US LAB

Ultrasound spectroscopy


 

Setup:

Heterodyne Setup:
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Highly constant frequency source, (Rohde&Schwarz, SMY01)
 

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RF Amplfier, 1-500MHz, switch (ISI) and heterodyne mixer (HP)
 

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Preamplifier/filter (Stanford Research)
 

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Puls- and Delaygenerator (Stanford Research)
 

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Boxcar Averager (EG&G, analog and digital)
 

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Superconducting magnet and bath cryostat

Pulse-Echo Setup:
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Pulsed RF sources, 0.9-20MHz, 10-90MHz, 90-300MHz (Matec)
 

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Synchronization, Log-Peakmeter (Matec)
 

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Frequency Counter

 

Sound velocity and sound attenuation experiments are perfomed on molecular and composite materials and magnets to study the interplay of lattice degrees of freedom with the nanostructure or magnetism of the material.

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Strong relaxation effects are observed in recently studied nanoscale composites from dielectric, metallic and ferromagnetic components.
 

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Excotic magnetic instabilities (e.g. Bose-Einstein transition of diluted magnons) are observed in weakly interacting molecular systems, see below.
 

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Sound velocity and attenuation are detected using a phase sensitive technique as function of temperature and magnetic field.

Patric Scheib at the ultrasound setup (3-150MHz) with a phase sensitive
heterodyne detection scheme and a superconducting magnet (9 Tesla).

 

 

 

A sound attenuation study of the
Bose-Einstein transition in TlCuCl3

E.Ya. Sherman, P. Lemmens, B. Busse, A. Oosawa, H. Tanaka,

Phys. Rev. Lett. 91, 057201 (2003)

 

 

We investigated experimentally and theoretically sound attenuation in the quantum spin system TlCuCl3 in magnetic fields at low temperatures. Near the point of Bose-Einstein condensation (BEC) of magnons a sharp peak is observed in the sound attenuation. This effect demonstrates a hysteresis as function of the magnetic field pointing to a first-order contribution to the transition. The sound damping has a Drude-like form arising as a result of hardcore magnon-magnon collisions. The strength of the coupling between the lattice and "relativistic" magnons is estimated from the experimental data. The puzzling relationship between the transition temperature and the concentration of magnons is explained by their "relativistic" dispersion.

Preprint

 

 

Webpages of Peter Lemmens

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p.lemmens at tu-bs.de, letzte Änderung: 23.02.2007