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Cheng-Li Wu

Publications and source records attributed to Cheng-Li Wu.

At least 19 recordsLinked to original sources
arXiv2020

Fermion dynamical symmetry and strongly-correlated electrons: a comprehensive model of high-temperature superconductivity

We review application of the SU(4) model of strongly-correlated electrons to cuprate and iron-based superconductors. A minimal self-consistent generalization of BCS theory to incorporate antiferromagnetism on an equal footing with pairing and strong Coulomb repulsion is found to account systematically for the major features of high-temperature superconductivity, with microscopic details of the parent compounds entering only parametrically. This provides a systematic procedure to separate essential from peripheral, suggesting that many features exhibited by the high-$T\tsub c$ data set are of interest in their own right but are not central to the superconducting mechanism. More generally, we propose that the surprisingly broad range of conventional and unconventional superconducting and superfluid behavior observed across many fields of physics results from the systematic appearance of similar algebraic structures for the emergent effective Hamiltonians, even though the microscopic Hamiltonians of the corresponding parent states may differ radically from each other.

cond-mat.supr-con↗
arXiv2014

Description of collective and quasiparticle excitations in deformed actinide nuclei: The first application of the Heavy Shell Model

The Heavy Shell Model (HSM) (Y. Sun and C.-L. Wu, Phys. Rev. C 68, 024315 (2003)) was proposed to take the advantages of two existing models, the projected shell model (PSM) and the Fermion Dynamical Symmetry Model (FDSM). To construct HSM, one extends the PSM by adding collective D-pairs into the intrinsic basis. The HSM is expected to describe simultaneously low-lying collective and quasi-particle excitations in deformed nuclei, and still keeps the model space tractable even for the heaviest systems. As the first numerical realization of the HSM, we study systematically the band structures for some deformed actinide nuclei, with a model space including up to 4-quasiparticle and 1-D-pair configurations. The calculated energy levels for the ground- state bands, the collective bands such as β - and γ -bands, and some quasiparticle bands agree well with known experimental data. Some low-lying quasiparticle bands are predicted, awaiting experimental confirmation.

nucl-th↗
arXiv2009

A Unified Description of Cuprate and Iron Arsenide Superconductors

We propose a unified description of cuprate and iron-based superconductivity. Consistency with magnetic structure inferred from neutron scattering implies significant constraints on the symmetry of the pairing gap for the iron-based superconductors. We find that this unification requires the orbital pairing formfactors for the iron arsenides to differ fundamentally from those for cuprates at the microscopic level.

cond-mat.supr-con↗
arXiv2008

Inhomogeneity, Dynamical Symmetry, and Complexity in High-Temperature Superconductors: Reconciling a Universal Phase Diagram with Rich Local Disorder

A model for high-temperature superconductors incorporating antiferromagnetism, d-wave superconductivity, and no double lattice-site occupancy can give energy surfaces exquisitely balanced between antiferromagnetic and superconducting order for specific ranges of doping and temperature. The resulting properties can reconcile a universal cuprate phase diagram with rich inhomogeneity, relate that inhomogeneity to pseudogaps, give a fundamental rationale for giant proximity effects and other emergent behavior, and provide an objective framework for separating essential from peripheral in the superconducting mechanism.

cond-mat.str-el↗
arXiv2008

Generalizing the Cooper-Pair Instability to Doped Mott Insulators

Copper oxides become superconductors rapidly upon doping with electron holes, suggesting a fundamental pairing instability. The Cooper mechanism explains normal superconductivity as an instability of a fermi-liquid state, but high-temperature superconductors derive from a Mott-insulator normal state, not a fermi liquid. We show that precocity to pair condensation with doping is a natural property of competing antiferromagnetism and d-wave superconductivity on a singly-occupied lattice, thus generalizing the Cooper instability to doped Mott insulators, with significant implications for the high-temperature superconducting mechanism.

cond-mat.str-el↗
arXiv2008

k-dependent SU(4) model of high-temperature superconductivity and its coherent-state solutions

We extend the SU(4) model [1-5] for high-Tc superconductivity to an SU(4)k model that permits explicit momentum (k) dependence in predicted observables. We derive and solve gap equations that depend on k, temperature, and doping from the SU(4)k coherent states, and show that the new SU(4)k model reduces to the original SU(4) model for observables that do not depend explicitly on momentum. The results of the SU(4)k model are relevant for experiments such as ARPES that detect explicitly k-dependent properties. The present SU(4)k model describes quantitatively the pseudogap temperature scale and may explain why the ARPES-measured T* along the anti-nodal direction is larger than other measurements that do not resolve momentum. It also provides an immediate microscopic explanation for Fermi arcs observed in the pseudogap region. In addition, the model leads to a prediction that even in the underdoped regime, there exist doping-dependent windows around nodal points in the k-space, where antiferromagnetism may be completely suppressed for all doping fractions, permitting pure superconducting states to exist.

cond-mat.supr-con↗
arXiv2008

Coexistence of normal, super-, and hyper-deformation in nuclei: A study with angular momentum projection

Angular-momentum-projected energy surface calculations for A~110 nuclei indicate three distinct energy minima occurring at different angular-momenta. These correspond to normal, super-, and hyper-deformed shapes coexisting in one nucleus. 110Pd is studied in detail, with a quantitative prediction on super- and hyper-deformed spectra by the Projected Shell Model calculation. It is suggested that several other neighboring nuclei in the A-110 mass region, with the neutron number around 64, also exhibit clear super- and hyper-deformation minima.

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arXiv2007

The Origin of Fermi Arcs in Cuprate Pseudogap States and Strong Constraints on Viable Theories of High-Temperature Superconductivity

A full Fermi surface exists in underdoped high-temperature superconductors if the temperature T lies above the pseudogap temperature T*. Below T* only arcs of Fermi surface survive, scaling with T/T* as T -> 0, with T* displaying strong doping dependence. There is no accepted explanation for this behavior. We show that generalizing the BCS theory of normal superconductivity to include d-wave pairs and antiferromagnetism leads to the origin and doping dependence of the T* scale, and a quantitative description of Fermi arcs. These results place strong constraints on viable theories of high-temperature superconductivity.

cond-mat.supr-con↗
arXiv2007

Pairing Gaps, Pseudogaps, and Phase Diagrams for Cuprate Superconductors

We use a symmetry-constrained variational procedure to construct a generalization of BCS to include Cooper pairs with non-zero momentum and angular momentum. The resulting gap equations are solved at zero and finite temperature, and the doping-dependent solutions are used to construct gap and phase diagrams. We find a pseudogap terminating at a critical doping that may be interpreted in terms of both competing order and preformed pairs. The strong similarity between observation and predicted gap and phase structure suggests that this approach may provide a unified description of the complex structure observed for cuprate superconductors.

cond-mat.supr-con↗
arXiv2005

Temperature-dependent gap equations and their solutions in the SU(4) model of high-temperature superconductivity

Temperature-dependent gap equations in the SU(4) model of high-Tc superconductivity are derived and analytical solutions are obtained. Based on these solutions, a generic gap diagram describing the features of energy gaps as functions of doping P is presented and a phase diagram illustrating the phase structure as a function of temperature T and doping P is sketched. A special doping point P_q occurs naturally in the solutions that separates two phases at temperature T = 0: a pure superconducting phase on one side (P > P_q) and a phase with superconductivity strongly suppressed by antiferromagnetism on the other (P < P_q). We interpret P_q as a quantum phase transition point. Moreover, the pairing gap is found to have two solutions for P < P_q: a small gap that is associated with competition between superconductivity and antiferromagnetism and is responsible for the ground state superconductivity, and a large gap without antiferromagnetic suppression that corresponds to a collective excited state. A pseudogap appears in the solutions that terminates at P_q and originates from the competition between d-wave superconductivity and antiferromagnetism. Nevertheless, this conclusion does not contradict the preformed pair picture conceptually if the preformed pairs are generally defined as any pairs formed before pairing condensation.

cond-mat.supr-con↗
arXiv2005

SU(4) Model of High-Temperature Superconductivity: Manifestation of Dynamical Symmetry in Cuprates

The mechanism that leads to high-temperature superconductivity in cuprates remains an open question despite intense study for nearly two decades. Here, we introduce an SU(4) model for cuprate systems having many similarities to dynamical symmetries known to play an important role in nuclear structure physics and in elementary particle physics. Analytical solutions in three dynamical symmetry limits of this model are found: an SO(4) limit associated with antiferromagnetic order; an SU(2) limit that may be interpreted as a d-wave pairing condensate; and an SO(5) limit that may be interpreted as a doorway state between the antiferromagnetic order and the superconducting order. It is demonstrated that with a slightly broken SO(5) but under constraint of the parent SU(4) symmetry, the model is capable of describing the rich physics that is crucial in explaining why cuprate systems that are antiferromagnetic Mott insulators at half filling become superconductors through hole doping.

cond-mat.supr-con↗
arXiv2003

Mott Insulators, No-Double-Occupancy, and Non-Abelian Superconductivity

SU(4) dynamical symmetry is shown to imply a no-double-occupancy constraint on the minimal symmetry description of antiferromagnetism and d-wave superconductivity. This implies a maximum doping fraction of 1/4 for cuprates and provides a microscopic critique of the projected SO(5) model. We propose that SU(4) superconductors are representative of a class of compounds that we term non-abelian superconductors. We further suggest that non-abelian superconductors may exist having SU(4) symmetry and therefore cuprate-like dynamics, but without d-wave hybridization.

cond-mat.supr-con↗
arXiv2003

Multi-Shell Shell Model for Heavy Nuclei

Performing a shell model calculation for heavy nuclei has been a long-standing problem in nuclear physics. Here we propose one possible solution. The central idea of this proposal is to take the advantages of two existing models, the Projected Shell Model (PSM) and the Fermion Dynamical Symmetry Model (FDSM), to construct a multi-shell shell model. The PSM is an efficient method of coupling quasi-particle excitations to the high-spin rotational motion, whereas the FDSM contains a successful truncation scheme for the low-spin collective modes from the spherical to the well-deformed region. The new shell model is expected to describe simultaneously the single-particle and the low-lying collective excitations of all known types, yet keeping the model space tractable even for the heaviest nuclear systems.

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arXiv2002

SO(5) as a Critical Dynamical Symmetry in the SU(4) Model of High-Temperature Superconductivity

An SU(4) model of high-temperature superconductivity and antiferromagnetism has recently been proposed. The SO(5) group employed by Zhang is embedded in this SU(4) as a subgroup, suggesting a connection between our SU(4) model and the Zhang SO(5) model. In order to understand the relationship between the the two models, we have used generalized coherent states to analyze the nature of the SO(5) subgroup. By constructing coherent-state energy surfaces, we demonstrate explicitly that the SU(4) supset SO(5) symmetry can be interpreted as a critical dynamical symmetry interpolating between superconducting and antiferromagnetic phases, and that this critical dynamical symmetry has many similarities to critical dynamical symmetries identified previously in other fields of physics. More generally, we demonstrate with this example that the mathematical techniques associated with generalized coherent states may have powerful applications in condensed matter physics because they provide a clear connection between microscopic many-body theories and their broken-symmetry approximate solutions. In addition, these methods may be interpreted as defining the most general Bogoliubov transformation subject to a Lie group symmetry constraint, thus providing a mathematical connection between algebraic formulations and the language of quasiparticle theory. Finally, we suggest that the identification of the SO(5) symmetry as a critical dynamical symmetry implies deep algebraic connections between high-temperature superconductors and seemingly unrelated phenomena in other field of physics.

cond-mat.supr-con↗
arXiv2002

Microscopic Description of Band Structure at Very Extended Shapes in the A ~ 110 Mass Region

Recent experiments have confirmed the existence of rotational bands in the A \~ 110 mass region with very extended shapes lying between super- and hyper-deformation. Using the projected shell model, we make a first attempt to describe quantitatively such a band structure in 108Cd. Excellent agreement is achieved in the dynamic moment of inertia J(2) calculation. This allows us to suggest the spin values for the energy levels, which are experimentally unknown. It is found that at this large deformation, the sharply down-sloping orbitals in the proton i_{13/2} subshell are responsible for the irregularity in the experimental J(2), and the wave functions of the observed states have a dominant component of two-quasiparticles from these orbitals. Measurement of transition quadrupole moments and g-factors will test these findings, and thus can provide a deeper understanding of the band structure at very extended shapes.

nucl-th↗
arXiv2001

SU(3) Symmetry and Scissors Mode Vibrations in Nuclei

We show that a nearly perfect SU(3) symmetry emerges from an extended Projected Shell Model. Starting from a deformed potential we construct separate bases for neutron and proton collective rotational states by exact angular momentum projection. These rotational states are then coupled by diagonalizing a residual pairing plus quadrupole interaction. The states obtained exhibit a one-to-one correspondence with an SU(3) spectrum up to high angular momentum and excitation, and their wave functions have a near-maximal overlap with the SU(3) states. They can also be classified as rotational bands built on spin-1$\hbar$ phonon excitations, which may correspond to a geometrical scissors mode and its generalizations. This work is a direct demonstration that numerical angular momentum projection theory extends the Elliott's original idea to heavy nuclear systems.

nucl-th↗
arXiv2000

An SU(4) Model of High-Temperature Superconductivity and Antiferromagnetism

We present an SU(4) model of high-temperature superconductivity having many similarities to dynamical symmetries known to play an important role in microscopic nuclear structure physics and in elementary particle physics. Analytical solutions in three dynamical symmetry limits of this model are found: an SO(4) limit associated with antiferromagnetic order; an SU(2) X SO(3) limit that may be interpreted as a d-wave pairing condensate; and an SO(5) limit that may be interpreted as a doorway state between the antiferromagnetic order and the superconducting order. The model suggests a phase diagram in qualitative agreement with that observed in the cuprate superconductors. The relationship between the present model and the SO(5) unification of superconductivity and antiferromagnetic order proposed by Zhang is discussed.

cond-mat.supr-con↗