## Condensed Matter Theory

### Theoretical Condensed Matter and Statistical Physics

http://www.cmpt.phys.tohoku.ac.jp/eng/

#### Staff

Professor : | Sumio Ishihara HP | Toshihiro Kawakatsu | Riichiro Saito HP | |

Associate Professor : | Nariya Uchida HP | Takashi Koretsune HP | Naokazu Shibata HP | |

Assistant Professor : | Wataru Izumida HP | Junya Otsuki HP | Tatsuya Nakajima | |

Ahmad Ridwan Tresna Nugraha | Takahiro Murashima | Hisatoshi Yokoyama |

#### Research

Theoretical Condensed Matter and Statistical Physics Group covers a large area of solid state physics and statistical physics, based on the two fundamental frameworks; quantum mechanics and statistical mechanics. Exotic, new and fundamental properties are of interest in highly correlated systems, soft materials, nano-materials, quasi-crystal and so on. Recently, research targets of the condensed mater theory are expanded to quantum information, topology, biomaterial, interdisciplinary region to the elementary particle physics, and general relativity. In addition to analytical approach, we perform numerical calculations for non-equilibrium systems or for electronic systems using sophisticated techniques. Domestic and international collaborations with experimental groups or industries stimulate our activities significantly. Every member of this group is available for discussions on a daily basis, thereby helping students to understand the many aspects of current physics.

The following lists the research subjects for each research group.

** Novel Quantum Phenomena in Correlated Electron Systems (Ishihara, Ohtsuki, Yokoyama) **

In strongly correlated electron system, highly non trivial phenomena emerge due to the many body and non-linear effects associated with the quantum statistical effects, which are unexpected from one-body picture such as the simple band structure calculation. High temperature superconductivity, metal-insulator transition due to electron interaction, and quantum spin liquid state are the examples. In this research group, microscopic origin of the novel quantum phenomena in strongly correlated electron systems are investigated. The followings are the recent research topics: Novel superconductivity, magnetism, dielectrics and optical properties in many electron systems. Non-equilibrium state and transient state in many electron systems. Competition and cooperation of electron-electron interaction and electron-phonon interaction.

High temperature superconductivity and cold atom system. Superconductivity and magnetism in heavy fermion systems.

** Non-equilibrium Statistical Physics of Soft Matter (Kawakatsu, Murashima) **

Soft matter is a class of materials that show soft mechanical properties caused by their multi-scale internal structures. Typical examples of soft matter are polymers, liquid crystals, surfactants and membranes. To study such systems we use theoretical models based on statistical physics as well as multi-scale simulation methods that combine microscopic scales (atomic scale) and acroscopic scales (hydrodynamics scale).

** Physics of Carbon nanotube, Graphene, Atomic Layer Materials （Saito, Izumida, Nakajima, Nugraha）**

Solid state physics and optical properties of one-dimensional (1D) and two-dimensional (2D) materials such as carbon nanotube (1D) and graphen or atomic layer materials (2D). Saito and Nugraha investigate the optical properties of nanotubes and atomic layer materials analytically and numerically with using first principles calculations. Valleytronics, excitons, surface plasmon, coherent phonons, resonance Raman spectroscopy, thermoelectric power have been developed. Izumida investigates nanotubes in quantum transport regime such as confinement effects and spin-orbit coupling by numerical and analytical methods. Nakajima investigates near-field effects and light propagation with negative refractive index in layered materials made of noble-metal spheres and split-ring resonators by using quantum-theoretical methods.

** Nonlinear Dynamics of Cells Far From Equilibrium (Uchida) **

Cells as the basic units of life exhibit various dynamic phenomena far from equilibrium.

The movements of flagella, cilia and the filaments composing the cytoskeleton are driven by molecular motors and self-organize into macroscopic motion of cells such as swimming, crawling and cell division. The interaction between cells induces emergent collective motion and morphogenesis. We study the dynamics of cells and their components using theoretical tools and concepts such as fluid mechanics, synchronization and phase transition.

** Materials science from first principles (Koretsune) **

First-principles approaches aim to compute physical properties starting from crystal structures as input and without using empirical models or parameters. Recently, a lot of physical phenomena have become able to discuss from first principles and now first-principles calculations are essential techniques to compare theories and experiments. We develop and apply the methods for various systems to explain and predict materials properties. Our targets include superconductivity, magnetic properties and transport properties. We also use data science approaches to design new functional materials from first-principles calculations.

** Low Dimensional Correlated Electron Systems (Shibata) **

Our research area includes strongly correlated electron systems such as fractional quantum Hall systems, heavy fermion systems, frustrated quantum spin systems. These many-body quantum systems are studied by the density matrix renormalization group (DMRG) method. The ground state phase diagram of 2D electron systems in a magnetic field, the multiple magnetization plateaus of Kagome Heisenberg quantum magnets, and the presence of the fractional quantum Hall effects in graphene and its bilayer have been clarified in our research group.