A tensor-network variational formalism of thermofield dynamics is introduced. The formalism relates the original Hilbert space with its tilde space by a product of two copies of a tensor network. Then, their interface becomes an event horizon, and the logarithm of the tensor rank corresponds to the black hole entropy. Eventually, a multiscale entanglement renormalization ansatz reproduces an anti-de Sitter black hole at finite temperature. Our finding shows rich functionalities of multiscale entanglement renormalization ansatz as efficient graphical representation of AdS/CFT correspondence.

We present a compelling response of a low-dimensional strongly correlated system to an external perturbation. Using the time-dependent Lanczos method we investigate a nonequilibrium evolution of the half-filled one-dimensional extended Hubbard model, driven by a transient laser pulse. When the system is close to the phase boundary, by tuning the laser frequency and strength, a sustainable charge order enhancement is found that is absent in the Mott insulating phase. We analyze the conditions and investigate possible mechanisms of emerging charge order enhancement. Feasible experimental realizations are proposed.

It is a fundamental task in strongly correlated electron systems to examine interdependence among spin, charge, orbital, and lattice degrees of freedom. Recently, the examinations of the interdependence under nonequilibrium conditions become quite urgent issues in various on-going subjects. Ultrafast transient optics is an approach exploring new functionalities of materials and observing properties hidden in equilibrium conditions. The most important subject is photoinduced phase transition of low-dimensional transition metal oxides and organic materials with strong electron correlation.One-dimensional (1D) Mott insulators, Sr 2 CuO 3 and halogen-bridged Ni compounds, show photoinduced insulator-to-metal transition accompanied with its picosecond recovery to the insulating state. 1,2 The time scale of the recovery is three order of magnitude faster than that for semiconductors. 3 The 1D Mott insulators also exhibit gigantic third-order optical nonlinearity, and because of these two properties, they are promising future opto-electronics materials. 4 The complete description of optically excited Mott insulators is thus desired, but key theoretical concepts in nonequilibrium are still under construction.The photocarriers of the 1D Mott insulators are called holon and doublon representing empty and doubly occupied sites, respectively. A holon and a doublon recombine by emitting energy to other elementary excitations. A problem is to clarify a pass way of energy dissipation due to the recombination. Two possible candidates are spin and phonon excitations, since antiferromagnetic (AF) exchange energy and highest phonon frequencies are of the same order (∼0.1 eV). 5 Since high-energy states created by optical excitation may violate the separation of spin and charge degrees of freedom inherent in 1D electron systems, 6 a pass way for energy dissipation through a spin channel can be expected. However, recent numerical studies have shown robustness of the spin-charge separation for nonequilibrium steady states. 7, 8 It is thus necessary to make clear a coupling of spin and charge degrees of freedom under photoirradiation. As for phonon relaxation, pump-probe experiments have been done for various TTF-TCNQ salts with different magnitudes of electron-phonon (EP) coupling. 9 K-and Rb-TCNQ show spin-Peierls (SP) transition at T c =395 K and 220 K, respectively, and their photocarriers are once localized as polarons at around 70 fs, and then recombine with a few ps. On the other hand, ET-F 2 TCNQ does not show SP transition, and metallic photocarriers decay within 200 fs. Therefore, a fundamental question to be answered is about what is driving force of ultrafast relaxation of the 1D Mott insulators when the charge carriers couple weakly with spin and lattice. Since EP coupling is also present in semiconductors, we need to answer another question why phonon relaxation in the Mott insulators is much faster than that in the semiconductors.In this Letter, we incorporate time dependent vector potential of laser pulse into density-m...

In quantum spin chains at criticality, two types of scaling for the entanglement entropy exist: one comes from conformal field theory (CFT), and the other is for entanglement support of matrix product state (MPS) approximation. On the other hand, the quantum spin-chain models can be mapped onto two-dimensional (2D) classical ones by the Suzuki-Trotter decomposition. Motivated by the scaling and the mapping, we introduce information entropy for 2D classical spin configurations as well as a spectrum, and examine their basic properties in the Ising and the three-state Potts models on the square lattice. They are defined by the singular values of the reduced density matrix for a Monte Carlo snapshot. We find scaling relations of the entropy compatible with the CFT and the MPS results. Thus, we propose that the entropy is a kind of "holographic" entanglement entropy. At T(c), the spin configuration is fractal, and various sizes of ordered clusters coexist. Then, the singular values automatically decompose the original snapshot into a set of images with different length scales, respectively. This is the origin of the scaling. In contrast to the MPS scaling, long-range spin correlation can be described by only few singular values. Furthermore, the spectrum, which is a set of logarithms of the singular values, also seems to be a holographic entanglement spectrum. We find multiple gaps in the spectrum, and in contrast to the topological phases, the low-lying levels below the gap represent spontaneous symmetry breaking. These contrasts are strong evidence of the dual nature of the holography. Based on these observations, we discuss the amount of information contained in one snapshot.

We examine the single-particle excitation spectrum in the one-dimensional Hubbard-Holstein model at half-filling by performing the dynamical density matrix renormalization group (DDMRG) calculation. The DDMRG results are interpreted as superposition of spectra for a spinless carrier dressed with phonons. The superposition is a consequence of robustness of the spin-charge separation against electron-phonon coupling. The separation is in contrast to the coupling between phonon and spin degrees of freedom in two-dimensional systems. We discuss implication of the results of the recent angle-resolved photoemission spectroscopy measurements on SrCuO2.PACS numbers: 71.10. Fd, 74.72.Jt The interplay between electron correlation and electron-phonon coupling is one of the hot topics in the field of strongly correlated electron systems such as high-T c cuprates. Particularly in an electron-removal process from the Mott insulators, the electron-phonon coupling occurs due to charge imbalance around a created hole. Thus, the angle-resolved photoemission spectroscopy (ARPES) is a direct tool, and observes a quasiparticle dressed with phonons [1]. For the twodimensional (2D) insulating cuprates, the ARPES experiments have revealed a broad low-energy peak that is interpleted as a result of disappearance of the quasiparticle weight due to the coupling [2,3,4].The quasiparticle in the 2D systems is not only dressed with phonon cloud, but also dressed with antiferromagnetic (AF) spin fluctuation. Thus, the effect of phonon on the spectrum is affected by the spin configuration of the background. However, in one-dimensional (1D) correlated electron systems, a photohole created by ARPES decays into spinon and holon due to the spin-charge separation [5]. Therefore, the effect of phonon on the spectrum strongly depends on whether the spin-charge separation is robust against the electron-phonon coupling. In this Letter, we examine the effect of phonon on the singleparticle excitation spectrum in 1D Mott insulators.Theoretically, the Hubbard-Holstein model is a basic model to study the interplay between electron correlation and electron-phonon coupling in cuprates [6,7]. However, we have only limited information on the singleparticle excitation spectrum in this model [4,8,9,10,11,12,13]. This is because it is hard to treat the infinite phononic degrees of freedom and electron correlation on an equal footing. In order to overcome the difficulty, we apply the dynamical density matrix renormalization group (DDMRG) method to the calculation of the spec- * Present adress:Department of Physics, Tohoku University, Sendai 980-8578, Japan; Electronic address: matsueda@cmpt.phys.tohoku.ac.jp tra in the half-filled systems.We find the following four characteristic features in the single-particle excitation spectrum: (i) a dip at highbinding energy side of the spinon branch, (ii) broad holon branch, (iii) decrease of the spectral weight of the spinon branch, and (iv) slight enhancement of the weight at lowbinding energy side of the spinon branch. ...

Photo-induced spin-state change in itinerant correlated-electron system is studied. The model Hamiltonians before and after photon-pumping are derived from the two-orbital Hubbard model with crystalline field splitting. A photon introduced in the low-spin band insulator induces a bound state of the high-spin state and a photoexcited hole. This bound state brings a characteristic peak in the optical absorption spectra in the photo-excited state. The present results well explain the recent experimental results of the ultrafast optical spectroscopy in perovskite cobaltites. 71.10.w, 71.30.+h, 78.20.Bh A number of electronic phases and the phase transition between them are one of the central issues in correlated electron systems. In particular, materials with multi-degrees of freedom, such as charge, spin, orbital and so on, exhibit various exotic phenomena related to the phase transition [1]. Recently developed ultrafast optical techniques open up a new frontier for research of the phase transition [2]. Irradiation of a pump laser pulse into a system in a vicinity of the phase boundary triggers an abrupt change in the electronic structures. This is the so-called photo-induced phase transition (PIPT). In contrast to the conventional phase transitions caused by controlling temperature (T ), doping carriers and so on, photoinduced phase is transient and highly nonequilibrium. Photoinduced phenomena in correlated electron systems offer large possibility of new hidden phases [3,4], which do not realize in the thermal equilibrium state, and prompt several theoretical challenges [5][6][7][8][9][10][11].The spin-state transition is one of the targets in recent PIPT studies. This is a transition between the states with different magnitudes of the spin-angular moment in transition-metal ions. Different spin states are realized owing to a delicate balance of the intra-ion Hund coupling and the crystallinefield splitting. Some examples have been seen in the insulating organometallic complexes, such as the Prussian-blue analog, where magnitudes of the localized spins in Co or Fe ions are switched by photon irradiation [12,13]. Another type of the photo-induced spin-state transition is suggested in the correlated electron systems, the cobalt oxides with a perovskite structure, R 1−x A x CoO 3 and RACo 2 O 6−δ (R: a rare-earth ion, A: an alkaline-earth ion). Possible three spin states in Co 3+ are the low-spin (LS) in the (e g ) 0 (t 2g ) 6 configuration, the intermediate-spin (IS) in (e g ) 1 (t 2g ) 5 , and the high-spin (HS) in (e g ) 2 (t 2g ) 4 [14,15]. Temperature induced spin-state transition from the low-T LS insulating state into the high-T HS or IS metallic one is commonly observed in these cobaltites. The ultrafast optical measurements in the low-T LS insulators show transient metallic spectra which are completely different from the spectra in the high-T phase [16][17][18]. The electron conduction and the spin-state are strongly coupled with each other, in highly contrast to the spin-crossover organometallic comple...

Motivated by photoinduced phase transition in manganese oxides, charge and spin dynamics induced by photoirradiation are examined. We calculate the transient optical absorption spectra of the extended double-exchange model by the density matrix renormalization group (DMRG) method. A charge-ordered insulating (COI) state becomes metallic just after photoirradiation, and the system tends to recover the initial COI state. The recovery is accompanied with remarkable suppression of an antiferromagnetic correlation in the COI state. The DMRG results are consistent with recent pump-probe spectroscopy data. PACS numbers: 78.20.Bh, 78.47.+p, 75.47.Lx Competition among multiple phases is a key issue to understand electronic states in strongly correlated electron systems. Nature of the phases is characterized by internal degrees of freedom of electrons, namely spin, charge, and orbital [1]. Low-lying fluctuations of these degrees of freedom grow up toward the phase boundary. Then, tiny amounts of external perturbations can dramatically change the electronic states. Prototypical examples are perovskite manganese oxides where a ferromagnetic metallic (FM) phase competes with a chargeordered insulating (COI) phase associated with antiferromagnetic (AF) and orbital orders [2]. In the COI phase, electric resistivity drastically decreases by magnetic field, pressure, impurity, and so on.Photoirradiation is known to be a powerful tool to induce such dramatic response. This is called photoinduced phase transition (PIPT) [3]. Ultrafast manipulation of femtosecond pulse laser is characteristic of the PIPT, and provides a striking difference between the PIPT and the other phase transitions. At early stages of the PIPT, only electronic part of the systems is strongly affected, and the spin, charge, and orbital degrees of freedom play major roles on relaxation dynamics. This is in contrast to the other phase transitions where structual deformation sometimes occurs. Thus, we expect novel relaxation processes of the PIPT and electronic structures that can not be created in equillibrium conditions.The manganese oxides provide good playgrounds for elucidating effects of the multiple degrees of freedom on the PIPT [4,5,6,7,8,10]. In particular, the photoirradiation into the COI phase strongly influences transport, magnetic, and optical properties. Remarkable decrease of electric resistivity has been observed in Pr 0.7 Ca 0.3 MnO 3 after the photoirradiation [4,5,6]. Metallic behavior has been also observed in pump-probe reflection spectroscopy measurements on Nd 0.5 Ca 0.5 MnO 3 [7]: spectral weight of optical conductivity above the COI gap is transfered into lower-energy region within 400 fs after the photoirradiation. Magneto-optical Kerr spectroscopy measurements performed on Nd 0.5 Sr 0.5 MnO 3 and Gd 0.55 Sr 0.45 MnO 3 suggest that a FM component is induced by the photoirradiation [8,9].The purpose of this Letter is to examine microscopic processes of the PIPT in a system where the charge and spin degrees of freedom strongly couple ...

Motivated by the recent angle-resolved photoemission spectroscopy (ARPES) measurements on one-dimensional Mott insulators, SrCuO2 and Na0.96V2O5, we examine the single-particle spectral weight of the one-dimensional (1D) Hubbard model at half-filling. We are particularly interested in the temperature dependence of the spinon and holon excitations. For this reason, we have performed the dynamical density matrix renormalization group and determinantal quantum Monte Carlo (QMC) calculations for the single-particle spectral weight of the 1D Hubbard model. In the QMC data, the spinon and holon branches become observable at temperatures where the short-range antiferromagnetic correlations develop. At these temperatures, the spinon branch grows rapidly. In the light of the numerical results, we discuss the spinon and holon branches observed by the ARPES experiments on SrCuO2. These numerical results are also in agreement with the temperature dependence of the ARPES results on Na0.96V2O5.