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The development of van der Waals heterostructures has introduced unconventional phenomena that emerge at atomically precise interfaces. For example, interlayer excitons in two-dimensional transition metal dichalcogenides show intriguing optical properties at low temperatures. Here we report on room-temperature observation of interface excitons in mixed-dimensional heterostructures consisting of two-dimensional tungsten diselenide and one-dimensional carbon nanotubes. Bright emission peaks originating from the interface are identified, spanning a broad energy range within the telecommunication wavelengths. The effect of band alignment is investigated by systematically varying the nanotube bandgap, and we assign the new peaks to interface excitons as they only appear in type-II heterostructures. Room-temperature localization of low-energy interface excitons is indicated by extended lifetimes as well as small excitation saturation powers, and photon correlation measurements confirm single-photon emission. With mixed-dimensional van der Waals heterostructures where band alignment can be engineered, new opportunities for quantum photonics are envisioned., Comment: 8 pages, 4 figures

Nanomaterials exhibit unique optical phenomena, in particular excitonic quantum processes occurring at room temperature. The low dimensionality, however, imposes strict requirements for conventional optical excitation, and an approach for bypassing such restrictions is desirable. Here we report on exciton transfer in carbon-nanotube/tungsten-diselenide heterostructures, where band alignment can be systematically varied. The mixed-dimensional heterostructures display a pronounced exciton reservoir effect where the longer-lifetime excitons within the two-dimensional semiconductor are funneled into carbon nanotubes through diffusion. This new excitation pathway presents several advantages, including larger absorption areas, broadband spectral response, and polarization-independent efficiency. When band alignment is resonant, we observe substantially more efficient excitation via tungsten diselenide compared to direct excitation of the nanotube. We further demonstrate simultaneous bright emission from an array of carbon nanotubes with varied chiralities and orientations. Our findings show the potential of mixed-dimensional heterostructures and band alignment engineering for energy harvesting and quantum applications through exciton manipulation., Comment: 9 pages, 4 figures

We have developed a simple and straightforward way to realize controlled post-doping towards 2D transition metal dichalcogenides (TMDs). The key idea is to use low-kinetic energy dopant beams and a high-flux chalcogen beam at the same time, leading to substitutional doping with controlled dopant densities. Atomic-resolution transmission electron microscopy has revealed that dopant atoms injected toward TMDs are incorporated substitutionally into the hexagonal framework of TMDs. Electronic properties of doped TMDs (Nb-doped WSe2) have shown drastic change, p-type action with more than two orders of magnitude increase in on current. Position-selective doping has also been demonstrated by the post-doping toward TMDs with a patterned mask on the surface. The post-doping method developed in this work can be a versatile tool for 2D-based next-generation electronics in the future.

In a nucleus which has two Hartree states deformed in quadrupole symmetry, i.e., prolate state and oblate state, the nuclear residual interaction derived in the present theory beyond the Hartree approximation acts as the restoring force for the spherical symmetry of the nuclear system to be recovered so that the deformed nucleus makes collective tunneling transitions between prolate states and oblate states. We derive the Hamiltonian that is effective not only for the Hartree states but also for the collective tunneling transitions between the Hartree states. Solving the secular equation for the effective Hamiltonian on the basis states expressed in terms of SU(5) algebra, we analyze the nuclear collective tunneling transitions between a prolate state and an oblate state in the real-time description. The collective tunneling transitions are discussed to proceed through tri-axially asymmetric deformed states., Comment: LaTex, 12 pages, 7 postscript figures

We derive the linear Langevin equation that describes the behavior of the fluctuations of the order parameter of the chiral phase transition above the critical temperature by applying the projection operator method to the Nambu-Jona-Lasinio model at finite temperature and density. The Langevin equation relaxes exhibiting oscillation, reveals thermalization and converges to the equilibrium state consistent with the mean-field approximation as time goes on. With the help of this Langevin equation, we further investigate the relaxation of the critical fluctuations. The relaxation time of the critical fluctuations increases at speed as the temperature approaches toward the critical temperature because of the critical slowing down. The critical slowing down is enhanced as the chemical potential increases because of the Pauli blocking. Furthermore, we find another enhancement of the critical slowing down around the tricritical point., Comment: Style file is changed, references are added, 42 pages, 14 figures, Nucl. Phys. A in press

We derive the linear Langevin equation that describes the behavior of critical fluctuation above the critical temperature of the chiral phase transition in the Nambu-Jona-Lasinio model. The Langevin equation relaxes exhibiting oscillation and shows thermalization. The relaxation becomes slower as the system approaches the critical point(the critical slowing down). The time correlation function also calculated using the Langevin equation shows that there exists the same soft mode as the previous calculation in the linear response theory., Comment: 4 pages, 3 figures

We apply the projection operator method (POM) to $\phi^4$ theory and derive both quantum and semiclassical equations of motion for the soft modes. These equations have no time-convolution integral term, in sharp contrast with other well-known results obtained using the influence functional method (IFM) and the closed time path method (CTP). However, except for the fluctuation force field terms, these equations are similar to the corresponding equations obtained using IFM with the linear harmonic approximation, which was introduced to remove the time-convolution integral. The quantum equation of motion in POM can be regarded as a kind of quantum Langevin equation in which the fluctuation force field is given in terms of the operators of the hard modes. These operators are then replaced with c-numbers using a certain procedure to obtain a semiclassical Langevin equation. It is pointed out that there are significant differences between the fluctuation force fields introduced in this paper and those introduced in IFM. The arbitrariness of the definition of the fluctuation force field in IFM is also discussed., Comment: 35pages,2figures, Prog. Theor. Phys. Vol. 107 No. 5 in press

Current quark mass effects on the chiral phase transition of QCD is studied in the improved ladder approximation. An infrared behavior of the gluon propagator is modified in terms of an effective running coupling. The analysis is based on a composite operator formalism and a variational approach. We use the Schwinger-Dyson equation to give a ``normalization condition'' for the Cornwall-Jackiw-Tomboulis effective potential and to isolate the ultraviolet divergence which appears in an expression for the quark-antiquark condensate. We study the current quark mass effects on the order parameter at zero temperature and density. We then calculate the effective potential at finite temperature and density and investigate the current quark mass effects on the chiral phase transition. We find a smooth crossover for $T>0$, $\mu=0$ and a first-order phase transition for $\mu>0$, T=0. Critical exponents are also studied and our model gives the classical mean-field values. We also study the temperature dependence of masses of scalar and pseudoscalar bosons. A critical end point in the $T$-$\mu$ plane is found at $T \sim 100$ MeV, $\mu \sim 300$ MeV., Comment: 19 pages, 13 figures

We derive a new expansion of the Heisenberg equation of motion based on the projection operator method proposed by Shibata, Hashitsume and Shing\=u. In their projection operator method, a certain restriction is imposed on the initial state. As a result, one cannot prepare arbitrary initial states, for example a coherent state, to calculate the time development of quantum systems. In this paper, we generalize the projection operator method by relaxing this restriction. We explain our method in the case of a Hamiltonian both with and without explicit time dependence. Furthermore, we apply it to an exactly solvable model called the damped harmonic oscillator model and confirm the validity of our method., Comment: 16 pages, revtex, no figures, To appear in Prog.Theor.Phys

The chiral phase transition at finite temperature T and/or chemical potential $\mu$ is studied using the QCD-like theory with a variational approach. The ``QCD-like theory'' means the improved ladder approximation with an infrared cutoff in terms of a modified running coupling. The form of Cornwall-Jackiw-Tomboulis effective potential is modified by the use of the Schwinger-Dyson equation for generally nonzero current quark mass. We then calculate the effective potential at finite T and/or $\mu$ and investigate the phase structure in the chiral limit. We have a second-order phase transition at $T_c=129$ MeV for $\mu=0$ and a first-order one at $\mu_c=422$ MeV for T=0. A tricritical point in the T-$\mu$ plane is found at T=107 MeV, $\mu=210$ MeV. The position is close to that of the random matrix model and some version of the Nambu-Jona-Lasinio model., Comment: 10 pages, 6 figures. Accepted for publication in Physical Review D

The chiral phase transition at high temperature is investigated using the effect ive potential in the framework of the QCD-like gauge theory with a variational a pproach. We have a second order phase transition at $T_c=136$MeV. We also investigate numerically the temperature dependence of condensate, $f_\pi$ a nd $a_2(T)$(coefficient of the quadratic term in the effective potential) and es timate the critical exponents of these quantities., Comment: 12 pages,7 figures

A new statistical model for multiparticle production in $e^+e^-$ annihilation is proposed based on the idea of the longitudinal phase space with limited transverse momentum. The longitudinal rapidity space is divided into cells of equal size in order to take into account the Bose-Einstein correlations(BEC) with a finite correlation length $\delta y$. The maximum entropy method is used to determine the probability distributions of the final state pions (or $\rho$ mesons) for a given mean multiplicity, mean transverse momentum and mean total energy. Event simulation based on our model is performed in two extreme cases, a $\pi$-model and a $\rho$-model. The $\pi$-model assumes that only pions are produced directly. On the other hand, the $\rho$-model assumes that only $\rho$ mesons are produced directly and they decay into pions. A good overall fit to experimental data from PETRA to LEP energy regions is obtained for $\delta y$ ranging from 0.6 to 1.2 in the $\pi$-model. We found that the BEC plays a very important role to reproduce various correlation data. In some correlation data, resonance effect and conservation laws are also important., Comment: 42pages including 20figures. submitted to Physical Reviw D

We study the time development of correlation functions at both zero and finite temperature with Shibata-Hashitsume's projection operator method and carry out the renormalization of ultraviolet divergence that appears in a time-dependent frequency shift, using a mass counter term. A harmless divergence of log t-type remains at an initial time t=0 after the lowest order renormalization., Comment: 9 pages, 4 figures, RevTeX

We study the nuclear stopping in high energy nuclear collisions using the constituent quark model. It is assumed that wounded nucleons with different number of interacted quarks hadronize in different ways. The probabilities of having such wounded nucleons are evaluated for proton-proton, proton-nucleus and nucleus-nucleus collisions. After examining our model in proton-proton and proton-nucleus collisions and fixing the hadronization functions, it is extended to nucleus-nucleus collisions. It is used to calculate the rapidity distribution and the rapidity shift of final state protons in nucleus-nucleus collisions. The computed results are in good agreement with the experimental data on $^{32}\mbox{S} +\ ^{32}\mbox{S}$ at $E_{lab} = 200$ AGeV and $^{208}\mbox{Pb} +\ ^{208}\mbox{Pb}$ at $E_{lab} = 160$ AGeV. Theoretical predictions are also given for proton rapidity distribution in $^{197}\mbox{Au} +\ ^{197}\mbox{Au}$ at $\sqrt{s} = 200$ AGeV (BNL-RHIC). We predict that the nearly baryon free region will appear in the midrapidity region and the rapidity shift is $\langle \Delta y \rangle = 2.22$., Comment: 40 pages, 16 Postscript figures, submitted to Phys. Rev. D