Quasiparticle Picture Research Articles

TOPOLOGICAL PHASE TRANSITIONS IN STRONGLY CORRELATED FERMI SYSTEMS

Opportunities for topological phase transitions in strongly correlated Fermi systems near a quantum critical point are explored as an alternative to collective scenarios for experimentally observed departures from standard Fermi-liquid behavior. Attention is focused on a quantum critical point at which the effective mass is divergent due to vanishing of the quasiparticle group velocity at the Fermi surface. Working within the original Landau quasiparticle theory, it is demonstrated that the quasiparticle picture can remain meaningful beyond the quantum critical point through rearrangements of the unstable normal Fermi surface and quasiparticle spectrum. Two possibilities emerge at zero temperature, depending on whether the quasiparticle interaction is regular or singular at zero momentum transfer. In the regular case, one type of topological phase transformation leads to a state with a multiconnected Fermi surface. In the singular case, another type of topological phase transition leads to an exceptional state containing a fermion condensate – the Fermi surface swells into a volume in momentum space, within which partial occupation prevails and quasiparticle energies are pinned to the chemical potential. As the temperature increases from zero to a characteristic value Tm, a crossover can occur from the state with multiple Fermi surfaces to that containing a fermion condensate.

Quasiparticle Transformation during a Metal-Insulator Transition in Graphene

Here we show, with simultaneous transport and photoemission measurements, that the graphene-terminated SiC(0001) surface undergoes a metal-insulator transition upon dosing with small amounts of atomic hydrogen. We find the room temperature resistance increases by about 4 orders of magnitude, a transition accompanied by anomalies in the momentum-resolved spectral function including a non-Fermi-liquid behavior and a breakdown of the quasiparticle picture. These effects are discussed in terms of a possible transition to a strongly (Anderson) localized ground state.

General properties ofCePd1-xRhxat quantum critical point

The heavy-fermion metal CePd1� xRhx evolves from ferromagnetism at x ¼ 0 to a non-magnetic state at some critical concentration xc. Utilizing the quasiparticle picture and the concept of fermion condensation quantum phase transition (FCQPT), we address the non-Fermi liquid (NFL) behavior of ferromagnet CePd1� xRhx and show that it coincides with that of antiferromagnets YbRh2ðSi0:95Ge0:05Þ2 and CeCoIn5, paramagnets CeRu2Si2 and CeNi2Ge2 and 2D 3 He. We show that the NFL behavior is independent of the peculiarities of specific alloy and universal. Our study of the NFL behavior allows us to calculate the magnetoresistance of CeCoIn5. Obtained results are in good agreement with facts and reveal a new scaling behavior of magnetoresistance.

Low-energy effective theory of the toric code model in a parallel magnetic field

We determine analytically the phase diagram of the toric code model in a parallel magnetic field which displays three distinct regions. Our study relies on two high-order perturbative expansions in the strong- and weak-field limits, as well as a large-spin analysis. Calculations in the topological phase establish a quasiparticle picture for the anyonic excitations. We obtain two second-order transition lines that merge with a first-order line, giving rise to a multicritical point as recently suggested by numerical simulations. We compute the values of the corresponding critical fields and exponents that drive the closure of the gap. We also give the one-particle dispersions of the anyonic quasiparticles inside the topological phase.

Inhomogeneous quantum quenches

We study the problem of a quantum quench in which the initial state is the ground state ofan inhomogeneous Hamiltonian, in two different models, conformal field theory andordinary free field theory, which are known to exhibit thermalization of finite regions in thehomogeneous case. We derive general expressions for the evolution of the energy flow andcorrelation functions, as well as the entanglement entropy in the conformal case.Comparison of the results from the two approaches in the regime of their common validityshows agreement up to a point further discussed. Unlike for the thermal analogue, theevolution in our problem is non-diffusive and can be physically interpreted using anintuitive picture of quasiparticles emitted from the initial time hypersurface andpropagating semiclassically.

THE SURPRISING PHENOMENON OF LEVEL MERGING IN FINITE FERMI SYSTEMS

When applied to a finite Fermi system having a degenerate single-particle spectrum, the Landau-Migdal Fermi-liquid approach leaves room for the possibility that different single-particle energy levels merge with one another. It will be argued that the opportunity for this behavior exists over a wide range of strongly interacting quantum many-body systems. An inherent feature of the mergence phenomenon is the presence of nonintegral quasiparticle occupation numbers, which implies a radical modification of the standard quasiparticle picture. Consequences of this alteration are surveyed for nuclear, atomic, and solid-state systems.

Topology of the Fermi surface beyond the quantum critical point

We examine the nature of phase transitions occurring in strongly correlated Fermi systems at the quantum critical point (QCP) associated with a divergent effective mass. Conventional scenarios for the QCP involving collective degrees of freedom are shown to have serious shortcomings. Working within the original Landau quasiparticle picture, we propose an alternative topological scenario for the QCP, in systems that obey standard Fermi liquid (FL) theory in advance of the QCP. Applying the technique of Poincar\'e mapping, we analyze the sequence of iterative maps generated by the Landau equation for the single-particle spectrum at zero temperature. It is demonstrated that the Fermi surface is subject to rearrangement beyond the QCP. If the sequence of maps converges, a multi-connected Fermi surface is formed. If it fails to converge, the Fermi surface swells into a volume that provides a measure of entropy associated with formation of an exceptional state of the system characterized by partial occupation of single-particle states and dispersion of their spectrum proportional to temperature. Based on this dual scenario, the thermodynamics of Fermi systems beyond the QCP exhibits striking departures from the predictions of standard FL theory. Mechanisms for the release of the entropy excess of the exceptional state are discussed.

Complex Quasiparticle Band Structure Induced by Electron-Phonon Interaction: Band Splitting in the1×1H/W(110)Surface

We show that the self-consistent solution of the complex Dyson equation for the electron-phonon (EP) problem introduces many body effects which are often observed in photoemission experiments. The formalism is applied to the H covered W(110) surface, using first-principles results for the electronic and vibrational structure. We demonstrate that the measured spin-polarized surface band splitting [Phys. Rev. Lett. 84, 2925 (2000)10.1103/PhysRevLett.84.2925; 89, 216802 (2002)] can be traced back to different quasiparticle (QP) states induced by EP coupling. Despite the breakdown of the single QP picture, the spectral functions are very well represented by the predicted multiple QP structure.

Spin wave dispersion based on the quasiparticle self-consistentGW method: NiO,MnO and α-MnAs

We present calculations of spin wave dispersions for antiferromagnetic MnO, NiO and ferromagneticα-MnAs based on the recently developed quasiparticle self-consistentGW method(QSGW); we have alreadyshown that QSGW gave a good quasiparticle picture for MnO and NiO in comparison withoptical experiments. To obtain the spin wave dispersions, we have developeda method to calculate the transverse dynamical spin susceptibility in therandom-phase approximation. This is a general method applicable not only toQSGW, but also to any first-principle method which gives the non-interacting (one-body)Hamiltonian to represent quasiparticles, e.g. the Kohn–Sham Hamiltonian in the densityfunctional theory. In the method, we first calculate the non-interacting spin susceptibilityfrom the supplied non-interacting Hamiltonian; then we obtain the spin susceptibilitywhere the size of the effective interaction is determined so as to satisfy a sum rule. ForMnO and NiO, the obtained spin wave dispersions show good agreement with experiments,in contrast to the cases in the local density approximation (LDA) and in theLDA+U. These results support our claim that the independent-particle picture is powerful enough even formaterials like NiO and MnO classified to the Mott insulator, that is, the quasiparticle pictures byQSGW work well to describe their linear responses. Forα-MnAs, we find a collinear ferromagnetic ground state inQSGW, while this phase is unstable in the LDA.

Flattening of single-particle spectra in strongly correlated electron systems and the violation of the Wiedemann-Franz law

The renormalization of the Wiedemann-Franz (WF) ratio in strongly correlated electron systems is analyzed within the Landau quasiparticle picture. We demonstrate that the WF law is violated: (i) at the quantum critical point, where the effective mass diverges, and (ii) beyond a point of fermion condensation, where the single-particle spectrum ∈(p) becomes flat. Results of the analysis are compared with available experimental data.

A Novel Quasi-Particle Picture of Massless and Massive Quarks Coupled with a Massive Boson at Finite Temperature and Its Effect on the Dilepton Production Rate

We investigate the spectral properties of massless and massive quarks coupled with scalar and pseudo-scalar bosons at finite temperature in Yukawa models at the one-loop order. We show that the quak spectral function has a three-peak structure even for a finite but small quark mass. The behavior of the spectral function and the pole structure of the propagator are analyzed as functions of temperature and masses of the quark and bosons. Effects of the anomalous quark spectrum on the dilepton pruduction rate are briefly mentioned.

Quantum critical point in [formula omitted] ferromagnet

The heavy-fermion metal CePd 1 - x Rh x can be tuned from ferromagnetism at x = 0 to non-magnetic state at the critical concentration x c . The non-Fermi liquid behavior at x ≃ x c is recognized by power law dependence of the specific heat C ( T ) given by the electronic contribution, susceptibility χ ( T ) and volume expansion coefficient α ( T ) at low temperatures: C / T ∝ χ ( T ) ∝ α ( T ) / T ∝ 1 / T . We show that this alloy exhibits a universal thermodynamic non-Fermi liquid behavior independent of magnetic ground state. This can be well understood utilizing the quasiparticle picture and the concept of fermion condensation quantum phase transition at the density ρ = p F 3 / 3 π 2 = ρ FC , p F is Fermi momentum.

Merging of single-particle levels and non-Fermi-liquid behavior of finite Fermi systems

Within the Landau quasiparticle approach, we analyze the possibility of merging of different single-particle levels in finite spherical Fermi systems. It is demonstrated that the opportunity for this behavior is widespread in quantum many-body systems. The salient feature of the phenomenon is the occurrence of nonintegral quasiparticle occupation numbers, leading to a radical alteration of the standard quasiparticle picture. Implications of this alteration are considered for atomic and solid-state systems.

FURTHER ANALYSIS OF EXCITATIONS OF QUARKS AT FINITE TEMPERATURE — MASS EFFECT AND POLE STRUCTURE

We calculate the spectral function of the massive quark at finite temperature ( T ) using a Yukawa model and show that the peak in the negative energy region among the three-peaks found in a previous work for the massless quark is largely suppressed. To explore the underlying mechanism of this behavior, we also investigate the pole structure of the retarded Green function of the quark. We will show the result only for the massless quark. We find the residues of the poles corresponding the three-peaks are all comparable at T ∼ mb. We also show that the multi-peak structure of the quark spectra is well described in the pole approximation which indicates that the quasi-particle picture is valid in this T region.

Calculation of Photoelectron Spectra of Molybdenum and Tungsten Complexes Using Green's Functions Methods

Green's functions calculations are presented for several complexes of molybdenum and tungsten, two metals that are similar structurally but display subtle, but significant, differences in electronic structure. Outer valence Green's functions IPs for M(CO)6, M(Me)6, MH6, [MCl4O](-), and [MO4](-) (M = Mo, W) are generally within +/-0.2 eV of available experimental photoelectron spectra. The calculations show that electrons in M-L bonding orbitals are ejected at lower energies for Mo while the detachment energy for electrons in d orbitals varies with metal and complex. For the metal carbonyls, the quasiparticle picture assumed in OVGF breaks down for the inner valence pi CO molecular orbitals due to the coupling of two-hole-one-particle charge transfer states to the one-hole states. Incorporation of the 2h1p states through a Tamm-Dancoff approximation calculation accurately represents the band due to detachment from these molecular orbitals. Though the ordering of IPs for Green's functions methods and DFT Koopmans' theorem IPs is similar for the highest IPs for most compounds considered, the breakdown of the quasiparticle picture for the metal carbonyls suggests that scaling of the latter values may result in a fortuitous or incorrect assignment of experimental VDEs.

Sum-over-states versus quasiparticle pictures of coherent correlation spectroscopy of excitons in semiconductors: Femtosecond analogs of multidimensional NMR

Two-dimensional correlation spectroscopy 2DCS, based on the nonlinear optical response of excitons to sequences of ultrafast pulses may provide some unique insights into carrier dynamics in semiconductors. The most prominent feature of 2DCS, cross peaks, can best be understood using a sum-over-states picture involving the many-body eigenstates. However, the optical response of semiconductors is usually calculated by solving truncated equations of motion for dynamical variables, which result in a quasiparticle picture. In this work we derive Green’s function expressions for the four wave mixing signals generated in various phase-matching directions and use them to establish the connection between the two pictures. The formal connection with Frenkel excitons hard-core bosons and vibrational excitons soft-core bosons is pointed out.

Universal behavior of CePd1−x Rh x ferromagnet at the quantum critical point

The heavy-fermion metal CePd1−x Rh x can be tuned from ferromagnetism at x = 0 to the nonmagnetic state at some critical concentration x c . The non-Fermi liquid behavior (NFL) at x ≃ x c is recognized by the power-law dependence of the specific heat C(T) given by the electronic contribution susceptibility X(T) and volume expansion coefficient α(T) at low temperatures: C/T ∝ X(T) ∝ α(T)/T∝ 1/ √T. We also demonstrate that the behavior of the normalized effective mass M *N observed in CePd1−x Rh x at x ≃ 0.8 agrees with that of M * N observed in paramagnetic CeRu2Si2 and conclude that these alloys exhibit the universal NFL thermodynamic behavior at their quantum critical points. We show that the NFL behavior of CePd1−x Rh x can be accounted for within the frameworks of the quasiparticle picture and fermion condensation quantum phase transition, while this alloy exhibits a universal thermodynamic NFL behavior that is independent of the characteristic features of the given alloy such as its lattice structure, magnetic ground state, dimension, etc.

Renormalized quasiparticles in antiferromagnetic states of the Hubbard model

We analyze the properties of the quasiparticle excitations of metallic antiferromagnetic states in a strongly correlated electron system. The study is based on dynamical mean field theory (DMFT) for the infinite dimensional Hubbard model with antiferromagnetic symmetry breaking. Self-consistent solutions of the DMFT equations are calculated using the numerical renormalization group (NRG). The low energy behavior in these results is then analyzed in terms of renormalized quasiparticles. The parameters for these quasiparticles are calculated directly from the NRG derived self-energy, and also from the low energy fixed point of the effective impurity. They are found to be in good agreement. We show that the main low energy features of the $\bf k$-resolved spectral density can be understood in terms of the quasiparticle picture. We also find that Luttinger's theorem is satisfied for the total electron number in the doped antiferromagnetic state.

Merging of Single-Particle Levels and Non-Fermi-Liquid Behavior of Finite Fermi Systems

We examine the problem of finite Fermi systems having a degenerate single-particle spectrum and show that the Landau approach, applied to such a system, admits the possibility of merging single-particle levels. It is demonstrated that the opportunity for this behavior is widespread in quantum many-body systems. The salient feature of the phenomenon is the occurrence of nonintegral quasiparticle occupation numbers, leading to a radical alteration of the standard quasiparticle picture. Implications of this alteration are considered for nuclear, atomic, and solid-state systems.

Quasiparticle picture of quarks near chiral transition at finite temperature

We investigate, using a chiral effective model, the quark spectrum in the critical region of the chiral transition focusing on the effect of the possible mesonic excitations in the quark-gluon plasma phase. We find that there appear a novel three-peak structure in the quark spectra. We elucidate the mechanism of the appearance of the multi-peak structure with the help of a Yukawa model with an elementary boson.