arXiv subjects

Zhiguang Xiao

Publications and source records attributed to Zhiguang Xiao.

At least 37 records · Page 2Linked to original sources

arXiv2021

Bandwidth-control orbital-selective delocalization of 4f electrons in epitaxial Ce films

The 4f-electron delocalization plays a key role in the low-temperature properties of rare-earth metals and intermetallics, including heavy fermions and mix-valent compounds, and is normally realized by the many-body Kondo coupling between 4f and conduction electrons. Due to the large onsite Coulomb repulsion of 4f electrons, the bandwidth-control Mott-type delocalization, commonly observed in d-electron systems, is difficult in 4f-electron systems and remains elusive in spectroscopic experiments. Here we demonstrate that the bandwidth-control orbital-selective delocalization of 4f electrons can be realized in epitaxial Ce films by thermal annealing, which results in a metastable surface phase with a reduced layer spacing. The resulting quasiparticle bands exhibit large dispersion with exclusive 4f character near E_F and extend reasonably far below the Fermi energy, which can be explained from the Mott physics. The experimental quasiparticle dispersion agrees surprisingly well with density-functional theory calculation and also exhibits unusual temperature dependence, which could be a direct consequence of the delicate interplay between the bandwidth-control Mott physics and the coexisting Kondo hybridization. Our work therefore opens up the opportunity to study the interaction between two well-known localization-delocalization mechanisms in correlation physics, i.e., Kondo vs Mott, which can be important for a fundamental understanding of 4f-electron systems.

cond-mat.str-el↗

arXiv2020

Relativistic Friedrichs-Lee model and quark-pair creation model

In this paper, we present how the Friedrichs-Lee model could be extended to the relativistic scenario and be combined with the relativistic quark pair creation model in a consistent way. This scheme could be applied to study the "unquenched" effect of the meson spectra. As an example, if the lowest $J^{PC}=0^{++}$ $(u\bar u+d\bar d)/\sqrt{2}$ bound state in the potential model is coupled to the $ππ$ continuum, two resonance poles could be found from the scattering amplitude for the continuum states. One of them could correspond to the $f_0(500)/σ$ and the other probably $f_0(1370)$. This scheme might shed more light on why extra states could appear in the hadron spectrum other than the prediction of the quark potential model.

hep-ph↗

arXiv2020

Decays of $X(3872)$ to $χ_{cJ}π^0$ and $J/ψπ^+π^-$

By describing the $X(3872)$ using the extended Friedrichs scheme, in which $D\bar D^*$ is the dominant component, we calculate the decay rates of the $X(3872)$ to $π^0$ and a $P$-wave charmonium $χ_{cJ}$ state with $J=0,1$, or $2$, and its decays to $J/ψπ^+π^-$ where $π^+π^-$ are assumed to be produced via an intermediate $ρ$ state. The decay widths of $X(3872)\toχ_{cJ}π^0$ for $J=0,1,2$ are of the same order. However, this model calculation exhibits that the decay rate of $X(3872)$ to $χ_{c1}π^0$ is one order of magnitude smaller than its decay rate to $J/ψπ^+π^-$.

hep-ph↗

arXiv2020

Does the bottomonium counterpart of $X(3872)$ exist?

A narrow line shape peak at about 10615 MeV, just above the threshold in the $B\bar B^*$ channel, which can be regarded as the signal of bottomonium counterpart of $X(3872)$, $X_b$, is predicted by using the extended Friedrichs scheme. Though a virtual state is found at about 10593 MeV in this scheme, we point out that the peak is contributed mainly by the coupling form factor, which comes from the convolution of the interaction term and meson wave functions including the one from $χ_{b1}(4P)$, but not mainly by the virtual-state pole. In this picture, the reason why $X_b$ signal is not observed in the $Υπ^+π^-$ and $Υπ^+π^-π^0$ channels can also be understood. The $χ_{b1}(4P)$ mass and width are found to be about 10771 MeV and 6 MeV, respectively and a dynamically generated broad resonance is also found with its mass and width at about 10672 MeV and 78 MeV, respectively. The line shapes of these two states are also affected by the form factor effect. Thus, this study also emphasizes the importance of the structure of the wave functions of high radial excitations in the analysis of the line shapes, and provides a caveat that some signals may be generated from the structures of the form factors rather than from poles.

hep-ph↗

arXiv2019

Possible molecular states in $B^{()}B^{()}$ scatterings

We present that, if unitarizing the $B^{(*)}B^{(*)}$ scattering amplitudes in the constituent interchange model, one can find two bound state poles for $(I_{tot},S_{tot})=(0,1)$ $BB^*$ and $B^*B^*$ system, which corresponds to two $I(J^P)=0(1^+)$ doubly bottomed molecular states. Furthermore, it is noticed that the virtual states in $(1,0)$ $BB$, $(1,1)$ $BB^*$, $(1,0)$ $B^*B^*$, and $(1,2)$ $B^*B^*$ systems could produce enhancements of the module squares of the scattering $T$-matrix just above the related thresholds, which might correspond to $I(J^P)=1(0^+)$, $1(1^+)$, and $1(2^+)$ doubly bottomed molecular states, respectively. The calculation may be helpful for searching for the doubly bottomed molecular state in future experiments.

hep-ph↗

arXiv2019

Understanding $X(3872)$ and its decays in the extended Friedrichs scheme

We present that the $X(3872)$ could be represented as a dynamically generated state in the extended Friedrichs scheme, in which the ratio of "elementariness" and "compositeness" of the different components in the $X(3872)$ is about $Z_{c\bar c}:X_{\bar D^0 D^{0*}}: X_{ D^+ D^{-*}}: X_{\bar D^* D^*}$ $= 1:(2.67\sim 8.85):(0.45\sim 0.46):0.04$. Furthermore, its decays to $π^0$ and a $P$-wave charmonium $χ_{cJ}$ state with $J=0,1$, or $2$, $J/ψπ^+π^-$, and $J/ψπ^+π^-π^0$ could be calculated out with the help of Barnes-Swanson model. The isospin breaking effects is easily understood in this scheme. This calculation also shows that the decay rate of $X(3872)$ to $χ_{c1}π^0$ is much smaller than its decay rate to $J/ψπ^+π^-$.

hep-ph↗

arXiv2019

Strong band kinks in magic-thickness Yb films arising from interfacial electron-phonon coupling

Interfacial electron-phonon coupling in ultrathin films has attracted much interest; it can give rise to novel effects and phenomena, including enhanced superconductivity. Here we report an observation of strong kinks in the energy dispersions of quantum well states in ultrathin Yb films grown on graphite. These kinks, arising from interfacial electron-phonon coupling, are most prominent for films with a preferred ("magic") thickness of 4 monolayers, which are strained and hole doped by the substrate. The energy position of the kinks agrees well with the optical phonon energy of graphite, and the extracted electron-phonon coupling strength λ shows a large subband dependence, with a maximum value up to 0.6. The kinks decay rapidly with increasing film thickness, consistent with its interfacial origin. The variation of λ is correlated with evolution of the electronic wave function amplitudes at the interface. A Lifshitz transition occurs just beyond the magic thickness where the heavy Yb 5d bands begin to populate right below the Fermi level.

cond-mat.mes-hall↗

arXiv2017

Comprehending Isospin breaking effects of $X(3872)$ in a Friedrichs-model-like scheme

Recently, we have shown that the $X(3872)$ state can be naturally generated as a bound state by incorporating the hadron interactions into the Godfrey-Isgur quark model using the Friedrichs-like model combined with the QPC model, in which the wave function for the $X(3872)$ as a combination of the bare $c\bar c$ state and the continuum states can also be obtained. Under this scheme, we now investigate the isospin breaking effect of $X(3872)$ in its decays to $J/ψπ^+π^-$ and $J/ψπ^+π^-π^0$. By Considering its dominant continuum parts coupling to $J/ψρ$ and $J/ψω$ through the quark rearrangement process, one could obtain the reasonable ratio of $\mathcal{B}(X(3872)\rightarrow J/ψπ^+π^-π^0)/\mathcal{B}(X(3872)\rightarrow J/ψπ^+π^-)\simeq (0.58\sim 0.92)$. It is also shown that the $\bar D D^*$ invariant mass distributions in the $B\rightarrow \bar D D^* K$ decays could be understood qualitatively at the same time. This scheme may provide more insight to understand the enigmatic nature of the $X(3872)$ state.

hep-ph↗

arXiv2017

Understanding $X(3862)$, $X(3872)$, and $X(3930)$ in a Friedrichs-model-like scheme

We developed a Friedrichs-model-like scheme in studying the hadron resonance phenomenology and present that the hadron resonances might be regarded as the Gamow states produced by a Hamiltonian in which the bare discrete state is described by the result of usual quark potential model and the interaction part is described by the quark pair creation model. In a one-parameter calculation, the $X(3862)$, $X(3872)$, and $X(3930)$ state could be simultaneously produced with a quite good accuracy by coupling the three P-wave states, $χ_{c2}(2P)$, $χ_{c1}(2P)$, $χ_{c0}(2P)$ predicted in the Godfrey-Isgur model to the $D\bar D$, $D\bar D^{*}$, $D^*\bar D^*$ continuum states. At the same time, we predict that the $h_c(2P)$ state is at about 3902 MeV with a pole width of about 54 MeV. In this calculation, the $X(3872)$ state has a large compositeness. This scheme may shed more light on the long-standing problem about the general discrepancy between the prediction of the quark model and the observed values, and it may also provide reference for future search for the hadron resonance state.

hep-ph↗

arXiv2017

Partial Wave Decomposition in Friedrichs Model With Self-interacting Continua

We consider the nonrelativistic model of coupling bare discrete states with continuum states in which the continuum states can have interactions among themselves. By partial-wave decomposition and constraint to the conserved angular momentum eigenstates, the model can be reduced to Friedrichs-like model with additional interactions between the continua. If a kind of factorizable form factor is chosen, the model can be exactly solvable, that is, the generalized discrete eigenstates including bound states, virtual states, and resonances, can all be represented using the original bare states, and so do the in-state and out-state. The exact $S$ matrix is thus obtained. We then discuss the behaviors of the dynamically generated $S$-wave and $P$-wave discrete states as the coupling is varying when there is only one self-interacting bare continuum state. We find that even when the potential is repulsive there could also be resonances and virtual states. In the $P$-wave cases with attractive interaction, we find that when there is a near-threshold bound state, there will always be an accompanying virtual state and we also give a more general argument of this effect.

hep-ph↗

arXiv2017

On Friedrichs Model with Two Continuum States

The Friedrichs model with one discrete state coupled to more than one continuum is studied. The exact eigenstates for the full Hamiltonian can be solved explicitly. The discrete state is found to generate more than one virtual state pole or more than one pair of resonance poles in different Riemann sheets in different situations. The form factors could also generate new states on different sheets. All these states can appear in the generalized completeness relation.

hep-ph↗

arXiv2016

Virtual states and generalized completeness relation in the Friedrichs Model

We study the well-known Friedrichs model, in which a discrete state is coupled to a continuum state. By examining the pole behaviors of the Friedrichs model in a specific form factor thoroughly, we find that, in general, when the bare discrete state is below the threshold of the continuum state, there should also be a virtual-state pole accompanying the bound-state pole originating from the bare discrete state as the coupling is turned on. There are also other second-sheet poles originating from the singularities of the form factor. We give a general argument for the existence of these two kinds of states. As the coupling is increased to a certain value, the second-sheet poles may merge and become higher-order poles. We then discuss the completeness relations incorporating bound states, virtual states, and resonant states corresponding to higher-order poles.

hep-ph↗

arXiv2015

Phase structures of the black D$p$-D$(p+4)$-brane system in various ensembles I: thermal stability

When the D$(p+4)$-brane ($p=0,1,2$) with delocalized D$p$ charges is put into equilibrium with a spherical thermal cavity, the two kinds of charges can be put into canonical or grand canonical ensemble independently by setting different conditions at the boundary. Using the thermal stability condition, we discuss the phase structures of various ensembles of this system formed in this way and find out the situations that the black brane could be the final stable phase in these ensembles. In particular, van der Waals-like phase transitions can happen when D0 and D4 charges are in different kinds of ensembles. Furthermore, our results indicate that the D$(p+4)$-branes and the delocalized D$p$-branes are equipotent.

hep-th↗

arXiv2015

Phase structures of the black D$p$-D$(p + 4)$-brane system in various ensembles II: electrical and thermodynamic stability

By incorporating the electrical stability condition into the discussion, we continue the study on the thermodynamic phase structures of the D$p$-D$(p + 4)$ black brane in GG, GC, CG, CC ensembles defined in our previous paper arXiv:1502.00261. We find that including the electrical stability conditions in addition to the thermal stability conditions does not modify the phase structure of the GG ensemble but puts more constraints on the parameter space where black branes can stably exist in GC, CG, CC ensembles. In particular, the van der Waals-like phase structure which was supposed to be present in these ensembles when only thermal stability condition is considered would no longer be visible, since the phase of the small black brane is unstable under electrical fluctuations. However, the symmetry of the phase structure by interchanging the two kinds of brane charges and potentials is still preserved, which is argued to be the result of T-duality.

hep-th↗

arXiv2015

Could the $X(3915)$ and the $X(3930)$ Be the Same Tensor State?

By using a combined amplitude analysis of the $γγ\rightarrow D\bar{D}$ and $γγ\rightarrow J/ψω$ data, we demonstrate that the $X(3915)$, which is quoted as a $J^{PC}=0^{++}$ state in the Particle Data Group table, is favored by the data to be a $J^{PC}=2^{++}$ state appearing in both channels, which means that the $X(3915)$ and the $X(3930)$ can be regarded as the same $J^{PC}=2^{++}$ state. Meanwhile, the data also prefer a large helicity-0 contribution of this tensor resonance to the amplitudes instead of the helicity-2 dominance assumed by {\it BABAR}, which may indicate a sizable portion of non-$q\bar q$ components in this state. Identifying the $X(3915)$ with the $X(3930)$ and abandoning the helicity-2 dominance for this tensor state are helpful for the further understandings of the properties of this state and also of the mysterious "$XYZ$" charmoniumlike resonances.

hep-ph↗

arXiv2015

Distinguishing cusp effects and near-threshold-pole effects

We make use of a unitarized coupled-channel model to analyze the mass distribution data of final states in production processes of $X(4260)$. By analyzing the analytical structures of the decay amplitudes, we find that the line shape of $Z_c(3900)$ signal is related to the combined effect of a pair of near-threshold "shadow" poles and the $(D\bar{D}^*)^\pm$ thresholds, in which the third-sheet pole might provide a dominant contribution. As all the coupled channels effects are tuning off, the trajectories of these two poles suggest that the $Z_c(3900)$ might originate from the attractive interaction of $(D^*\bar{D}^*)^\pm$ through a long-distance interaction, $e.g.$ $π$-exchange interaction, as a "deuteron-like" state. There is no nearby pole structure corresponding to the $Z_c(4025)$ signal in the $(D^*\bar{D}^*)^\pm$ mass distribution.

hep-ph↗

arXiv2014

Baryons in the Sakai-Sugimoto model in the D0-D4 background

The baryon spectrum in the Sakai-Sugimoto model in the D4 background with smeared D0 charges is studied. We follow the instanton description of baryons by Hata et al.[Prog. Theor. Phys. 117, 1157]. The background corresponds to an excited state with nonzero glue condensate $\langle {\rm tr} (F_{μν}\tilde F^{μν})\rangle$ which is proportional to the D0 charge density. The baryon size shrinks when we turn on small D0 charge density. But for larger D0 charge density where massive modes in the gauge theory may also take effect, the size of baryons will grow. The difference between baryon masses will become smaller when D0 charge density increases. There may also be indications that the baryon will become unstable and cannot exist for sufficiently large D0 density.

hep-th↗

arXiv2013

Comprehending heavy charmonia and their decays by hadron loop effects

We present that including the hadron loop effects could help us to understand the spectrum of the heavier charmonium-like states and their decays simultaneously. The observed states could be represented by the poles on the complex energy plane. By coupling to the opened thresholds, the pole positions are shifted from the bare states predicted in the quenched potential model to the complex energy plane. The pole masses are generally pulled down from the bare masses and the open-charm decay widths are related to the imaginary parts of the pole positions. Moreover, we also analyze the pole trajectory of the $χ_{c1}(2P)$ state while the quark pair production rate from the vacuum changes in its uncertainty region, which indicates that the enigmatic X(3872) state may be regarded as a $1^{++}$ $c\bar{c}$ charmonium-dominated state dressed by the hadron loops as the others.

hep-ph↗