arXiv subjects

K. Petukhov

Publications and source records attributed to K. Petukhov.

10 recordsLinked to original sources

arXiv2007

Energy Level Lifetimes in the Single-Molecule Magnet Fe_8 : Experiments and Simulations

We present pump-probe measurements on the single-molecule magnet Fe_8 with microwave pulses having a length of several nanoseconds. The microwave radiation in the experiments is located in the frequency range between 104 GHz and 118 GHz. The dynamics of the magnetization of the single Fe_8 crystal is measured using micrometer-sized Hall sensors. This technique allows us to determine the level lifetimes of excited spin states, that are found to be in good agreement with theoretical calculations. The theory, to which we compare our experimental results, is based on a general spin-phonon coupling formalism, which involves spin transitions between nearest and next-nearest energy levels. We show that good agreement between theory and experiments is only obtained when using both the Delta m_S = +-1 transition as well as Delta m_S = +-2, where Delta m_S designates a change in the spin quantum number m_S. Temperature dependent studies of the level lifetimes of several spin states allow us finally to determine experimentally the spin-phonon coupling constants.

cond-mat.mes-hall↗

arXiv2007

Pump-Probe Experiments on the Single-Molecule Magnet Fe8 : Measurement of Excited Level Lifetimes

We present magnetization measurements on the single molecule magnet Fe8 in the presence of pulsed microwave radiation. A pump-probe technique is used with two microwave pulses with frequencies of 107 GHz and 118 GHz and pulse lengths of several nanoseconds to study the spin dynamics via time-resolved magnetization measurements using a Hall probe magnetometer. We find evidence for short spin-phonon relaxation times of the order of one microsecond. The temperature dependence of the spin-phonon relaxation time in our experiments is in good agreement with previously published theoretical results. We also established the presence of very short energy diffusion times, that act on a timescale of about 70 ns.

cond-mat.mes-hall↗

arXiv2007

Magnetization dynamics in the single-molecule magnet Fe8 under pulsed microwave irradiation

We present measurements on the single molecule magnet Fe8 in the presence of pulsed microwave radiation at 118 GHz. The spin dynamics is studied via time resolved magnetization experiments using a Hall probe magnetometer. We investigate the relaxation behavior of magnetization after the microwave pulse. The analysis of the experimental data is performed in terms of different contributions to the magnetization after-pulse relaxation. We find that the phonon bottleneck with a characteristic relaxation time of 10 to 100 ms strongly affects the magnetization dynamics. In addition, the spatial effect of spin diffusion is evidenced by using samples of different sizes and different ways of the sample's irradiation with microwaves.

cond-mat.mes-hall↗

arXiv2005

Electron paramagnetic resonance detected via magnetization measurements

Presented are magnetization measurements on a crystal of Fe8 single-molecule magnets using a Hall probe magnetometer. Irradiation with microwaves at frequencies of 92 and 110-120 GHz leads to the observation of electron paramagnetic resonance (EPR) detected via magnetization measurements. A quantitative analysis of the results are introduced by means of the spin temperature. It is shown that pulsed microwave experiments allow a better control over the spin excitation.

cond-mat.mes-hall↗

arXiv2004

Anisotropy and Field-Dependence of the Spin-Density-Wave Dynamics in the Quasi One-Dimensional Conductor (TMTSF)_2PF_6

The anisotropic and non-linear transport properties of the quasi one-dimensional organic conductor (TMTSF)_2PF_6 have been studied by dc, radiofrequency, and microwave methods. Microwave experiments along all three axes reveal that collective transport, which is considered to be the fingerprint of the spin-density-wave condensate, also occurs in the perpendicular b' direction. The pinned mode resonance is present in the $a$ and b'-axes response, but not along the least conducting c* direction. The ac-field threshold, above which the spin-density-wave response is non-linear, strongly decreases as the temperature drops below 4 K. With increasing strength of the microwave electric field and of the radiofrequency signal, the pinned mode and the screened phason loss-peak shift to lower frequencies. In the non-linear regime, in addition to the phason relaxation mode with Arrhenius-like resistive decay, an additional mode with very long and temperature-independent relaxation time appears below 4 K. We attribute the new process to short-wavelength spin-density-wave excitations associated with discommensurations in a random commensurate N=4 domain structure.

cond-mat.str-el↗

arXiv2004

Scaling Behavior of the Longitudinal and Transverse Transport in Quasi One-Dimensional Organic Conductors

We report on dc and microwave experiments of the low-dimensional organic conductors (TMTSF)$_2$PF$_6$ and (TMTSF)$_2$ClO$_4$ along the $a$, $b^{\prime}$, and $c^*$ directions. In the normal state of (TMTSF)$_2$PF$_6$ below T=70 K, the dc resistivity follows a power-law with $ρ_a$ and $ρ_{b^{\prime}}$ proportional to $T^2$ while $ρ_{c^*}\propto T$. Above $T = 100$ K the exponents extracted from the data for the $a$ and $c^*$ axes are consiste1nt with what is to be expected for a system of coupled one-dimensional chains (Luttinger liquid) and a dimensional crossover at a temperature of about 100 K. The $b^\prime$ axis shows anomalous exponents that could be attributed to a large crossover between these two regimes. The contactless microwave measurements of single crystals along the $b^{\prime}$-axis reveal an anomaly between 25 and 55 K which is not understood yet. The organic superconductor (TMTSF)$_2$ClO$_4$ is more a two-dimensional metal with an anisotropy $ρ_a/ρ_{b^{\prime}}$ of approximately 2 at all temperatures. Such a low anisotropy is unexpected in view of the transfer integrals. Slight indications to one-dimensionality are found in the temperature dependent transport only above 200 K. Even along the least conducting $c^*$ direction no region with semiconducting behavior is revealed up to room temperature.

cond-mat.str-el↗

arXiv2004

Evidence for the S=9 excited state in Mn12-bromoacetate measured by electron paramagnetic resonance

We present high-frequency high-field electron paramagnetic resonance (EPR) measurements on the dodecanuclear manganese complex Mn12-bromoacetate (Mn12-BrAc). The crystal-field parameters are found to be identical to those of the original compound Mn12-acetate (Mn12-Ac). A detailed analysis of the frequency and temperature dependence of anomalous peaks observed in the EPR spectra of Mn12-BrAc enables us to locate the S=9 manifold at about 40 K above the Ms= +/-10 ground state of this nominally S=10 system. This is very close to the Ms = +/-6 state of the S = 10 manifold, thus suggesting pathways for the thermally assisted magnetization dynamics and related properties. Finally, the EPR fine structures recently attributed to disorder associated with the acetic acid of crystallization in Mn12-Ac are absent in the present measurements, thus suggesting that the Mn12-BrAc complex represents a more suitable candidate for measurements of quantum effects in high symmetry S=10 SMMs.

cond-mat.mes-hall↗

arXiv2003

Low-Frequency Optical Properties of $β^{\prime\prime}$-(BEDO- TTF)$_5$[CsHg(SCN)$_4$]$_2$: Indications of Electronic Correlations in a 1/5-Filled Two-Dimensional Conductor

The polarized reflectivity of $β^{\prime\prime}$-(BEDO-TTF)$_5$[CsHg(SCN)$_4$]$_2$ is studied in the infrared range between 60 cm-1 and 6000 cm-1 from room temperature down to 10 K. Already at T=300 K a pseudogap in the optical conductivity is present of about 300 cm-1; the corresponding maximum in the spectrum shifts to lower frequencies as the temperature decreases. In contrast to quarter-filled BEDT-TTF-based conductors of the $β^{\prime\prime}$-phase a robust Drude component in the conductivity spectra is observed which we ascribe to the larger fraction of charge carriers associated with the 1/5-filling of the conduction band. This observation is corroborated by exact diagonalization calculations on an extended Hubbard model on a square lattice for different fillings. A broad band at 4000 cm-1 which appears for the electric field polarized parallel to the stacks of the BEDO-TTF molecules is associated to structural modulations in the stacks; these modulations lead to a rise of the dc and microwave resistivity in 100 to 30 K temperature range.

cond-mat.str-el↗

arXiv2002

Correlation gap in the heavy-fermion antiferromagnet UPd_2Al_3

The optical properties of the heavy-fermion compound UPd$_2$Al$_3$ have been measured in the frequency range from 0.04 meV to 5 meV (0.3 to 40 cm$^{-1}$) at temperatures $2 {\rm K}<T< 300$ K. Below the coherence temperature $T^*\approx 50$ K, the hybridization gap opens around 10 meV. As the temperature decreases further ($T\leq 20$ K), a well pronounced pseudogap of approximately 0.2 meV develops in the optical response; we relate this to the antiferromagnetic ordering which occurs below $T_N\approx 14$ K. The frequency dependent mass and scattering rate give evidence that the enhancement of the effective mass mainly occurs below the energy which is associated to the magnetic correlations between the itinerant and localized 5f electrons. In addition to this correlation gap, we observe a narrow zero-frequency conductivity peak which at 2 K is less than 0.1 meV wide, and which contains only a fraction of the delocalized carriers. The analysis of the spectral weight infers a loss of kinetic energy associated with the superconducting transition.

cond-mat.str-el↗

arXiv2002

The Nature of Heavy Quasiparticles in Magnetically Ordered Heavy Fermions

The optical conductivity of the heavy fermions UPd2Al3 and UPt3 has been measured in the frequency range from 10 GHz to 1.2 THz (0.04 meV to 5 meV) at temperatures 1 K < T < 300 K. In both compounds a well pronounced pseudogap of less than a meV develops in the optical response at low temperatures; we relate this to the antiferromagnetic ordering. From the energy dependence of the effective electronic mass and scattering rate we derive the energies essential for the heavy quasiparticle. We find that the enhancement of the mass mainly occurs below the energy which is related to magnetic correlations between the local magnetic moments and the itinerant electrons. This implies that the magnetic order in these compounds is the pre-requisite to the formation of the heavy quasiparticle and eventually of superconductivity.

cond-mat.str-el↗