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.
