**General**

The microwave absorption laboratory has been founded as a part of the new facilities of the Physics Department at the Science Hill in 1997. It was established by prof. dr. sc. Antonije Dulčić and prof. dr. sc. Miroslav Požek in cooperation with the group of prof. dr. sc. Michael Mehring of Stuttgart (Germany). Ever since it was founded, the group has been researching superconductors – classical BCS and HTSCs. Most of the work has been done on fluctuation effects in cuprates, influence of the magnetic field on suppression of T_{c}, and effects of reduced dimensions in thin films.

Currently, the microwave absorption measurements are performed by:

prof. dr. sc. Antonije Dulčić adulcic-at-phy.hr

prof. dr. sc. Miroslav Požek mpozek-at-phy.hr

doc. dr. sc. Dalibor Paar dpaar-at-phy.hr

dr. sc. Mihael Grbić mgrbic-at-phy.hr

In photo from left to right: M. S. Grbić, M. Požek, A. Dulčić,

M. Bosiočić (NMR) and D. Paar

We are always open for collaboration and __feel free to contact any of us__.

__Students__ who wish to gain some knowledge in this technique, make a seminar or a diploma thesis are always welcome – we usually have more ideas than available hands.

Our recent publications are available here

A more concise overwiev of our research topics can be found here:

**Fluctuation effects in cuprates** - to fully understand the properties of the underdoped region of the cuprates where an energy (pseudo)gap defines the physics of the normal state, we have explored the properties of ac conductivity close to *T*_{c}. By varying the strength of the external magnetic field and chemical doping of the system at hand we were able to determine the properties of the fluctuating superconducting order parameter.

**Ruthenates -** in a hybrid compund RuSr_{2}Gd_{2}Cu_{2}O_{8} we studied the intriguing properties of coexisting magnetism and superconductivity. By combinig dc trasnport measurements, ac and dc susceptibility and microwave absorption measurements we found a decoupled behaviour of RuO_{2} and CuO_{2} planes. While the CuO_{2} planes form a superconducting state, the RuO_{2} planes remain in the normal state and order magnetically. From two doping exchange studies: exchanging Sr to La (superconductivity suppressed, magnetism enhanced) and Ru to Sn (superconductivity enhanced, magnetism suppressed) we find evidence of collossal magnetoresistance in the RuO_{2} planes.

- M. Požek, I. Kupčić, A. Dulčić, A. Hamzić, D. Paar, M. Basletić, E. Tafra, G. V. M. Williams, "Microwave and magnetotransport properties of RuSr
_{2}GdCu_{2}O_{8}(R = Eu, Gd) doped with Sn", Phys. Rev. B**77**, 214514 (2008). - M. Požek, A. Dulčić, D. Paar, A. Hamzić, M. Basletić, E. Tafra, G. V. M. Williams, S. Krämer, "Decoupled CuO
_{2}and RuO_{2}layers in superconducting and magnetically ordered RuSr_{2}GdCu_{2}O_{8}", Phys. Rev. B**65**, 174514 (2002). -
M. Požek, A. Dulčić, D. Paar, G. V. M. Williams, and S. Krämer, "Transport and microwave study of superconducting and magnetic RuSr

_{2}EuCu_{2}O_{8}", Phys. Rev. B**64**, 064508 (2001).

**Finite frequency cutoff **- by considering the contributions of the Gaussian processes in the superconducting fluctuations to ac conductivity, we introduce a lower limit for the relaxation time of the charge carriers. Through it, we reveal the finite-size effects in (cuprate) superconductors visible as the relaxation time slows down when approaching *T*_{c}.

- D.-N. Peligrad, M. Mehring, A. Dulčić, "Critical fluctuations and pseudogap observed in the microwave conductivity of Bi
_{2}Sr_{2}CaCu_{2}O_{8+δ}, Bi_{2}Sr_{2}Ca_{2}Cu_{3}O_{10+δ}, and YBa_{2}Cu_{3}O_{7-}_{}δ thin films", Phys. Rev. B**69**, 144516 (2004).

**Reduced dimensionality effects in niobium **- niobium is a classical BCS type II. supercodnuctor whose properties make it particularly usefull for industrial applications. The large (bulk) coherence length of ~20 nm enables us to observe the change in the properties of the system as the thickness of the film becomes of comparable size. Besides the change in *H _{c2||}*,

*T*

_{c}and vortex-depininning frequency, the reduced dimensionality broadens the critical fluctuation region which reveals the properties of the fluctuating order parameter.

**Microwave response of materials**

Frequency dependant measurements have always been important for studying electrodynamical properties of solids. At low frequencies (kHz up to MHz), one can place contacts on the sample and study the complex response by the use of phase amplifiers, network analyzers, impedance analyzers etc. However those methods are not well suited for measurements at gigaherz frequencies since the wavelength becomes comparable with the cable dimensions, and parasitic capacities and inductivites can screen the sample properties.

Under “microwaves” we consider the electromagnetic waves of frequencies between 1 GHz and 1 THz (higher than radio frequencies and lower than infrared). In experimental solid state physics, the microwave apsorption measurements are useful to study high-frequency transport properties. Frequencies are high enough to avoid the application of electrical contacts, but still low enough to avoid excitations of phonons or e.g. quasiparticles in superconductors.

There are several possible experimental arrangements for the measurements of microwave response of a sample of interest. We use the method of perturbation of a resonant microwave cavity, which is basically a box made of very conducting material whose dimensions are comparable to the wavelength (~3 cm). When in resonance, standing electromagnetic wave is formed in the cavity. When a sample is placed into the cavity, it causes a weak perturbation to the cavity configuration. The resonant frequency and the Q-factor of the cavity are slightly changed. These changes can be measured, and one obtains valuable information about conductivity, permitivity and/or permeability of the studied sample.

Resonant frequency and Q-factor of empty cavity (left) and the change when a sample perturbs the cavity (right)

Distribution of microwave magnetic (blue) and electric (red) fields in a _{e}TE_{112} mode.

**Our microwave cavity**

We have constructed an elliptical copper microwave cavity with base dimensions 13x21 mm^{2}, and height 28 mm. The lowest resonant mode is _{e}TE_{111} with a resonant frequency ~9.5 GHz. In this mode, the sample is at maximum of the microwave electric field. Similarly, for higher modes one can obtain several different polarizations of microwave magnetic or electric field at the sample position.

The fact that the cavity is made of copper enables us to apply an external magnetic field. Sometimes the cavity is made of superconducting material to have lower noise - but in that case one cannot use the magnetic field as probing parameter. Our experimental configuration includes static magnetic field up to **16(18) T**. The sample temperature can be varied between 3 and 323 K. This configuration ensures broad phase space for investigation of electric and magnetic properties of materials

Homogeneous distribution of the microwave

magnetic field around the sample volume

The laboratoy has a microwave absorption measurement setup operating at discrete frequencies **9 - 20 GHz** (hopefully soon to expand to 35 GHz) which gives us access to four modes: _{e}TE_{111}, _{e}TE_{112}, _{e}TE_{211} and _{e}TE_{113}. Each mode has a certain distribution of the microwave fields inside the cavity, and by placing the sample in the proper position we can obtain a response to either electric or magnetic driving field.

For more details about the method, please visit the the websites of Dalibor Paar and Mihael Grbić.