vol. 80 | iTHES

Vol. 80, May 18th, 2015


  • Upcoming Events
  • Event report
  • Paper of the week

Paper of the week

Yoshitomo Kamiya, Yasuyuki Kato, Joji Nasu, Yukitoshi Motome


Magnetic Three States of Matter: A Quantum Monte Carlo Study of Spin Liquids

From the Office

The office of iTHES assistant, Ms.Chikako Oota is situated at the second floor of the main research building, room # 246. The extension number is 3261. She will be at the office from 10 a.m. to 16 p.m.

Upcoming Events

Prof.Yasuhiro Masuda (Institute of Particle and Nuclear Studies, KEK)
EDM Measurement with Ultracold Neutrons
Fri 29th May 2015 11:00-12:00
Main research building, 5th floor, 535,537
CP violation is one of important issues in particle physics. The CP violation induces an electric dipole moment in the neutron (nEDM). With nEDM, many theories of particle physics have been tested. Although the standard model predicts very small values of nEDM, new physics such as supersymmetric theory as well as multi-Higgs model, which is beyond the standard model, predicts nEDM values in a 10^{-27} e cm region. The present state of the arts nEDM measurement is at Grenoble, which shows the upper limit of 3×10^{-26} e cm. In this measurement, ultracold neutrons (UCN) was used. UCN are very low energy neutrons, which can be confined in a measurement cell. The precision of the measurement was limited by the number of UCN in the cell, but the improvement of the number of UCN was strongly limited by Liouville’s theorem. Many institutes have been developing super thermal UCN sources, which get rid of this limitation. Here, we discuss our approach to this problem. Our UCN source became world competitive. We also discuss the present status and the future direction of our nEDM measurement.

Kazunori Kohri (KEK)
Close Encounters of the Dark Matter
12:30-13:30, May 29.
Room 535-537 in the Main Research Build.
I review the current status of the dark matter research for non-specialists.

June 4 (Thur.) from 2:00 pm
Dr. William Witczak-Krempa (https://www.perimeterinstitute.ca/people/william-witczak-krempa )
Constraining quantum critical dynamics: 2+1D Ising model & beyond
Rm 154&156, Main Research Building, Wako campus, RIKEN
Quantum critical (QC) phase transitions generally lead to the destruction of quasiparticles. The resulting correlated quantum fluid, when thermally excited, displays rich universal dynamics. We establish non-perturbative constraints on the linear-response dynamics of QC systems at finite temperature, in spatial dimensions above one. Specifically, we analyze the large frequency/momentum asymptotics of observables, which we use to derive powerful sum rules. The general results are applied to the O(N) Wilson-Fisher fixed point (CFT), describing the QC Ising and XY models when N=1,2, respectively. We'll contrast the results with Dirac fermions. Our focus will be on the order parameter susceptibility, conductivity, and shear viscosity. Connections to quantum Monte Carlo simulations, experiments and AdS/CFT will be made.
It's based on my recent paper (http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.177201 ), and other works with Subir Sachdev.

June 15 (Mon), 2015
RIKEN Wako Main Campus
"Symposium on the Present and Future of iTHES"

It has passed 2 years since the RIKEN iTHES started on June 1, 2013. At this symposium, we will discuss the current status and future perspective of iTHES both from scientific and organizational points of view.
All the iTHES members/researchers/associates are urged to attend this important event.

Event report

We had a 5th iTHES Academic-Industrial Lecture: "Artificial Intelligence (AI) : Present and Future". The speakers were Dr. Yutaka Matsuo (U Tokyo) & Dr. K. Takahashi (RIKEN QBiC). Dr. Matsuo gave a talk on “Deep learning: present and future”. He presented a possible very bright near future where our social lives (e.g. autonomous car) can be improved drastically by AI. Dr. Takahashi gave a talk on “AI and Science/Technology” where he presented a possible near future where progress of many academic fields of research can be accelerated not with a help of AI but by AI itself. This means that our economic activities (e.g. GDP) can be increased drastically because they depend on innovations in academic fields. Their talks were very impressive and we recognized the importance of AI for our bright future.

Paper of the week

Quantum spin liquids (QSLs) are novel magnetic states, strongly correlated and remaining disordered down to zero temperature. They may be characterized by a topological nonlocal order parameter or nontrivial entanglement properties, but not with a conventional order parameter of the Ginzburg-Landau type. Possibilities of such exotic states have been debated for a long time (both theoretically and experimentally) in frustrated magnets, where localized magnetic moments are subject to competing interactions.
In our recent preprint (arXiv: 1504.06911), we reported quantum Monte Carlo results of theoretical models (Kitaev’s toric codes) in (2+1) and (3+1) dimensions. These models have an exactly solvable point realizing a QSL ground state, hence providing us opportunities to investigate properties of QSLs responding to additional magnetic interactions as well as thermal effects. In this study, we extended the model by a ferromagnetic interaction. Consequently, the obtained phase diagram (see the attached figure from our paper) features magnetic analogue of “three states of matter,” namely, QSL (“liquid”), paramagnetic (“gas”), and the ferromagnetic ordered (“solid”) phases.
(*) This work was done in collaboration with Y. Kato (Univ. of Tokyo), J. Nasu (Tokyo Institute of Technology), and Y. Motome (Univ. of Tokyo).