arXiv subjects

Akihiro Tanaka

Publications and source records attributed to Akihiro Tanaka.

28 records · Page 2Linked to original sources

arXiv2010

Photoinduced transition between conventional and topological insulators in two-dimensional electronic systems

Manipulating the topological properties of insulators, encoded in invariants such as the Chern number and its generalizations, is now a major issue for realizing novel charge/spin responses in electron systems. We propose that a simple optical means --subjecting to a driving laser field with circular polarization, can be fruitfully incorporated to this end. Taking as a prototypical example the two-band insulator first considered by Haldane, we show how the electron system can be tuned through phases associated with different Chern numbers as the laser intensity is adiabatically swept; i.e., a photo-induced analog of the quantum Hall plateau transition. The implications of our findings includes the possibility of laser-tuning a conventional insulator into a quantum spin Hall system.

cond-mat.mtrl-sci↗

arXiv2008

Geometric phases and the magnetization process in quantum antiferromagnets

The physics underlying the magnetization process of quantum antiferromagnets is revisited from the viewpoint of geometric phases. A continuum variant of the Lieb-Schultz-Mattis-type approach to the problem is put forth, where the commensurability condition of Oshikawa {\it et al} derives from a Berry connection formulation of the system's crystal momentum. %, similar to that developed by Haldane for ferromagnets. %Building on the physical picture which arises, We then go on to formulate an effective field theory which can deal with higher dimensional cases as well. We find that a topological term, whose principle function is to assign Berry phase factors to space-time vortex objects, ultimately controls the magnetic behavior of the system. We further show how our effective action maps into a ${\bf Z}_2$ gauge theory under certain conditions, which in turn allows for the occurrence of a fractionalized phase with topological order.

cond-mat.stat-mech↗

arXiv2008

Ultra fast quantum key distribution over a 97 km installed telecom fiber with wavelength-division multiplexing clock synchronization

We demonstrated ultra fast BB84 quantum key distribution (QKD) transmission at 625 MHz clock rate through a 97 km field-installed fiber using practical clock synchronization based on wavelength-division multiplexing (WDM). We succeeded in over-one-hour stable key generation at a high sifted key rate of 2.4 kbps and a low quantum bit error rate (QBER) of 2.9%. The asymptotic secure key rate was estimated to be 0.78-0.82 kbps from the transmission data with the decoy method of average photon numbers 0, 0.15, and 0.4 photons/pulse.

quant-ph↗

arXiv2007

Experimental Decoy State Quantum Key Distribution with Unconditional Security Incorporating Finite Statistics

We propose the improved decoy state quantum key distribution incorporating finite statistics due to the finite code length and report on its demonstration. In our experiment, four different intensities including the vacuum state for optimal pulses are used and the key generation rate of 200 bps is achieved in the 20 km telecom optical fiber transmission keeping the eavesdropper's mutual information with the final key less than 2^{-9}.

quant-ph↗

arXiv2006

Effective field theory with a $θ$-vacua structure for 2d spin systems

We construct a nonlinear sigma (NL$σ$) model description of 2+1d spin systems, by coupling together antiferromagnetic spin chains via interchain exchange terms. Our mapping incorporates methods developed recently by ourselves and by Senthil and Fisher, which aim at describing competition between antiferromagnetic and valence-bond-solid orders in quantum magnets. The resulting 2+1d $O$(4) NL$σ$ model contains a topological $θ$-term whose vacuum angle $θ$ varies continuously with $δ$, the bond-alternation strength of the interchain interaction. This implies that the $θ$-vacua structure for this NL$σ$ model can be explored by tuning $δ$ in a suitable 2d spin system, which is strongly reminiscent of the situation for 1+1 AF spin chains with bond-alternation.

cond-mat.str-el↗

arXiv2005

Competition between Hidden Spin and Charge Orderings in Stripe Phase

The correlation between charge and spin orderings in hole-doped antiferromagnets is studied within an effective model of quantum strings fluctuating in an antiferromagnetic background. In particular, we perform the direct estimation of the charge and spin long-range-order parameters by means of the quantum Monte Carlo simulation. A hidden spin long-range order is found to be governed by a competition between the two trends caused by increasing hole mobility: the enhancement of the two-dimensional spin-spin correlation mediated by hole motions and the reformation of a strong stripe order.

cond-mat.str-el↗

arXiv2005

Many-body spin Berry phases emerging from the $π$-flux state: antiferromagnetic/valence-bond-solid competition

We uncover new topology-related features of the $π$-flux saddle-point solution of the $D$=2+1 Heisenberg antiferromagnet. We note that symmetries of the spinons sustain a built-in competition between antiferromagnetic (AF) and valence-bond-solid (VBS) orders, the two tendencies central to recent developments on quantum criticality. An effective theory containing an analogue of the Wess-Zumino-Witten term is derived, which generates quantum phases related to AF monopoles with VBS cores, and reproduces Haldane's hedgehog Berry phases. The theory readily generalizes to $π$-flux states for all $D$.

cond-mat.str-el↗

arXiv2003

Possible spin triplet superconductivity in Na$_x$CoO$_{2}\cdot y$H$_{2}$0

Combining symmetry based considerations with inputs from available experimental results, we make the case that a novel spin-triplet superconductivity triggered by antiferromagnetic fluctuations may be realized in the newly discovered layered cobaltide Na$_x$CoO$_{2}\cdot y$H$_2$O. In the proposed picture, unaccessable via resonating-valence-bond physics extrapolated from half-filling, the pairing process is similar to that advanced for Sr$_{2}$RuO$_4$, but enjoys a further advantage coming from the hexagonal structure of the Fermi-surface which gives a stronger pairing tendency.

cond-mat.str-el↗

arXiv2001

Quantal phases, disorder effects and superconductivity in spin-Peierls systems

In view of recent developments in the investigation on cuprate high-T${}_{\rm c}$ superconductors and the spin-Peierls compound CuGeO${}_{3}$, we study the effect of dilute impurity doping on the spin-Peierls state in quasi-one dimensional systems. We identify a common origin for the emergence of antiferromagnetic order upon the introduction of static vacancies, and superconductivity for mobile holes.

cond-mat.str-el↗

arXiv1994

Numerical Study of the $S=1$ Antiferrromagnetic Spin Chain with Bond Alternation

We study the $S=1$ quantum spin chain with bond alternation ${\cal H}=\sum _i (1-(-1)^iδ)\vect{S}_i\cdot \vect{S}_{i+1}$ by the density matrix renormalization group method recently proposed by Steven R. White (\PRL{69}{3844}{1993}). We find a massless point at $δ_c =0.25 \pm 0.01$. We also find the edge states in the region $δ<δ_c$ under the open boundary condition, which disappear in the region $δ>δ_{c}$. At the massless point, the spin wave velocity $v_s$ is $3.66 \pm 0.10$ and the central charge $c$ is $1.0\pm 0.15$. Our results indicate that a continuous phase transition occurs at the massless point $δ=δ_c $ accompanying breaking of the hidden $Z_2\times Z_2$ symmetry.

cond-mat↗