Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E45: Mott Insulators |
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Sponsoring Units: DCMP Chair: Wenxin Ding, Kavli Institute on Theoretical Sciences, Beijing Room: LACC 505 |
Tuesday, March 6, 2018 8:00AM - 8:12AM |
E45.00001: Non-trivial Pairing Separated by a Mott Core on the t-J Ladder Chen Cheng, Rubem Mondaini, Marcos Rigol We investigate the ground state properties of the t-J ladder by using the large-scale density matrix renormalization group method. Applying a strong chemical potential to the inner legs of the lattice, to emulate the effects of a confining trap, the central part of the system behaves as a Mott insulator, while on the outer legs the system is in a metallic-like phase. In this case, we demonstrate a real-space separated super-conducting pairing, with two paired-particles being split by the Mott insulator core. By examining the corresponding pairing correlation functions for both the singlet and the triplet channels, we find a regime where the singlet pairing dominates and decays as a power-law when the Mott core has an even number of legs. Therefore, there is a non-trivial singlet superconducting pairing displaying quasi-long-range order on this ladder in the presence of a Mott core. We expect that our findings can be confirmed by the cold atoms/molecules in optical lattices experiments. |
Tuesday, March 6, 2018 8:12AM - 8:24AM |
E45.00002: Local Density of States of Impurities in Mott Insulators Wenxin Ding, Qimiao Si Motivated by recent scanning tunneling microscope measurements of local density of states on impurities in parent compounds and very lightly doped, insulating compounds of the cuprates high-$T_C$ superconductors (C. Ye et. al. Nat. Commun. {\bf 4}, 1365 (2013), and P. Cai et. al. Nat. Phys. {\bf 12}, 1047 (2016)), we start with the slave rotor description of the Mott insulating phase, and solve the local density of states of the impurities sites using a $T$-matrix approach. Excellent agreement with the experimental results is found. Our calculations account for all the following key features of the observed local density of states: i) positions and amplitudes of the in-gap spectral weights of a single impurity; ii) the spectral weight transfer from the upper Hubbard band to the lower Hubbard band; iii) the difference between two different types of impurities. By generalizing the $T$-matrix solutions to the finite-doping, insulating case, we suggest that the observed suppression of spectral weight at the Fermi energy can be attributed to the protection from the bulk insulating phase. |
Tuesday, March 6, 2018 8:24AM - 8:36AM |
E45.00003: Parity-time symmetry breaking physics of dissipative Mott insulators Vikram Tripathi, Alexey Galda, Himadri Barman, Valerii Vinokour An applied electric field causes dielectric breakdown to an insulator, leading to finite current after the field crosses a certain threshold value. This fact has been understood through the dissipative Landau-Zener tunneling mechanism which is analogous to the Schwinger mechanism for vacuum polarization in Quantum Electrodynamics. However, the problem becomes challenging in case of the correlated Mott insulators. |
Tuesday, March 6, 2018 8:36AM - 8:48AM |
E45.00004: Mott Transitions in the Multiband Hubbard Model through Variational Wave Functions Caterina De Franco, Luca Fausto Tocchio, Federico Becca We study the multi-band Hubbard model with two or three degenerate bands by using correlated variational wave functions and Monte Carlo methods. Here, the interplay among electronic Coulomb repulsion, kinetic energy and the Hund's coupling gives rise to a rich phase diagram. Imposing a paramagnetic wave function, we have obtained the Mott metal-insulator transitions when the electronic filling is commensurate (i.e., for n=1, 2, and 3) at finite values of the Coulomb repulsions U_c(n). In agreement with previous Dynamical Mean-Field theory results, we observe that the inclusion of the Hund's coupling J lowers the value of U_c(n) at half filling, while it leads to an increase of U_c(n) for other fillings. At half-filling, the transition is first order for both J equal and different from 0. At non-commensurate filling and finite J, we report the existence of a large region of spin-triplet superconductivity with inter-band pairing. By contrast, a (singlet) d-wave intra-band pairing is present only when the intra-band Coulomb repulsion is larger than the inter-band one. |
Tuesday, March 6, 2018 8:48AM - 9:00AM |
E45.00005: Filling-driven Mott transition in SU(N) Hubbard models Seung-Sup Lee, Jan Von Delft, Andreas Weichselbaum We study the filling-driven Mott transition involving the metallic and paramagnetic insulating phases in SU(N) Fermi-Hubbard models, using dynamical mean-field theory (DMFT) and the numerical renormalization group (NRG) as impurity solver. The compressibility shows a striking temperature dependence: near the critical temperature, it is strongly enhanced in the metallic phase close to the insulating phase. We demonstrate that this compressibility enhancement is associated with the thermal suppression of the quasiparticle peak in the local spectral functions. We also explain that the asymmetric shape of the quasiparticle peak originates from the asymmetry in the underlying doublon-holon dynamics. |
Tuesday, March 6, 2018 9:00AM - 9:12AM |
E45.00006: Photoinduced absorptions inside the Mott gap in two-dimensional extended Hubbard model Kazuya Shinjo, Takami Tohyama We theoretically investigate pump-probe optical responses in a two-dimensional extended Hubbard model describing cuprates by using a time-dependent Lanczos method. At half filling, pumping generates photoinduced absorptions inside the Mott gap. A part of low-energy absorptions is attributed to the independent propagation of photoinduced holon and doublon. The spectral weight just below the Mott gap increases with decreasing the on-site Coulomb interaction U. We find that the next-nearest-neighbor Coulomb interaction V_{1} enhances this U dependence, indicating the presence of biexcitonic contributions formed by two holon-doublon pairs. Photo pumping in hole-doped systems also induces spectral weights below remnant Mott-gap excitations, being consistent with recent experiment. The induced weights are less sensitive to V_{1} and may be related to formation of a biexcitonic state in the presence of hole carriers. |
Tuesday, March 6, 2018 9:12AM - 9:24AM |
E45.00007: Critical behavior of an impurity at the boson superfluid-Mott insulator transition Seth Whitsitt, Subir Sachdev We present a universal theory for the critical behavior of an impurity at the two-dimensional superfluid-Mott insulator transition. Our analysis is motivated by a numerical study of the Bose-Hubbard model with an impurity site by Huang et al. (Phys. Rev. B {\bf 94}, 220502 (2016)); they found an impurity phase transition as a function of the trapping potential, while the bulk was critical. The bulk theory is described by the $O(2)$ symmetric Wilson-Fisher conformal field theory, and we model the impurity by a localized spin-1/2 degree of freedom. We also consider a generalized model by considering an O(N) symmetric bulk theory coupled to a spin-S degree of freedom. We study this field theory using the ε = 3 - d expansion, where the impurity-bulk interaction flows to an infrared stable fixed point at the critical trapping potential. We determine the scaling dimensions of the impurity degree of freedom and the associated critical exponents near the critical point. We also determine the universal contribution of the impurity to the finite temperature compressibility of the system at criticality. Our results are compared with numerical simulations. |
Tuesday, March 6, 2018 9:24AM - 9:36AM |
E45.00008: Role of screening and charge transfer for a Mott transition in a Hydrogen lattice Victor Quito, Darko Tanaskovic, Sudeshna Sen, Wei Ku, Eduardo Miranda, Vladimir Dobrosavljevic A lattice of hydrogenic atoms may be regarded as a prototypical model system displaying a Mott metal-insulator transition as the density is varied at zero temperature. According to early ideas of Mott, however, this transition may be accompanied by appreciable charge transfer from the hydrogenic impurity band to the conduction band, giving rise to a significant screening of the ionic binding potential, possibly leading to a first order Mott transition at T=0. Based on very recent ab-initio modeling [1] of the hydrogenic lattice, we here propose a minimal model system to investigate the relevant screening effects, which are usually ignored in standard single-band Hubbard model description of the Mott transition. We investigate the appropriate (two band) charge-transfer model, supplemented by an inter-band Coulomb interaction, which is solved using a combination of slave-boson (Gutzwiller) methods and DMFT. The results should have strong relevance for the long-standing puzzle of the metal-insulator transition in doped semiconductors, such as Si:P. |
Tuesday, March 6, 2018 9:36AM - 9:48AM |
E45.00009: Strain-induced continuous transition from Weak localization regime to Strong localization regime in a doped Mott Insulator Ravindra Bisht, Gopinath Daptary, Aveek Bid, Arup Raychaudhuri We report electrical transport properties of the Nd_{0.7}La_{0.3}NiO_{3} films (thickness 80 nm) measured down to 300 mK which were grown on LaAlO_{3 }(100), SrTiO_{3 }(100), and NdGaO_{3} (100) single crystal substrates by pulsed laser deposition. We show that strain/strain inhomogeneity induced by the substrates can lead to a continuous transition from the weak localized regime to a strong localized regime and likelihood of occurrence of a non- Fermi liquid behavior. We observed that the metallic state of a film under compressive strain shows the non- Fermi liquid behavior while the film that experiences a tensile strain shows a metal-insulator transition and a Fermi liquid behavior. The disorder induced continuous transition has been proposed in the context of half-filled Anderson Hubbard model at a finite temperature where the quenched disorder as well auxiliary field fluctuations is responsible for non-Fermi liquid scaling ^{1} . It is likely that strain inhomogeneity in such films acts as a quenched disorder. |
Tuesday, March 6, 2018 9:48AM - 10:00AM |
E45.00010: Disorder Driven Mott Gap Collapse accompanied by the Emergence of a Pseudogap Zhenyu Wang, Yoshinori Okada, Jared O’ Neal, Wenwen Zhou, Daniel Walkup, Chetan Dhital, Tom Hogan, Patrick Clancy, Young-June Kim, Luiz Santos, Stephen Wilson, Nandini Trivedi, Vidya Madhavan The origin of the pseudogap in doped Mott insulators remains one of the biggest mysteries in correlated electron systems. Of the potential causes for a pseudogap, disorder, while ubiquitous, is least invoked and most poorly understood. To investigate the role of disorder in destabilizing a Mott state and realizing a pseudogap state, we use scanning tunneling microscopy and spectroscopy to study isovalent Ru substitutions in Sr_{3}(Ir_{1-x}Ru_{x})_{2}O_{7 }(0≤x≤0.5). Ru creates disorder without the complication of additional order parameters and drives the insulator into an antiferromagnetic, metallic state with a pseudogap similar to that seen in other iridates and cuprates. Nanoscale data show that disorder mediates the transition by locally collapsing the Mott gap and this effect can be captured by a minimal Mott-Hubbard model. Our work establishes a novel mechanism for an insulator to metal transition and establishes a central role for disorder in realizing an emergent pseudogap state in doped Mott systems. |
Tuesday, March 6, 2018 10:00AM - 10:12AM |
E45.00011: Melting of the Mott insulating state in the lightly doped iridate (Sr_{1-x}La_{x})_{2}IrO_{4} Irene Battisti, Koen Bastiaans, Vitaly Fedoseev, Alberto De la Torre, Anna Tamai, Emily Hunter, Robin Perry, Jan Zaanen, Felix Baumberger, Milan Allan High temperature superconductivity as it manifests in the cuprates was for long time suspected to be strongly related to the copper oxide layers, and therefore specific to only this family of materials. Using spectroscopic-imaging scanning tunneling microscopy, we visualize the electronic states of the iridate (Sr_{1-x}La_{x})_{2}IrO_{4,} which is chemically radically different from the cuprates but also an effective Mott insulator. Above a certain doping threshold, we observe the emergence of a phase separated state, with the nucleation of pseudogap puddles and local charge order around clusters of dopant atoms [1]. At lower doping, we measure fully gapped Mott spectra, with a gap value that is in disagreement with photoemission and optical experiments. We find that this is evidence for poor electronic screening in the lightly doped iridates, and we develop an algorithm able to extract the intrinsic value of the gap, reconciling our measurement with literature and illustrating the importance of considering field penetration when performing STM experiments on poorly screened quantum materials [2]. |
Tuesday, March 6, 2018 10:12AM - 10:24AM |
E45.00012: Metallicity beyond Mott-Ioffe-Regel limit in n-doped strontium titanate Kamran Behnia, Xiao Lin, Willem Rischau, Benoit Fauque |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E45.00013: Abstract Withdrawn The Mott insulator Ca_{2}RuO_{4} becomes a good metal by various stimuli such as chemical doping [1], pressure, temperature or electric field [2]. We find that moderate DC current strongly suppresses the insulating behavior and it alters the magnetic nature dramatically in Ca_{2}RuO_{4}. By the application of 1.5-A/cm^{2} DC current, the resistivity is reduced by more than 5 orders of magnitude and ρ-T curve exhibits semimetallic nature. The antiferromagnetic ordering is completely suppressed with 1-A/cm^{2} current. Surprisingly, the current induced semimetallic state exhibits giant diamagnetism below 40 K [3]. We attribute this diamagnetism to the emergent “Mott semimetal” state accompanied by light-mass quasiparticles originating from the partial Mott gap closing. Such emergence of a novel electronic state thereby demonstrates that DC current can be a powerful tuning parameter for some Mott insulators. |
Tuesday, March 6, 2018 10:36AM - 10:48AM |
E45.00014: Orbital-selective Mott transition in Sr_{2}Mn_{3}As_{2}O_{2} Vaideesh Loganathan, Andriy Nevidomskyy, David Senechal Sr_{2}Mn_{3}As_{2}O_{2} is a layered material composed of alternating cuprate-like MnO_{2} layers and MnAs layers similar to iron pnictides [1]. A quasi-2D Neel-AFM ordering in the MnO_{2} layer accompanies the material's low-temperature insulating behavior, whereas the MnAs layer orders above room temperature and has a negligible effect on the transport properties. To better understand the experimental findings, we have performed first-principles DFT+U calculations to explore the electronic structure. We find both layers to be Mott insulating for sufficiently high Hubbard interaction U, with the MnO_{2} layer requiring a larger value of U to become insulating. We observe the Mott transition in the MnO_{2} layer to arise from the e_{g} (d_{x}^{2}_{−y}^{2} and d_{z}^{2}) orbitals. We also notice orbital selectivity, namely the d_{z}^{2} orbital forming a Mott gap at a lower Hubbard U than the d_{x}^{2}_{−y}^{2} orbital. We construct an effective multi-orbital Hubbard model using Wannier functions to parametrize the DFT bands, and then study this model using Variational Cluster Approximation (VCA) and Cellular Dynamic Mean Field Theory (CDMFT). Our results show the metal-insulator phase transition is indeed orbital selective. |
Tuesday, March 6, 2018 10:48AM - 11:00AM |
E45.00015: Impurities induced Mott insulating state revealed by magneto-optical spectrum Jin Peng, Xiaoshan Wu, Xueli Xu, zhe Qu, Z. G. Sheng, Zhiqiang Mao Insulator-metal transition IMT) that accompanied by huge resistivity change is one of the most studied phenomena in condensed matter physics area. While there are many observations of IMT upon tuning of the one-electron bandwidth W or band filling by doping, there are few reports of the opposite i.e., of the formation of a Mott-like insulating state upon the addition of impurities in a metal. One example of such impurity induced Mott transition is found in Cr doped V_{2}O_{3}. Another is the Mn or Ti doped Ruddleson-Popper-type layered ruthenates. Common features of these two systems include the widespread phase separation regions and strong lattice-orbital coupling. In this talk, we present the temperature and magnetic field dependent heat capacity and optical spectral measurements. We established two types of phase separation regions vary with magnetic field or doping concentration. Strong enhancement of both sommerfeld coefficient γ and long-wavelength excitation coefficient B were observed in the phase separation regions. Ferro-orbital-ordering, as well as lattice softening were observed in the Mott insulating state. These studies confirmed our previous proposed d^{0}/d^{3} rule and the lattice-orbital coupling assisted mechanism for the impurities induced Mott transitions. |
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