https://libjncir.jncasr.ac.in/xmlui/handle/10572/1496 2026-04-04T05:31:50Z https://libjncir.jncasr.ac.in/xmlui/handle/10572/2582 Site-disorder driven superconductor-insulator transition: a dynamical mean field study Kamar, Naushad Ahmad; Vidhyadhiraja, N. S. We investigate the effect of site disorder on the superconducting state in the attractive Hubbard model within the framework of dynamical mean field theory. For a fixed interaction strength (U), the superconducting order parameter decreases monotonically with increasing disorder (x), while the single-particle spectral gap decreases for small x, reaches a minimum and keeps increasing for larger x. Thus, the system remains gapped beyond the destruction of the superconducting state, indicating a disorder-driven superconductor-insulator transition. We investigate this transition in depth considering the effects of weak and strong disorder for a range of interaction strengths. In the clean case, the order parameter is known to increase monotonically with increasing interaction, saturating at a finite value asymptotically for U -> infinity. The presence of disorder results in destruction of superconductivity at large U, thus drastically modifying the clean case behaviour. A physical understanding of our findings is obtained by invoking particle-hole asymmetry and the probability distributions of the order parameter and spectral gap. Restricted Access 2014-01-01T00:00:00Z https://libjncir.jncasr.ac.in/xmlui/handle/10572/2581 Preformed excitons, orbital selectivity, and charge density wave order in 1T-TiSe2 Koley, S.; Laad, M. S.; Vidhyadhiraja, N. S.; Taraphder, A. Traditional routes to charge density wave (CDW) in transition-metal dichalcogenides, relying on Fermi surface nesting or Jahn-Teller instabilities, have recently been brought into question. While this calls for exploration of alternative views, a paucity of theoretical guidance sustains lively controversy on the origin of, and interplay between, CDW and superconductive orders in transition-metal dichalcogenides. Here, we explore a preformed excitonic liquid route, heavily supplemented by modern correlated electronic-structure calculations, to an excitonic CDW order in 1T-TiSe2. We show that orbital-selective dynamical localization arising from preformed excitonic liquid correlations is somewhat reminiscent of states proposed for d and f band quantum criticality at the border of magnetism. Excellent quantitative explication of a wide range of spectral and transport responses in both normal and CDW phases provides strong support for our scenario, and suggests that soft excitonic liquid fluctuations mediate superconductivity in a broad class of transition-metal dichalcogenides on the border of CDW. This brings the transition-metal dichalcogenides closer to the bad actors (where the metallic state is clearly not a Fermi liquid) in d and f band systems, where anomalously soft fluctuations of electronic origin are believed to mediate unconventional superconductivity on the border of magnetism. Restricted Access 2014-01-01T00:00:00Z https://libjncir.jncasr.ac.in/xmlui/handle/10572/2580 Kondo-hole substitution in heavy fermions: Dynamics and transport Kumar, Pramod; Vidhyadhiraja, N. S. Kondo-hole substitution is a unique probe for exploring the interplay of interactions, f-electron dilution, and disorder in heavy-fermion materials. Within the diluted periodic Anderson model, we investigate the changes in single-particle dynamics as well as response functions, as a function of Kondo-hole concentration (x) and temperature. We show that the spectral weight transfers due to Kondo-hole substitution have characteristics that are different from those induced by temperature; the dc resistivity crosses over from a highly nonmonotonic form with a coherence peak in the x -> 0 limit to a monotonic single-impurity-like form that saturates at low temperature. The thermopower exhibits a characteristic maximum as a function of temperature, the value of which changes sign with increasing x, and its location is shown to correspond to a low energy scale of the system. The Hall coefficient also changes sign with increasing x at zero temperature and is highly temperature dependent for all x. As x is increased beyond a certain x(c), the Drude peak and the mid-infrared peak in the optical conductivity vanish almost completely; a peak in the optical scattering rate melts and disappears eventually. We discuss the above-mentioned changes in the properties in terms of a crossover from coherent, Kondo lattice behavior to single-impurity-like, incoherent behavior with increasing x. A comparison of theory with experiments carried out for the dc resistivity and the thermopower of Ce1-xLaxB6 yields good agreement. Restricted Access 2014-01-01T00:00:00Z https://libjncir.jncasr.ac.in/xmlui/handle/10572/2279 Feasibility of a metamagnetic transition in correlated systems Acharya, Swagata; Medhi, Amal; Vidhyadhiraja, N. S.; Taraphder, A. The long-standing issue of the competition between the magnetic field and the Kondo effect, favoring, respectively, triplet and singlet ground states, is addressed using a cluster slave-rotor mean-field theory for the Hubbard model and its spin-correlated, spin-frustrated extensions in two dimensions. The metamagnetic jump is established and compared with earlier results of dynamical mean-field theory. This approach also reproduces the emergent super-exchange energy scale in the insulating side. A scaling is found for the critical Zeeman field in terms of the intrinsic coherence scale just below the metal-insulator transition, where the critical spin fluctuations are soft. The conditions required for metamagnetism to appear at a reasonable field are also underlined. Gutzwiller analysis on the two-dimensional Hubbard model and a quantum Monte Carlo calculation on the Heisenberg spin system are performed to check the limiting cases of the cluster slave-rotor results for the Hubbard model. Low-field scaling features for magnetization are discussed. Restricted Access 2016-01-01T00:00:00Z