergoes a superfluid-to-charge-density-wave transition without any signature of supersolidity. Our analyses is based on extensive calculations of the structure factor, the superfluid fraction, the Bose-Einstein condensate fraction, and the system energy at various fillings. We present a phase diagram for this system and compare it to that of the xxz-model. We also demonstrate explicitly that the next-nearest-neighbor hopping (in the absence of nearest-neighbor hopping) in the effective Hamiltonian leads only to a single transition, i.e., a first-order superfluid-to-supersolid transition.

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