arXiv subjects

R. Chitra

Publications and source records attributed to R. Chitra.

At least 55 records · Page 3Linked to original sources

arXiv2017

Sensing Floquet-Majorana fermions via heat transfer

Time periodic modulations of the transverse field in the closed XY spin-1/2 chain generate a very rich dynamical phase diagram, with a hierarchy of topological phases characterized by differing numbers of Floquet-Majorana modes. We show that this rich phase diagram survives when the system is coupled to dissipative end reservoirs. Circumventing the obstacle of preparing and measuring quasi energy configurations endemic to Floquet-Majorana detection schemes, we show that stroboscopic heat transport and spin density are robust observables to detect both the dynamical phase transitions and Majorana modes. In particular, we find that the derivative of the heat current, with respect to a control parameter, changes sign at the boundaries separating topological phases with different numbers of Floquet Majorana modes. We present a simple scheme to directly count the number of Floquet-Majorana modes in a phase from the Fourier transform of the local spin density profile. Our results are valid provided the anisotropies are not strong and can be easily implemented in quantum engineered systems.

cond-mat.str-el↗

arXiv2016

Single spin probe of Many-Body Localization

We use an external spin as a dynamical probe of many body localization. The probe spin is coupled to an interacting and disordered environment described by a Heisenberg spin chain in a random field. The spin-chain environment can be tuned between a thermalizing delocalized phase and non-thermalizing localized phase, both in its ground- and high-energy states. We study the decoherence of the probe spin when it couples to the environment prepared in three states: the ground state, the infinite temperature state and a high energy Néel state. In the non-thermalizing many body localized regime, the coherence shows scaling behaviour in the disorder strength. The long-time dynamics of the probe spin shows a logarithmic dephasing in analogy with the logarithmic growth of entanglement entropy for a bi-partition of a many-body localized system. In summary, we show that decoherence of the probe spin provides clear signatures of many-body localization.

cond-mat.dis-nn↗

arXiv2016

Parametric symmetry breaking in a nonlinear resonator

Much of the physical world around us can be described in terms of harmonic oscillators in thermodynamic equilibrium. At the same time, the far from equilibrium behavior of oscillators is important in many aspects of modern physics. Here, we investigate a resonating system subject to a fundamental interplay between intrinsic nonlinearities and a combination of several driving forces. We have constructed a controllable and robust realization of such a system using a macroscopic doubly clamped string. We experimentally observe a hitherto unseen double hysteresis in both the amplitude and the phase of the resonator's response function and present a theoretical model that is in excellent agreement with the experiment. Our work provides a thorough understanding of the double-hysteretic response through a symmetry breaking of parametric phase states that elucidates the selection criteria governing transitions between stable solutions. Our study motivates applications ranging from ultrasensitive force detection to low-energy computing memory units.

cond-mat.mes-hall↗

arXiv2016

Signatures of topological quantum phase transitions in driven and dissipative qubit-arrays

We study photonic signatures of symmetry broken and topological phases in a driven, dissipative circuit QED realization of spin-1/2 chains. Specifically, we consider the transverse-field XY model and a dual model with 3-spin interactions. The former has a ferromagnetic and a paramagnetic phase, while the latter features, in addition, a symmetry protected topological phase. Using the method of third quantization, we calculate the non-equilibrium steady-state of the open spin chains for arbitrary system sizes and temperatures. We find that the bi-local correlation function of the spins at both ends of the chain provides a sensitive measure for both symmetry-breaking and topological phase transitions of the systems, but no universal means to distinguish between the two types of transitions. Both models have equivalent representations in terms of free Majorana fermions, which host zero, one and two topological Majorana end modes in the paramagnetic, ferromagnetic, and symmetry protected topological phases, respectively. The correlation function we study retains its bi-local character in the fermionic representation, so that our results are equally applicable to the fermionic models in their own right. We propose a photonic realization of the dissipative transverse-field XY model in a tunable setup, where an array of superconducting transmon qubits is coupled at both ends to a photonic microwave circuit.

quant-ph↗

arXiv2016

Ultrasensitive hysteretic force sensing with parametric nonlinear oscillators

We propose a novel method for linear detection of weak forces using parametrically driven nonlinear resonators. The method is based on a peculiar feature in the response of the resonator to a near resonant periodic external force. This feature stems from a complex interplay between the parametric drive, external force and nonlinearities. For weak parametric drive, the response exhibits the standard Duffing-like single jump hysteresis. For stronger drive amplitudes, we find a qualitatively new double jump hysteresis which arises from stable solutions generated by the cubic Duffing nonlinearity. The additional jump exists only if the external force is present and the frequency at which it occurs depends linearly on the amplitude of the external force, permitting a straightforward ultrasensitive detection of weak forces. With state-of-the-art nanomechanical resonators, our scheme should permit force detection in the atto-newton range.

nlin.CD↗

arXiv2015

Hydrogen Bond Symmetrization in Glycinium Oxalate under Pressure

We report here the evidences of hydrogen bond symmetrization in the simplest amino acid- carboxylic acid complex, glycinium oxalate, at moderate pressures of 8 GPa using in-situ infrared and Raman spectroscopic investigations combined with first-principles simulations. The protonation of the semioxalate units through dynamic proton movement results in infinite oxalate chains. At pressures above 12 GPa, the glycine units systematically reorient with pressure to form hydrogen bonded supramolecular assemblies held together by these chains.

cond-mat.mtrl-sci↗

arXiv2015

Dynamical many-body phases of the parametrically driven, dissipative Dicke model

The dissipative Dicke model exhibits a fascinating out-of-equilibrium many-body phase transition as a function of a coupling between a driven photonic cavity and numerous two-level atoms. We study the effect of a time-dependent parametric modulation of this coupling, and discover a rich phase diagram as a function of the modulation strength. We find that in addition to the established normal and super-radiant phases, a new phase with pulsed superradiance which we term dynamical normal phase appears when the system is parametrically driven. Employing different methods, we characterize the different phases and the transitions between them. Specific heed is paid to the role of dissipation in determining the phase boundaries. Our analysis paves the road for the experimental study of dynamically stabilized phases of interacting light and matter.

cond-mat.quant-gas↗

arXiv2014

Heat Current Characteristics in Nanojunctions with Superconducting Leads

As a fundamental requisite for thermotronics, controlling heat flow has been a longstanding quest in solid state physics. Recently, there has been a lot of interest in nanoscale hybrid systems as possible candidates for thermal devices. In this context, we study the heat current in the simplest hybrid device of a two level system weakly coupled to two heat baths. We use the reduced density matrix approach together with a simple Born-Markov approximation to calculate the heat current in the steady state. We consider different kinds of reservoirs and show that the nature of the reservoir plays a very important role in determining the thermal characteristics of the device. In particular, we investigate the effectiveness of a conventional superconductor as a reservoir with regard to manipulating the heat current. In the emergent temperature characteristics, we find that superconductivity in the reservoirs leads to enhanced thermal currents and that the superconducting phase transition is clearly visible in the heat current. We observe negative differential thermal conductance and a pronounced rectification of the heat current, making this a good building block for a quantum thermal diode.

cond-mat.mes-hall↗

arXiv2013

Effect of a gap on the decoherence of a qubit

We revisit the problem of the decoherence and relaxation of a central spin coupled to a bath of conduction electrons. We consider both metallic and semiconducting baths to study the effect of a gap in the bath density of states (DOS) on the time evolution of the density matrix of the central spin. We use two weak coupling approximation schemes to study the decoherence. At low temperatures, though the temperature dependence of the decoherence rate in the case of a metallic bath is the same irrespective of the details of the bath, the same is not true for the semiconducting bath. We also calculate the relaxation and decoherence rates as a function of external magnetic fields applied both on the central spin and the bath. We find that in the presence of the gap, there exists a certain regime of fields, for which surprisingly, the metallic bath has lower rates of relaxation and decoherence than the semiconducting bath.

cond-mat.mes-hall↗

arXiv2013

Decoherence induced by an ordered environment

This Letter deals with the time evolution of a qubit weakly coupled to a reservoir which has a symmetry broken state with long range order at finite temperatures. In particular, we model the ordered reservoir by a standard BCS superconductor with s-wave pairing. We study the reduced density matrix of a qubit using both the time-convolutionless and Nakajima-Zwanzig approximations. We study different kinds of couplings between the qubit and the superconducting bath. We find that ordering in the superconducting bath generically leads to an unfavorable non- Markovian faster-than-exponential decay of the qubit coherence. On the other hand, a coupling of the qubit to the non-ordered sector of the bath can result in a Markovian decoherence of the qubit with a drastic reduction of the decoherence rate. Since these behaviors are endemic to the ordered phase, qubits can serve as useful probes of continuous phase transitions in their environment. We also briefly discuss the validity of our main result, faster than exponential decay of the qubit coherences, for a qubit coupled to a generic ordered bath with a spontaneously broken continuous symmetry at finite temperatures.

cond-mat.mes-hall↗

arXiv2012

Phase diagram of the asymmetric Hubbard model and an entropic chromatographic method for cooling cold fermions in optical lattices

We study the phase diagram of the asymmetric Hubbard model (AHM), which is characterized by different values of the hopping for the two spin projections of a fermion or equivalently, two different orbitals. This model is expected to provide a good description of a mass-imbalanced cold fermionic mixture in a 3D optical lattice. We use the dynamical mean field theory to study various physical properties of this system. In particular, we show how orbital-selective physics, observed in multi-orbital strongly correlated electron systems, can be realized in such a simple model. We find that the density distribution is a good probe of this orbital selective crossover from a Fermi liquid to a non-Fermi liquid state. Below an ordering temperature $T_o$, which is a function of both the interaction and hopping asymmetry, the system exhibits staggered long range orbital order. Apart from the special case of the symmetric limit, i.e., Hubbard model, where there is no hopping asymmetry, this orbital order is accompanied by a true charge density wave order for all values of the hopping asymmetry. We calculate the order parameters and various physical quantities including the thermodynamics in both the ordered and disordered phases. We find that the formation of the charge density wave is signaled by an abrupt increase in the sublattice double occupancies. Finally, we propose a new method, entropic chromatography, for cooling fermionic atoms in optical lattices, by exploiting the properties of the AHM. To establish this cooling strategy on a firmer basis, we also discuss the variations in temperature induced by the adiabatic tuning of interactions and hopping parameters.

cond-mat.quant-gas↗

arXiv2012

Orbital selective crossover and Mott transitions in an asymmetric Hubbard model of cold atoms in optical lattices

We study the asymmetric Hubbard model at half-filling as a generic model to describe the physics of two species of repulsively interacting fermionic cold atoms in optical lattices. We use Dynamical Mean Field Theory to obtain the paramagnetic phase diagram of the model as function of temperature, interaction strength and hopping asymmetry. A Mott transition with a region of two coexistent solutions is found for all nonzero values of the hopping asymmetry. At low temperatures the metallic phase is a heavy Fermi-liquid, qualitatively analogous to the Fermi liquid state of the symmetric Hubbard model. Above a coherence temperature, an orbital-selective crossover takes place, wherein one fermionic species effectively localizes, and the resulting bad metallic state resembles the non-Fermi liquid state of the Falicov-Kimball model. We compute observables relevant to cold atom systems such as the double occupation, the specific heat and entropy and characterize their behavior in the different phases.

cond-mat.str-el↗

arXiv2010

Weak coupling study of decoherence of a qubit in disordered magnetic environments

We study the decoherence of a qubit weakly coupled to frustrated spin baths. We focus on spin-baths described by the classical Ising spin glass and the quantum random transverse Ising model which are known to have complex thermodynamic phase diagrams as a function of an external magnetic field and temperature. Using a combination of numerical and analytical methods, we show that for baths initally in thermal equilibrium, the resulting decoherence is highly sensitive to the nature of the coupling to the environment and is qualitatively different in different parts of the phase diagram. We find an unexpected strong non-Markovian decay of the coherence when the random transverse Ising model bath is prepared in an initial state characterized by a finite temperature paramagnet. This is contrary to the usual case of exponential decay (Markovian) expected for spin baths in finite temperature paramagnetic phases, thereby illustrating the importance of the underlying non-trivial dynamics of interacting quantum spinbaths.

cond-mat.mes-hall↗

arXiv2008

Impurity effects in the quantum kagome system ZnCu3(OH)6Cl2

Motivated by the recent experiments on the new spin half Kagome compound ZnCu3(OH)6Cl2, we study a phenomenological model of a frustrated quantum magnet. The model has a spin liquid groundstate and is constructed so as to mimic the macroscopically large quasi-degeneracies expected in the low-lying energy structure of a Kagome system. We use numerical studies of finite size systems to investigate the static as well as the dynamical response at finite temperatures. The results obtained using our simple model are compatible with a large number of recent experiments including neutron scattering data. Our study suggests that many of the anomalous features observed in experiments have a natural interpretation in terms of a spin-1/2 defects (impurities) coupled to an underlying Kagome-type spin liquid.

cond-mat.str-el↗

arXiv2008

Disorder effects in the quantum kagome antiferromagnet ZnCu3(OH)6Cl2

The recent NMR experiments on ZnCu3(OH)6Cl2 motivate our study of the effect of non- magnetic defects on the antiferromagnetic spin-1/2 kagome lattice. We use exact diagonalization methods to study the effect of two such defects on finite size systems. Our results, obtained without adjustable parameters, are in good quantitative agreement with recent Oxygen 17 NMR data. They provide support for the experimental interpretation of the presence of defects within the kagome layers due to Zn/Cu substitutions. Our results also show that disorder effects become relevant at lower temperatures, raising questions about the experimental evidence for the absence of an intrinsic spin gap in the kagome 2D layers.

cond-mat.str-el↗

arXiv2006

Effect of random interactions in spin baths on decoherence

We study the decoherence of a central spin 1/2 induced by a spin bath with intrabath interactions. Since we are interested in the cumulative effect of interaction and disorder, we study baths comprising Ising spins with random ferro- and antiferromagnetic interactions between the spins. Using the resolvent operator method which goes beyond the standard Born-Markov master equation approach, we show that, in the weak coupling regime, the decoherence of the central spin at all times is entirely determined by the local-field distribution or equivalently, the dynamical structure factor of the Ising bath. We present analytic results for the Ising spin chain bath at arbitrary temperature for different distributions of the intrabath interaction strengths. We find clear evidence of non-Markovian behavior in the low temperature regime. We also consider baths described by Ising models on higher-dimensional lattices. We find that interactions lead to a significant reduction of the decoherence. An important feature of interacting spinbaths is the saturation of the asymptotic Markovian decay rate at high temperatures, as opposed to the conventional Ohmic boson bath.

cond-mat.mes-hall↗

arXiv2006

Electronic state of a doped Mott-Hubbard insulator at finite temperatures studied using the dynamical mean-field theory

We study the electronic state of the doped Mott-Hubbard insulator within Dynamical Mean Field Theory. The evolution of the finite temperature spectral functions as a function of doping show large redistributions of spectral weight in both antiferromagnetic and paramagnetic phases. In particular, a metallic antiferromagnetic state is obtained with a low frequency Slater-splitted quasiparticle peak coexisting with Hubbard bands. In the high temperature paramagnetic metallic phase, upon reducing doping, the system has a crossover through a ``bad metal'' state characterized by an anomalous shift of the quasiparticle peak away from the Fermi energy. We find that the {\it charge} compressibility of the antiferromagnetic metal is dramatically enhanced upon approaching the second order Néel line.

cond-mat.str-el↗

arXiv2006

Coulombian disorder in Charge Density Waves

When unscreened charged impurities are introduced in charge density wave system the long wavelength component of the disorder is long ranged and dominates static correlation functions. We calculate the x-ray intensity and find that it is identical to the one produced by thermal fluctuations in a disorder-free smectic-A. We suggest that this effect could be observed in the blue bronze material K$_{0.3}$MoO$_3$ doped with charged impurities such as Vanadium.

cond-mat.dis-nn↗