Anomalous Frequency Dependence Research Articles

Dipolar Noise in Fluorinated Molecular Wires.

We demonstrate a strategy to directly map and quantify the effects of dipole formation on electrical transports and noises in the self-assembled monolayers (SAMs) of molecular wires. In this method, the SAM patterns of fluorinated molecules with dipole moments were prepared on conducting substrates, and a conducting probe in contact-mode atomic force microscopy was utilized to map currents and noises through the probe on the molecular patterns. The maps were analyzed to extract the characteristic parameters of dipolar noises in SAMs, and the results were compared with those of hydrogenated molecular patterns without dipole moments. At rather low bias conditions, the fluorinated molecular junctions exhibited a tunneling conduction and a resistance value comparable to that of the hydrogenated molecules with a six-times-longer length, which was attributed to stronger dipoles formation in fluorinated molecules. Interestingly, conductance (G) in different regions of fluorinated molecular patterns exhibited a strong correlation with a noise power spectral density of SI/I2 like SI/I2 ∝ G−2, which can be explained by enhanced barrier fluctuations produced by the dipoles of fluorinated molecules. Furthermore, we observed that the noise power spectral density of fluorinated molecules showed an anomalous frequency (f) dependence like SI/I2 ∝ 1/f1.7, possibly due to the slowing down of the tunneling of carriers from increased barrier fluctuations. In rather high bias conditions, conductions in both hydrogenated and fluorinated molecules showed a transition from tunneling to thermionic charge transports. Our results provide important insights into the effects of dipoles on mesoscopic transport and resistance-fluctuation in molecules and could have a significant impact on the fundamental understanding and applications in this area.

Anomalous frequency dependence of magneto-electric effect in doped DTN

The anomalous frequency dependence observed for the magneto-electric effect in the Bose glass state of dichloro-tetrakis-thoiurea-nickel doped with bromine is analyzed in terms of the hierarchical droplet model of spin glasses. In this model low energy barriers must be overcome before accessing higher energy states leading to scaling laws for the spin correlation functions and an associated frequency dependence for transition rates.

Theory of quantum work in metallic grains

We generalize Anderson's orthogonality determinant formula to describe the statistics of work performed on generic disordered, non-interacting fermionic nanograins during quantum quenches. The energy absorbed increases linearly with time, while its variance exhibits a superdiffusive behavior due to Pauli's exclusion principle. The probability of adiabatic evolution decays as a stretched exponential. In slowly driven systems, work statistics exhibits universal features, and can be understood in terms of fermion diffusion in energy space, generated by Landau-Zener transitions. This diffusion is very well captured by a Markovian symmetrical exclusion process, with the diffusion constant identified as the energy absorption rate. The energy absorption rate shows an anomalous frequency dependence at small energies, reflecting the symmetry class of the underlying Hamiltonian. Our predictions can be experimentally verified by calorimetric measurements performed on nanoscale circuits.

Magnetoelectric spin-glass transition(s) in pure and disordered BiFeO3

We present here the results of a comparative study of the spin glass (SG) transition in pure and disordered (0.80BiFeO3-0.20BaTiO3 or 0.80BF-0.20BT) BiFeO3. It is shown that the anomalous frequency dependence of ac susceptibility in the SG phase of BF is not linked with the spin cycloid as it is present even after its destruction in 0.80BF-0.20BT solid solution. Our neutron scattering studies on disordered BF reveal the presence of long-range ordered (LRO) antiferromagnetic (AFM) peak and decrease in the ordered magnetic moment from the overall Brillouin function behaviour around the SG transition suggesting that some spin components are getting detached from the AFM phase. Further, the SG transition is accompanied with anomalies in the unit cell volume and ferroelectric polarization around the SG transition temperature, highlighting the presence of magnetoelastic and magnetoelectric couplings, respectively. All these results suggest that the SG phase of BF is intrinsic to the system and is not due to nano-sized impurities.

Anomalous frequency-dependent ionic conductivity of lesion-laden human-brain tissue

We study the effect of lesions on our four-electrode measurements of the ionic conductivity of (∼1 cm3) samples of human brain excised from patients undergoing pediatric epilepsy surgery. For most (∼94%) samples, the low-frequency ionic conductivity rises upon increasing the applied frequency. We attributed this behavior to the long-range (∼0.4 mm) diffusion of solvated sodium cations before encountering intrinsic impenetrable blockages such as cell membranes, blood vessels, and cell walls. By contrast, the low-frequency ionic conductivity of some (∼6%) brain-tissue samples falls with increasing applied frequency. We attribute this unusual frequency-dependence to the electric-field induced liberation of sodium cations from traps introduced by the unusually severe pathology observed in samples from these patients. Thus, the anomalous frequency-dependence of the ionic conductivity indicates trap-producing brain lesions.

Magnetic phase transition in ɛ-In x Fe2 − x O3 nanowires

This paper reports on a study of magnetic properties of ordered arrays of ɛ-In x Fe2 − x O3 (x = 0.24) nanowires possessing a high room-temperature coercive force of 6 kOe. Lowering the temperature below 190 K brings about a sharp decrease of the coercive force and magnetization of nanowires driven by the magnetic phase transition from the ferrimagnetic into antiferromagnetic phase. The transition is accompanied by a decrease of the magnetic anisotropy constant, which accounts for the anomalous frequency dependence of the position of the maximum in the temperature dependence of dynamic magnetic susceptibility. In the low-temperature phase, a spin-flop transition in the magnetic field of 28 kOe has been observed at T = 2 K. Lines related to the high-temperature hard-magnetic and low-temperature phases have been identified in electron spin resonance spectra of the nanowires. A line lying near zero magnetic field and evolving from the nonresonant signal related to the microwave magnetoresistance of the sample has also been detected.

Coupling- and Persistent-Current Magnetizations of $ \hbox{Nb}_{3}\hbox{Sn}$ Rutherford Cables With Cores of Stainless Steel and Woven Glass-Fiber Tape Measured by Pick-Up Coil Magnetometry

Coupling- and persistent-current magnetization of $\hbox{Nb}_{3}\hbox{Sn}$ Rutherford cables with cores of stainless steel (SS) and woven glass-fiber tape were measured by pick-up coil magnetometry in transverse magnetic fields of amplitude 400 mT and frequencies of up to 90 mHz applied in the face-on orientation. Analysis of the results yielded the interstrand contact resistances as well as estimates of the face-on coupling magnetizations generated by fields ramping at the LHC-specified 6.5 mT/s. Distinct differences were discovered in the behaviors of the SS-cored and glass-fiber-cored cables. The cable with a wide SS core had suitable coupling properties; that with a narrow off-center SS core provided excessively large coupling magnetization. The glass-fiber-cored cables exhibited anomalous ac-loss frequency dependence indicative of loose Rutherford-like assemblies of strands with relatively few points of low-resistance interstrand contact.

Anomalous low-temperature magnetic and magnetotransport properties in Ru-deficient SrRuO3

SrRuO3 is a well-known itinerant ferromagnet with many intriguing characteristics. The Ru deficiency in this system is believed to play a pivotal role in influencing many of its magnetic and transport properties. The present study involves the magnetic and transport properties of the Ru-deficient SrRu0.93O3 sample to gain more insight into the unusual low-temperature behavior. The ac susceptibility study reveals a sharp ferromagnetic transition at 150 K followed by a hump at Th ∼ 50 K, which has anomalous frequency dependence. Besides, the Th shifts to lower temperatures with an increase in the superposed dc-biasing field and adheres to H2 dependence, in accordance with the Gabay and Toulouse line for the Heisenberg spin glass systems. We also observe a pronounced memory effect toward the low-temperature side, signifying the characteristic of glassy behavior. The temperature-dependent magnetoresistance indicates the signature of an additional ordering toward the low-temperature side. All of the interesting findings combined unveil the existence of low-temperature cryptic magnetic phase in SrRu0.93O3.

Influence of the rheological properties on the electrical impedance of hydrogels

We show that the anomalous frequency dependence of the real part of the electrical impedance of hydrogels can be interpreted by taking into account their non-Newtonian character, according to which the effective viscosity, η, depends on the frequency of the imposed deformation. Our experimental results are in good agreement with the Poisson–Nernst–Planck model if the ionic viscosity is taken proportional to the macroscopic one, measured by means of a viscositymeter, and the diffusion coefficient is assumed frequency dependent according to the formula of Stokes D = KBT/(κη), where κ is a shape coefficient. From this result it follows that the non-Newtonian character of complex liquid can be investigated by means of the impedance spectroscopy technique.

Anomalous frequency dependence of scattering of the radio emission from the Crab pulsar

The frequency dependence of scattering of the radio emission from the Crab pulsar at the low frequencies 111, 63, and 44 MHz has been measured and analyzed during sporadic enhancements of scattering and dispersion measure in October–December 2006 and December 2008. The frequency dependence of the scattering differs from the generally accepted dependence, τsc(ν) ∝ νγ, where γ = −4.0 for Gaussian and γ = −4.4 for power-law Kolmogorov distributions of inhomogeneities of the scattering medium. In intervals of enhancement, the exponent of the frequency dependence γ decreased to −3.2(0.2) at the above frequencies. A model is proposed in which this is due to the presence of a dense plasma structure in the nebula in the line of sight toward the pulsar, in which scattering of the radio emission on turbulence differs from scattering in the interstellar medium. It is shown that the frequency dependence of scattering of the radio emission can be weaker in a dense plasma than in the rarefied interstellar medium.

Sediment shear as a perturbation in geoacoustic inversions and an explanation of the anomalous frequency dependence of the attenuation.

The depth dependent shear wave speed in marine sediments is much less than both the compressional and water column sound speeds. The neglect of shear in geoacoustic inversions is usually justifiable, but at frequencies less than 300 Hz, the loss of acoustic energy from the water column because of nonreturning radiation of shear waves into the bottom dominates the loss due to the intrinsic attenuation of the sediment. To account for this in a simple manner, a perturbation theory that takes advantage of the small shear speed has been devised. The canonical problem addressed takes the disturbance as having a horizontal trace velocity ω/k in the x direction, this being somewhat less than the compressional speed in the bottom but substantially larger than the depth‐dependent shear wave speed. To lowest order the stress components σzz and σxx satisfy the same reduced Helmholtz–Bergmann equation as if the bottom were an inhomogeneous fluid. The shear stress σxz satisfies an approximately uncoupled equation governing a shear wave propagating vertically downward. Coupling occurs at the interface, and the power carried off by the shear wave can be approximately determined. Results succinctly and quantitatively explain why geoacoustic inversions yield attenuation frequency exponents less than 2.

Dielectric relaxation behaviour of nanostructured Mn–Zn ferrite

Dielectric measurements and modulus analysis have been made to investigate the effects of grain size, frequency and temperature for nanostructured Mn–Zn ferrite. The anomalous frequency dependence of dielectric loss (tan δ) can be attributed to the resonance effect and also to the presence of both n- and p-type charge carriers. The tan δ for 59 and 69 nm grain size samples is found to be an order of magnitude smaller than those of bulk particles. Dielectric relaxation studies using modulus formalism have shown the presence of the non-Debye type of dielectric relaxation in these materials.

Grain size effect on the dielectric behavior of nanostructured Ni0.5Zn0.5Fe2O4

The dielectric properties of nanocrystalline Ni0.5Zn0.5Fe2O4 spinel ferrite with various grain sizes, obtained by mechanically milling, have been studied using impedance measurements in the frequency range from 1 Hz to 10 MHz and in the temperature range from 300 to 823 K. The effect of milling, frequency, and temperature on the dielectric properties of nanocrystalline Ni-Zn ferrite is discussed. The real part of dielectric constant (ε′) for the 14 nm grain size sample is found to be about an order of magnitude smaller than that of the bulk nickel zinc ferrite. The anomalous frequency dependence of ε′ has been explained on the basis of hopping of both electrons and holes. The unusual increase in dielectric loss with milling is because of the increase in electrical conductivity due to oxygen vacancies introduced upon milling. The electrical modulus has been fitted to a stretched exponential function and the results clearly reveal the presence of the non-Debye type of dielectric relaxation in this material. The relaxation frequency is observed to decrease with milling due to increasing interaction between charge carriers arising out of oxygen vacancies introduced by milling. The mechanism for the electrical conduction is found to be the same as that for the dielectric polarization.

Spectroscopic evidences of quantum critical charge fluctuations in cuprates

We calculate the optical conductivity in a clean system of quasiparticles coupled to charge-ordering collective modes. The absorption induced by these modes may produce an anomalous frequency and temperature dependence of low-energy optical absorption in some cuprates. However, the coupling with lattice degrees of freedom introduces a non-universal energy scale leading to scaling violation in low-temperature optical conductivity.

Modeling of anomalous frequency and bias dependences of effective gate resistance in RF CMOS

This paper explains the frequency and bias dependences of the effective gate resistance (real part of h/sub 11/) by considering the effect of the gate-to-body capacitance, gate-to-source/drain overlap capacitances, fringing capacitances, and nonquasi-static (NQS) effect. A new method of separating the physical gate resistance and the NQS channel resistance is proposed. Separating the gate-to-source parasitic capacitances from the gate-to-source inversion capacitance is found to be necessary for an accurate modeling of all the Y-parameters.

ET and TTF Salts with Lanthanide Complex Ions Showing Frequency-Dependent ac Magnetic Susceptibility

We measured conducting and magnetic properties of (ET)4[Ln(NCS)6]·CH2Cl2 (Ln = Dy, Ho, Er, and Yb), (ET)5[Ln(NCS)6NO3]·EtOH (Ln = Tb and Dy), and (TTF)6[Dy(NCS)6]. All of the complexes behaved as semiconductors irrespective of Ln ions used, and also exhibited anomalous frequency-dependence of ac magnetic susceptibility below ca. 10 K, especially for Dy derivatives, indicating blocking behavior as found in single-molecule magnets.

Shallow seismic attenuation and shear-wave splitting in the short period range of Deception Island volcano (Antarctica)

The occurrence of a seismic series in Deception Island volcano (Antarctica), composed of hundreds of local volcano–tectonic earthquakes, has permitted us to study the seismic attenuation of such a volcanic environment in the short-distance and high-frequency range. This study has been performed using P-waves, S-waves and coda-waves and applying different, frequency dependent and independent, techniques. The methods used for this analysis have been: spectral and broadening-of-the-pulse, for direct P- and S-waves, coda normalization for S-waves, and single back-scattering model for coda-waves. The results show that, in general, Q values are significantly smaller for the entire frequency range used (6–30 Hz) than those found in other volcanic and tectonic areas. The attenuation for P-waves is greater than for S-waves in the frequency independent methods, with a Q β / Q P ratio that ranges between 1.9 and 3.2. Comparing the Q-factor obtained for S-waves we have observed clear differences as a function of the method used; the coda normalization method has supplied significantly higher Q values ( Q d ) than the other two methods ( Q β ). We have interpreted this discrepancy as an effect of the methods: coda normalization and single back-scattering methods eliminate the contribution of the near-surface attenuation in their Q values. Comparing both Q β and Q d we have estimated the near-surface attenuation under the recording site, named Q κ . On the other hand, we have observed that Q d has anomalous frequency dependence, with a minimum value at 21 Hz. This pattern is interpreted as an effect of strong scattering of the seismic waves in the source area of the earthquakes. Q c values depend clearly with frequency and lapse time and the lapse time dependence could be interpreted as a depth dependence of the seismic attenuation in Deception Island volcano, Antarctica. The obtained Q values have allowed us to separate the contribution of intrinsic and scattering attenuation, deriving that the scattering attenuation is predominant over the intrinsic effects. Finally, in order to investigate how the heterogeneous medium of the volcanic island could produce other effects, we have checked whether it produces polarization of the shear-waves. The preliminary results of the polarization direction indicate a main E–W strain direction. All these evidences reveal the strongly heterogeneous structure of Deception Island volcano.

Effect of surface roughness on interfacial reaction–diffusion admittance

We develop a theory for the electrical admittance of a rough interface under diffusion-limited and partial diffusion-limited charge transfer conditions. The large and small-frequency expansion brings out the relationships between the various morphological features of the rough electrode i.e. area, curvature and width, and the average admittance. The diffusion-limited reaction admittance is determined on an approximately self-affine corrugated random surface fractal. We obtain exact results for the low roughness and the asymptotic results (in three frequency regions) for the arbitrary and large roughness surfaces. These results show an anomalous frequency dependence for the mean admittance and the mean excess admittance for the large and small roughness surfaces, respectively. The intermediate frequency behavior of the reaction admittance for small roughness interfaces has the following form: 〈 Y〉∼(i ω) 1/2+const.(i ω) 3/2− H , where H is Hurst's exponent, for the large roughness interfaces it has same form as predicted earlier: 〈 Y〉∼(i ω) 1− H/2 . This non-universality and dependence of intermediate-frequency behavior on the strength of interfacial fractality has not been previously conceived. We also show the localization of the active zones in the presence of roughness. Finally, these results unravel the connection between the total reaction admittance and the crossover frequency to the roughness characteristics likes the fractal dimension, lower and upper fractal cutoff lengths, and the amplitude of the fluctuations of the fractal.

Phonon anomaly at the charge ordering transition in1T−TaS2

The infrared reflectance of the transition-metal chalcogenide $1T\ensuremath{-}{\mathrm{TaS}}_{2}$ has been measured at temperatures from 30 K to 360 K over $30--45000{\mathrm{cm}}^{\ensuremath{-}1}$ (4 meV--5.5 eV). The optical conductivity was obtained by Kramers-Kronig analysis. At 360 K only modest traces of the phonon lines are noticeable. The phonon modes are followed by a pseudo-gap-like increase of the optical conductivity, with direct optical transitions observed at frequencies above 1 eV. As the temperature decreases, the low-frequency conductivity also decreases, phonon modes become more pronounced, and a pseudogap develops into a gap at $800{\mathrm{cm}}^{\ensuremath{-}1}$ (100 meV). We observe an anomalous frequency dependence of the $208{\mathrm{cm}}^{\ensuremath{-}1}$ infrared-active phonon mode. This mode demonstrates softening as the temperature decreases below the 180-K transition. The same mode demonstrates strong hysteresis of the frequency and linewidth changes, similar in its temperature behavior to the hysteresis in the dc resistivity. We discuss a possible relation of the observed softening of the mode to the structural changes and changes in electronic properties associated with the 180-K transition.

Anomalous behavior in a three-dimensional frustrated itinerant electron magnet

We present a simple microscopic theory for the magnetic and electronic properties of a three-dimensional frustrated itinerant electron magnet. Based on the single-band Hubbard model, it is investigated in the weak-correlation limit at a half-filling on the pyrochlore lattice. A doubly-degenerate flat band in the bare spectrum emerges as a character of the fully frustrated geometry. The random-phase-approximation dynamical susceptibility reproduces some characters in the low-frequency regime that were found in the neutron inelastic-scattering experiment of the cubic Laves compound Y(Sc)Mn2, especially in the spectral intensity distribution in the reciprocal space and in the almost irrelevant feature of the distribution to frequency. The quasi-particle self-energy shows a peculiarity, resulting from an anomalous frequency dependence in the non-interacting dynamical susceptibility, and then it leads to the breakdown of the Fermi-liquid picture. PACS No.: 75.25+z