Modern calorimetry: going beyond tradition

Calorimetry has been a traditional tool for obtaining invaluable thermodynamic information of matter, the free energy. We describe recent efforts to go beyond this traditional calorimetry: After introducing dynamic heat capacity, we present the various experimental methods to measure it. Applications and future prospects are also given.Comment: 13 pages, 1 figur

Robustness of the in-degree exponent for the world-wide web

We consider a stochastic model for directed scale-free networks following power-laws in the degree distributions in both incoming and outgoing directions. In our model, the number of vertices grow geometrically with time with growth rate p. At each time step, (i) each newly introduced vertex is connected to a constant number of already existing vertices with the probability linearly proportional to the in-degree of a selected vertex, and (ii) each existing vertex updates its outgoing edges through a stochastic multiplicative process with mean growth rate of outgoing edges g and variance $\sigma^2$. Using both analytic treatment and numerical simulations, we show that while the out-degree exponent $\gamma_{\rm out}$ depends on the parameters, the in-degree exponent $\gamma_{\rm in}$ has two distinct values, $\gamma_{\rm in}=2$ for $p > g$ and 1 for $p < g$, independent of different parameters values. The latter case has logarithmic correction to the power-law. Since the vertex growth rate p is larger than the degree growth rate g for the world-wide web (www) nowadays, the in-degree exponent appears robust as $\gamma_{\rm in}=2$ for the www

Graphene nanosystems and low-dimensional Chern-Simons topological insulators

A graphene nanoribbon is a good candidate for a $(1+1)$ Chern-Simons topological insulator since it obeys particle-hole symmetry. We show that in a finite semiconducting armchair ribbon, which has two zigzag edges and two armchair edges, a $(1+1)$ Chern-Simons topological insulator is indeed realized as the length of the armchair edges becomes large in comparison to that of the zigzag edges. But only a quasi-topological insulator is formed in a metallic armchair ribbon with a pseudogap. In such systems a zigzag edge acts like a domain wall, through which the polarization changes from $0$ to $e/2$, forming a fractional charge of one-half. When the lengths of the zigzag edges and the armchair edges are comparable a rectangular graphene sheet (RGS) is realized, which also possess particle-hole symmetry. We show that it is a $(0+1)$ Chern-Simons topological insulator. We find that the cyclic Berry phase of states of a RGS is quantized as $\pi$ or $0$ (mod $2\pi$), and that the Berry phases of the particle-hole conjugate states are equal each other. By applying the Atiyah-Singer index theorem to a rectangular ribbon and a RGS we find that the lower bound on the number of nearly zero energy end states is approximately proportional to the length of the zigzag edges. However, there is a correction to this index theorem due to the effects beyond the effective mass approximation.Comment: 7 pages, 9figure

Topological end states and Zak phase of rectangular armchair ribbon

We consider the end states of a half-filled rectangular armchair graphene ribbon (RAGR) in a staggered potential. Taking electron-electron interactions into account we find that, as the strength of the staggered potential varies, three types of couplings between the end states can occur: antiferromagnetic without or with spin splitting, and paramagnetic without spin-splitting. We find that a spin-splitting is present only in the staggered potential region $0<\Delta<\Delta_c$. The transition from the antiferromagnetic state at $\Delta=0$ to the paramagnetic state goes through an intermediate spin-split antiferromagnetic state, and this spin-splitting disappears suddenly at $\Delta_c$. For small and large values of $\Delta$ the end charge of a RAGR can be connected to the Zak phase of the periodic armchair graphene ribbon (PARG) with the same width, and it varies continuously as the strength of the potential changes

Peltier ac calorimeter

A new ac calorimeter, utilizing the Peltier effect of a thermocouple junction as an ac power source, is described. This Peltier ac calorimeter allows to measure the absolute value of heat capacity of small solid samples with sub-milligrams of mass. The calorimeter can also be used as a dynamic one with a dynamic range of several decades at low frequencies.Comment: 12 pages, 4 figure

Gap states and edge properties of rectangular graphene quantum dot in staggered potential

We investigate edge properties of a gapful rectangular graphene quantum dot in a staggered potential. In such a system gap states with discrete and closely spaced energy levels exist that are spatially located on the left or right zigzag edge. We find that, although the bulk states outside the energy gap are nearly unaffected, spin degeneracy of each gap state is lifted by the staggered potential. We have computed the occupation numbers of spin-up and -down gap states at various values of the strength of the staggered potential. The electronic and magnetic properties of the zigzag edges depend sensitively on these numbers. We discuss the possibility of applying this system as a single electron spintronic device

Transverse Acoustic Phonon Transistor Based on Asymmetric Potential Distribution

We experimentally demonstrate a transverse acoustic (TA) phonon transistor. Phonons are coherently initiated by femtosecond photocarrier screening on potential gradients. Although translational symmetry within the isotropic plane normally prohibits optical generation of TA phonons, we show that the combined application of an external bias in the vertical and lateral directions can break the selection rules, generating the forbidden TA mode. The amplitude and on-state time of the TA mode can be modulated by the external field strength and size of the laterally biased region. The observed frequency shift with an external bias as well as the strong geometrical dependence confirm the role of the asymmetric potential distribution in electrically manipulating the crystal symmetry to control and activate the transistor.Comment: 5 pages, 5 figure

The Commercial Stainless Steel Tube Enveloping Technique for MgB2

A commercial stainless steel tube was employed to synthesize MgB2. The specimen was prepared by a stoichiometric mixture of Mg and B. The specimen that had been enveloped in the commercial stainless steel tube was synthesized for 2 hours at 1193 K. X-ray spectra showed there were no second phases like MgO. The transition temperature of the specimen was 37.5 K with a sharp transition width within 1K. The specimen showed a good connection between grains and critical current density as calculated with the Bean model is more than 100,000 A/cm2 at 20 K and in zero field.Comment: 6 pages, 5 figure

Phase transition in the Ising model on a small-world network with distance-dependent interactions

We study the collective behavior of an Ising system on a small-world network with the interaction $J(r) \propto r^{-\alpha}$, where $r$ represents the Euclidean distance between two nodes. In the case of $\alpha = 0$ corresponding to the uniform interaction, the system is known to possess a phase transition of the mean-field nature, while the system with the short-range interaction $(\alpha\to\infty)$ does not exhibit long-range order at any finite temperature. Monte Carlo simulations are performed at various values of $\alpha$, and the critical value $\alpha_c$ beyond which the long-range order does not emerge is estimated to be zero. Thus concluded is the absence of a phase transition in the system with the algebraically decaying interaction $r^{-\alpha}$ for any nonzero positive value of $\alpha$

Topological gap states of semiconducting armchair graphene ribbons

In semiconducting armchair graphene ribbons a chiral lattice deformation can induce pairs of topological gap states with opposite energies. Near the critical value of the deformation potential these kink and antikink states become almost degenerate with zero energy and have a fractional charge one-half. Such a semiconducting armchair ribbon represents a one-dimensional topological insulator with nearly zero energy end states. Using data collapse of numerical results we find that the shape of the kink displays an anomalous power-law dependence on the width of the local lattice deformation. We suggest that these gap states may be probed in optical measurements. However, "metallic" armchair graphene ribbons with a gap induced by many-electron interactions have no gap states and are not topological insulators.Comment: 4 pages, 3 figure