Doc.dr. Dalibor Paar

Department of Physics
Faculty of Science
Bijenicka c. 32, P.P. 331
HR-10002 Zagreb
Croatia

Email: dpaar@phy.hr
Home Page: www.phy.hr/~dpaar
Tel: 385-1-4605542
Fax: 385-1-4680336

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Europski projekt SOLeNeMaR
 (Framework 7, koordinator M.Požek)

Izvješće o završenom projektu:
“Istraživanja visokotemperaturnih supravodiča
nuklearnom magnetskom rezonancijom”

Curriculum Vitae

Summary of Research Activities in Croatian Karst and Work on Karst Conservation

 

European project: Strengthening the SOLid-state research capacities in Zagreb by the introduction of the Nuclear Magnetic Resonance method

Coordinator: Miroslav Požek

Duration: 03/2009-02/2012

Project web page: nmr.phy.hr

The main goal of the project is the introduction of solid state nuclear magnetic resonance method (ssNMR) at Department of Physics, Faculty of Science, University of Zagreb (PDFS), which proved to be one of the key methods for the microscopic investigations of strongly correlated electron systems, high temperature superconductors, quantum magnets, conducting polymers, carbon nanotubes, and many other problems of condensed matter physics. We plan the acquisition of an ssNMR spectrometer suitable for measurements of broad spectral lines in strongly correlated systems, measurements at low temperatures, and in variable magnetic fields. Also, zero-field measurements of nuclear quadrupole resonance (NQR), that give very important information about local charge distribution, can be conducted by the use of this spectrometer.

Selected publications

Lang, G; Grafe, H.-J.; Hammerath, F.; Manthey, K.; Paar, Dalibor; Behr, G.; Werner, J.; Hamann- Borrero, J.; Büchner, B. Probing of the charge distribution in iron pnictides. // Physica. C, Superconductivity. 470 (2010) , S1; S454-S455

Lang, G.; Grafe, H.- J.; Paar, Dalibor; Hammerath, F.; Manthey, K.; Behr, G.; Werner, J.; Buchner, B. Nanoscale electronic order in iron pnictides. // Physical Review Letters. 104 (2010) , 9; 097001-1-097001-4

Paar, Dalibor; Grafe, H.-J.; Lang, G.; Hammerath, F.; Manthey, K.; Behr, G.; Werner, J.; Büchner, B. NMR study of the electronic properties of superconducting LaO0.9F0.1FeAs. // Physica. C, Superconductivity. 470 (2010) , S1; S468-S469

Grafe, H.-J.; Lang, G.; Hammerath, F.; Paar, Dalibor; Manthey, K.; Koch, K.; Rosner, H.; Curro, N. J.; Behr, G.; Werner, J.; Leps, N.; Klingeler, R.; Klaus, H.-H.; Litterst, F. J.; Büchner, B. Electronic properties of LaO1− xFxFeAs in the normal state probed by NMR/NQR. // New Journal of Physics. 11 (2009) ; 035002-1-035002-14

Grafe, H.-J.; Paar, Dalibor; Lang, G.; Curro, N. J.; Behr, G.; Werner, J.; Hamann-Borrero, J.; Hess, C.; Leps, N.; Klingeler, R.; Büchner, B., 75As NMR studies of superconducting LaFeAsO0.9F0.1. // Physical Review Letters. 101 (2008) ; 0473003-1-0473003-4

Summary of Research Activities in Solid State Physics (Microwave spectroscopy)

D.Paar, Microwave properties of YBa2Cu3Ox single crystal with different doping, Ph.D. dissertation, Faculty of Science, Univ. of Zagreb, 2006.

The experimental measurements of the microwave surface impedance of an optimally doped (Tc = 90K) and underdoped (Tc = 59 K) single crystal YBa2Cu3Ox were carried out in the temperature range 5-150 K and external magnetic field 0-16 T. We have measured a change in the Q-factor and resonant frequency of the microwave cavity loaded with sample and determined a surface impedance change.
With measurement in five resonant modes (9.33 GHz – 17.48 GHz) anisotropic properties of the samples were analyzed. Experimental measurements showed clear difference between the properties of optimally doped and underdoped sample. In underdoped sample in the normal state, a semiconductive behavior of the surface resistance was observed in the c-axis direction, and metallic dependence in ab plane. The temperature dependence of the complex conductivity in ab plane and in c-axis direction was determined. The London penetration depths in ab plane and in c-axis direction were found ab-lambda(0)=144 nm, c-lambda(0)=1.06 microm for optimally doped sample and ab-lambda(0)=259 nm, c-lambda(0)=11.4 microm for underdoped sample. Using the effective conductivity model we have analyzed magnetic field dependence of the surface impedance, for the determination of the upper critical field and the study of vortex dynamics. Upper critical field in B||ab orientation, close to the temperature of superconductive transition, showed =-10.9 T/K for optimally doped sample, and = -14.6 T/K for underdoped sample. In both samples in BDC||c orientation, we have observed magnetic dependence of the surface resistance with inflection point that moves to higher fields at lower temperatures. Possible explanation is vortex glass-to-liquid transition in the mixed state. For the parts of the curves in the glassy phase in the optimally doped sample, the upper critical field showed the slope dBc2/dT = - 5.1 T/K, that is unexpectedly higher than dBc2/dT = -2.6 T/K determined by conventional methods for the phase transition in the normal state.


Microwave currents with microwave magnetic field parallel to c-axis


The real and imaginary part of the surface resistance of optimally doped YBCO single crystal.


The real and imaginary part of the surface resistance of underdoped YBCO single crystal.



D.Paar, Investigation of superconductive and magnetic properties of RuSr2XCu2O8, X=Eu,Gd by microwave cavity perturbation method, M.Sc. thesis, Faculty of Science, Univ. of Zagreb, 2002.


Ru1212:Eu sample on sapphire rod prepared for microwave measurements

We have performed temperature and magnetic field dependence measurements of a microwave absorption in a sintered ceramic sample and a powdered sample dispersed in epoxy resin of the superconductive hybrid ruthenocuprates RuSr2EuCu2O8 and RuSr2GdCu2O8. Measurements were performed with a new method for the precise measurement of Q-factor and resonance frequency of a microwave cavity. Microwave resistance was measured in temperature range from 5 K up to 200 K in magnetic field from 0 to 8 T.
In both ceramic samples a peak was found near a magnetic ordering temperature at ~130 K. It is superimposed on a broad maximum which extends approximately 50 K above and below the magnetic ordering temperature. The microwave resistance decreases with increasing magnetic field in this region. The intergranular effects were separated from the intragranular effects by performing microwave measurements on the sintered ceramic sample as well as on the powdered sample. The powdered sample does not show semiconductor-like upturn below 120 K, so in the ceramic samples it is due to the intergranular conduction paths. The powdered sample also exhibits the peak at the magnetic ordering temperature, so it is an intrinsic property.
In the superconductive state of RuSr2EuCu2O8 (Tc=32 K) we found the evidence of numerous superconducting weak-links in the ceramic sample which are all drive normal for the magnetic fields greater than 0.5 T at 5 K.

In the superconductive state of RuSr2GdCu2O8 (Tc=50 K) the microwave absorption in zero field at 5 K is higher than in 8 T. The Gadolinium ESR line is superimposed on the microwave resistance which is dominated by the oscillations of the vortices at higher fields.
A detailed analysis of the microwave data on both, ceramic and powder samples show that the penetration depth remains frequency dependent and larger than the London penetration depth even at low temperatures. A consistent interpretation can be made with a model which base the coexistence of superconductivity and magnetic order on decoupled RuO2 and CuO2 planes. The conductivity in the RuO2 planes remains normal even when a superconducting order is developed in the CuO2 planes below the critical temperature.

This work was made in cooperation with D.-N. Peligrad, B. Nebendahl and M. Mehring from 2. Physikalische institut, Universität Stuttgart, Deutschland, and G.M.W. Williams from Industrial Research Limited, Lower Hutt, New Zealand.

Experimental method

Experimental method was developed and measurements were performed in the Laboratory for high frequency measurements, Department of Physics, Faculty of Science, Univ. of Zagreb in the cooperation with prof. M. Mehring, D.-N. Peligrad and B. Nebendahl from 2. Physikalische institut, Universität Stuttgart, Deutschland.


Microwave resonant cavity (copper).
The sample is placed in the centre of the cavity.


The modes of microwave resonant cavity

The microwave measurements were made in an elliptical eTE111 copper cavity operating at frequencies between 9 and 20 GHz. For the purpose of the present study it is essential to have a system with high stability so that very small changes of the Q factor can be reproducibly measured over long time scales. Therefore, the body of the microwave cavity was kept at liquid helium temperature. The unloaded cavity had a Q factor of about 25 000. The sample was mounted on a sapphire sample holder and positioned in the cavity center where the microwave electric field has maximum. The temperature of the sample could be varied from liquid helium to room temperature. The cryostat with the microwave cavity was placed in a superconducting magnet so that the sample could be exposed to a dc magnetic field of up to 16 T. The changes in the properties of the sample caused by either temperature variation or magnetic field were detected by a corresponding change in the Q factor of the cavity and a resonant frequency shift. The quantity 1/2Q represents the total losses of the cavity and the sample. The experimental uncertainty in the determination of 1/2Q was about 0.03 ppm. We present our data as the difference D(1/2Q) between the measured values with and without the sample in the cavity. The resonant frequency of the cavity loaded with the sample was measured with a microwave frequency counter and the results are expressed as Df / f , where f is the frequency at the beginning of the measurement and Df is the frequency shift. If the microwave penetration depth is much less than the sample thickness and the measured quantities are simply related to the surface impedance of the material. Both quantities can be temperature and field dependent. The sample is placed in the center of the cavity where the magnetic component of the microwave field has a node in the empty cavity. However, the wavelength in the conducting sample is much shorter than in vacuum so that a magnetic microwave field is also present within the skin depth from the sample surface. The total microwave impedance comprises both nonresonant resistance and resonant spin contributions.
 

No.

RESONANT MODE

FREQUENCY (5 K)

1.

eTE111

9.33 GHz

2.

eTE112

13.15 GHz

3.

eTM010

13.78 GHz

4.

eTE211

15.14 GHz

5.

eTE113

17.48 GHz

 

Laboratory for high frequency measurements
Department of Physics, Faculty of Science, University of Zagreb, Croatia

Prof. Dr. M.Požek, Prof. Dr. A.Dulčić, Dr. D.Paar
Ph.D. students: M.S. Grbić

Experimental equipment
8 T and 16 T Oxford superconductive magnets
Rohde&Schwarz signal generator (1-20 GHz)
Sigma-meter (Universität Stuttgart)
Cryostats with the microwave cavity

Agilent 53150A - 20 GHz microwave frequency counter
Stanford Research Systems SR380 DSP Lock-in amplifier
Lake shore 340 temperature controller
Leybold PT-50 turbo-molecular vacuum pump

Selected publications

Grbić, Mihael Srđan; Požek, Miroslav; Paar, Dalibor; Hinkov, Vladimir; Raichle, Markus; Haug, Daniel; Keimer, Bernd; Barišić, Neven; Dulčić, Antonije. Temperature range of superconducting fluctuations above Tc in YBa2Cu3O7-d single crystals. // Physicsl Review B - Condensed Matter and Materials Physics. 83 (2011) , 14; 144508-1-144508-12

Narduzzo, A.; Grbić, Mihael Srđan; Požek, Miroslav; Dulčić, Antonije; Paar, Dalibor; Kondrat, A.; Hess, C.; Hellmann, I.; Klingeler, R.; Werner, J.; Köhler, A.; Behr, G.; Büchner, B. Upper critical field, penetration depth, and depinning frequency of the high-temperature superconductor LaFeAs0.9O0.1 studied by microwave surface impedance. // Physical Review B - Condensed Matter and Materials Physics. 78 (2008) , 1; 012507-1-012507-4.

Požek, Miroslav; Kupčić, Ivan; Dulčić, Antonije; Hamzić, Amir; Paar, Dalibor; Basletić, Mario; Tafra, Emil; Williams, Grant V. M., Microwave and magnetotransport properties of RuSr2RCu2O8 (R = Eu, Gd) doped with Sn. // Physical Review B - Condensed Matter and Materials Physics. 77 (2008) , 21; 214514-1-214514-9.

Požek, Miroslav; Grbić, Mihael S.; Janjušević, Dragan; Dulčić, Antonije; Paar, Dalibor; Wagner, Thomas., Mixed state conductivity of thin niobium films in perpendicular magnetic fields. // Physica C. 460-462, (2007) 1291-1292.

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. Physical Review B. 74 (2006.) , 10; 104501.

A. Dulčić, M. Požek, D. Paar, E.-M. Choi, H.-J. Kim, W.N. Kang, S.-I. Lee,  Anisotropy in MgB2 thin film studied by magnetic field dependent complex microwave conductivity. Physica C. 408-410 (2004) ; 662-663.

A. Dulčić, M. Požek, D. Paar, E.-M. Choi, H.-J. Kim, W.N. Kang, S.-I. Lee. Coherence lengths and anisotropy in MgB2 superconductor. Physical Review B. 67 (2003) , 02; 020507(R).

A. Dulčić, D. Paar, M. Požek,  G.V. M. Williams, S. Krämer, Magnetization and Microwave Study of Superconducting MgB2. Physical Review B. 66 (2002) , 01; 014505.

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. Physical Review B. 65 (2002) ; 174514.

D.-N. Peligrad, B. Nebendahl, M. Mehring, A. Dulčić, M. Požek, D. Paar, General solution for the complex frequency shift in microwave measurements of thin films. Physical Review B. 64 (2001) ; 224504.

A. Dulčić, D. Paar, M. Požek,  G.V. M. Williams, S. Krämer, A Transport and Microwave Study of Superconducting and Magnetic RuSr_2EuCu_2O_8. Physical Review B. 64 (2001), 6406; 4508-4508.

D.-N. Peligrad, B. Nebendahl, C. Kessler, M. Mehring, A. Dulčić, M. Požek, D. Paar, Cavity Perturbation by Superconducting Films in Microwave Magnetic and Electric Fields. Physical review B 58 (1998) , 17; 11652-11671.