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1. Ground-state phase diagram of the t-t ′ -J model

   https://doi.org/10.1073/pnas.2109978118  

   Jiang, Shengtao ; Scalapino, Douglas J. ; White, Steven R. ( October 2021 , Proceedings of the National Academy of Sciences)

   We report results of large-scale ground-state density matrix renormalization group (DMRG) calculations on t-$t^{\prime }$-J cylinders with circumferences 6 and 8. We determine a rough phase diagram that appears to approximate the two-dimensional (2D) system. While for many properties, positive and negative$t^{\prime }$values ($t^{\prime }/t=\pm 0.2$) appear to correspond to electron- and hole-doped cuprate systems, respectively, the behavior of superconductivity itself shows an inconsistency between the model and the materials. The$t^{\prime }<0$(hole-doped) region shows antiferromagnetism limited to very low doping, stripes more generally, and the familiar Fermi surface of the hole-doped cuprates. However, we find$t^{\prime }<0$strongly suppresses superconductivity. The$t^{\prime }>0$(electron-doped) region shows the expected circular Fermi pocket of holes around the$\left(\pi ,\pi \right)$point and a broad low-doped region of coexisting antiferromagnetism and d-wave pairing with a triplet p component at wavevector$\left(\pi ,\pi \right)$induced by the antiferromagnetism and d-wave pairing. The pairing for the electron low-doped system with$t^{\prime }>0$is strong and unambiguous in the DMRG simulations. At larger doping another broad region with stripes in addition to weaker d-wave pairing and striped p-wave pairing appears. In a small doping region near$x=0.08$for$t^{\prime }\sim -0.2$, we find an unconventional type of stripe involving unpaired holes located predominantly on chains spaced three lattice spacings apart. The undoped two-leg ladder regions in between mimic the short-ranged spin correlations seen in two-leg Heisenberg ladders.

    

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2. Valence transition model of the pseudogap, charge order, and superconductivity in electron-doped and hole-doped copper oxides

   https://doi.org/DOI: 10.1103/PhysRevB.98.205153  

   Mazumdar, S. ( October 2018 , Physical review. B, Condensed matter)

   We present a valence transition model for electron- and hole-doped cuprates, within which there occurs a discrete jump in ionicity Cu2+ -> Cu1+ in both families upon doping, at or near optimal doping in the conventionally prepared electron-doped compounds and at the pseudogap phase transition in the hole-doped materials. In thin films of the T' compounds, the valence transition has occurred already in the undoped state. The phenomenology of the valence transition is closely related to that of the neutral-to-ionic transition in mixed-stack organic charge-transfer solids. Doped cuprates have negative charge-transfer gaps, just as rare-earth nickelates and BaBiO3. The unusually high ionization energy of the closed shell Cu1+ ion, taken together with the dopingdriven reduction in three-dimensional Madelung energy and gain in two-dimensional delocalization energy in the negative charge transfer gap state drives the transition in the cuprates. The combined effects of strong correlations and small d-p electron hoppings ensure that the systems behave as effective 1/2-filled Cu band with the closed shell electronically inactive O2- ions in the undoped state, and as correlated two-dimensional geometrically frustrated 1/4-filled oxygen hole band, now with electronically inactive closed-shell Cu1+ ions, in the doped state. The model thus gives microscopic justification for the two-fluid models suggested by many authors. The theory gives the simplest yet most comprehensive understanding of experiments in the normal states. The robust commensurate antiferromagnetism in the conventional T' crystals, the strong role of oxygen deficiency in driving superconductivity and charge carrier sign corresponding to holes at optimal doping are all manifestations of the same quantum state. In the hole-doped pseudogapped state, there occurs a biaxial commensurate period 4 charge density wave state consisting of O1- -Cu l(1+)-O1- spin singlets that coexists with broken rotational C-4 symmetry due to intraunit cell oxygen inequivalence. Finite domains of this broken symmetry state will exhibit twodimensional chirality and the polar Kerr effect. Superconductivity within the model results from a destabilization of the 1/4-filled band paired Wigner crystal [Phys. Rev. B 93, 165110 (2016) and ihid. 93, 205111 (2016)]. We posit that a similar valence transition, Ir4+ -> Ir3+, occurs upon electron doping Sr2IrO4. We make testable experimental predictions in cuprates including superoxygenated La2CuO4+delta and iridates. Finally, as indirect evidence for the valence bond theory of superconductivity proposed here, we note that there exist an unusually large number of unconventional superconductors that exhibit superconductivity proximate to exotic charge ordered states, whose band fillings are universally 1/4 or 3/4, exactly where the paired Wigner crystal is most stable. 

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3. Absence of Superconductivity in the Hubbard Dimer Model for κ-(BEDT-TTF)2X

   https://doi.org/10.3390/cryst11060580  

   Roy, Dipayan ; Clay, R. Torsten ; Mazumdar, Sumit ( June 2021 , Crystals)

   null (Ed.)

   In the most studied family of organic superconductors κ-(BEDT-TTF)2X, the BEDT-TTF molecules that make up the conducting planes are coupled as dimers. For some anions X, an antiferromagnetic insulator is found at low temperatures adjacent to superconductivity. With an average of one hole carrier per dimer, the BEDT-TTF band is effectively 12-filled. Numerous theories have suggested that fluctuations of the magnetic order can drive superconducting pairing in these models, even as direct calculations of superconducting pairing in monomer 12-filled band models find no superconductivity. Here, we present accurate zero-temperature Density Matrix Renormalization Group (DMRG) calculations of a dimerized lattice with one hole per dimer. While we do find an antiferromagnetic state in our results, we find no evidence for superconducting pairing. This further demonstrates that magnetic fluctuations in the effective 12-filled band approach do not drive superconductivity in these and related materials. 

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4. Substitutional transition metal doping in MoS 2 : a first-principles study

   https://doi.org/10.1088/2632-959X/ab7cb3  

   Yoshimura, Anthony ; Koratkar, Nikhil ; Meunier, Vincent ( March 2020 , Nano Express)

   Single-layer MoS₂is a direct-gap semiconductor whose band edges character is dominated by the d-orbitals of the Mo atoms. It follows that substitutional doping of the Mo atoms has a significant impact on the material’s electronic properties, namely the size of the band gap and the position of the Fermi level. Here, density functional theory is used along with the G₀W₀method to examine the effects of substituting Mo with four different transition metal dopants: Nb, Tc, Ta, and Re. Nb and Ta possess one less valence electron than Mo does and are therefore p-type dopants, while Re and Tc are n-type dopants, having one more valence electron than Mo has. Four types of substitutional structures are considered for each dopant species: isolated atoms, lines, three-atom clusters centered on a S atom (c3s), and three-atom clusters centered on a hole (c3h). The c3h structure is found to be the most stable configuration for all dopant species. However, electronic structure calculations reveal that isolated dopants are preferable for efficient n- or p-type performance. Lastly, it is shown that photoluminescence measurements can provide valuable insight into the atomic structure of the doped material. Understanding these properties of substitutionally-doped MoS₂can allow for its successful implementation into cutting-edge solid state devices.

    

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5. Crossover of high-energy spin fluctuations from collective triplons to localized magnetic excitations in Sr14−xCaxCu24O41 ladders

   https://doi.org/10.1038/s41535-022-00502-1  

   Tseng, Y. ; Thomas, J. ; Zhang, W. ; Paris, E. ; Puphal, P. ; Bag, R. ; Deng, G. ; Asmara, T. C. ; Strocov, V. N. ; Singh, S. ; et al ( December 2022 , npj Quantum Materials)

   Abstract We studied the magnetic excitations in the quasi-one-dimensional (q-1D) ladder subsystem of Sr 14−x Ca x Cu 24 O 41 (SCCO) using Cu L 3 -edge resonant inelastic X-ray scattering (RIXS). By comparing momentum-resolved RIXS spectra with high ( x  = 12.2) and without ( x  = 0) Ca content, we track the evolution of the magnetic excitations from collective two-triplon (2 T) excitations ( x  = 0) to weakly-dispersive gapped modes at an energy of 280 meV ( x  = 12.2). Density matrix renormalization group (DMRG) calculations of the RIXS response in the doped ladders suggest that the flat magnetic dispersion and damped excitation profile observed at x  = 12.2 originates from enhanced hole localization. This interpretation is supported by polarization-dependent RIXS measurements, where we disentangle the spin-conserving Δ S  = 0 scattering from the predominant Δ S  = 1 spin-flip signal in the RIXS spectra. The results show that the low-energy weight in the Δ S  = 0 channel is depleted when Sr is replaced by Ca, consistent with a reduced carrier mobility. Our results demonstrate that off-ladder impurities can affect both the low-energy magnetic excitations and superconducting correlations in the CuO 4 plaquettes. Finally, our study characterizes the magnetic and charge fluctuations in the phase from which superconductivity emerges in SCCO at elevated pressures. 

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