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.

• M. S. Grbić, M. Požek, D. Paar, V. Hinkov, M. Raichle, D. Haug, B. Keimer, N. Barišić, A. Dulčić, "Temperature range of superconducting fluctuations above T_c in YBa₂Cu₃O_7-δ single crystals", Phys. Rev. B 83, 144508, (2011).

• M. S. Grbić, N. Barišić, A. Dulčić, I. Kupčić, Y. Li, X. Zhao, G. Yu, M. Dressel, M. Greven, M. Požek, "Microwave measurements of the in-plane and c-axis conductivity in HgBa₂CuO_4+δ: Discriminating between superconducting fluctuations and pseudogap effects", Phys. Rev. B 80, 094511 (2009).

Ruthenates - in a hybrid compund RuSr₂Gd₂Cu₂O₈ 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 RuO₂ and CuO₂ planes. While the CuO₂ planes form a superconducting state, the RuO₂ 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 RuO₂ 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 ₂GdCu₂O₈ (R = Eu, Gd) doped with Sn", Phys. Rev. B 77, 214514 (2008).

• M. Požek, A. Dulčić, A. Hamzić, M. Basletić, E. Tafra, G. V. M. Williams, S. Krämer, "Magnetotransport of lanthanum doped RuSr₂GdCu₂O₈ - the role of gadolinium", Eur. Phys. J. 57, 1 (2007).

• M. Požek, A. Dulčić, D. Paar, A. Hamzić, M. Basletić, E. Tafra, G. V. M. Williams, S. Krämer, "Decoupled CuO₂ and RuO₂ layers in superconducting and magnetically ordered RuSr₂GdCu₂O₈", 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 RuSr₂EuCu₂O₈", 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 Bi₂Sr₂CaCu₂O_8+δ, Bi₂Sr₂Ca₂Cu₃O_10+δ, and YBa₂Cu₃O_7-δ thin films", Phys. Rev. B 69, 144516 (2004).

• D.-N. Peligrad, M. Mehring, A. Dulčić, "Short-wavelength cutoff effects in the ac fluctuation conductivity of superconductors", Phys. Rev. B 67, 174515 (2003).

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. 

• D. Janjušević, M. S. Grbić, M. Požek, A. Dulčić, D. Paar, B. Nebendahl, T. Wagner, "Microwave response of thin niobium films in the perpendicular static magnetic fields", Phys. Rev. B 74, 104501 (2006).

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 _eTE₁₁₂ mode.

Our microwave cavity
 

  We have constructed an elliptical copper microwave cavity with base dimensions 13x21 mm², and height 28 mm. The lowest resonant mode is _eTE₁₁₁ 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: _eTE₁₁₁, _eTE₁₁₂, _eTE₂₁₁ and _eTE₁₁₃. 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.