Laboratory for the physics of transport phenomena

Research

Charge and heat transport in new materials

Complex metallic alloys - CMAs

Complex metallic alloys (CMAs) are formed with crystal structures based on giant unit cells containing many tens, up to more than a thousand atoms per cell. Limiting cases are the quasicrystals for which the unit cells extend to infinity. Most often the structure of the CMAs, and their unit cells, can be considered to be built from, more or less interpenetrating, large atomic clusters which may posses the axes of rotational symmetries of 5, 8, 10 or 12 order. For this reason the properties of CMAs are the result of the interplay between long range order and the local order on the range of the first and second neighbours. As a result, these materials can offer unique combinations of properties, which are excluded in conventional materials.

CMAs are divided into two main groups which are regular crystals and quasicrystals

Regular crystals are described with a finite unit cell which is repeated infinitely through the space and are through this equal to the conventional crystals.

Quasicrystals (QCs) are materials with the sharp diffraction patterns which exhibit the rotational symmetries forbidden for periodic structures. Therefore they are perfectly ordered but do not have a unit cell which is repeated periodically. Quasicrystals are divided into three-dimensional icosahedral quasicrystals, i-QCs which are apperiodic in all three directions in space, and polygonal (decagonal, octagonal, dodecagonal) quasicrystals which are apperiodic in a particular plane but are periodic in a direction perpendicular to that plane.

Our main interest

is in the charge and heat transport in Al-Transition Metal based decagonal quasicrystalls (d-QCs) and a specific class of regular crystals which consists of the approximants to these decagonal quasicrystalls.

Approximant phases are characterized by large unit cells which periodically repeat in space, and by one set of the atomic planes which correspond to the quasiperiodic atomic planes of d-QCs in the sense that they show locally similar patterns. This means that their structure, on the scale of near-neighbor atoms, closely resemble each other. Further, the periodicity lengths along the stacking direction of these planes in the approximant phases are almost identical to those along the periodic direction of d-QCs. Therefore, decagonal approximants offer valid comparison to the d-QCs. Here it is important, that the translational periodicity of decagonal approximants may enable the straightforward theoretical simulation of the physical properties.

A consequence of the anisotropic and layered structure of both d-QCs and their approximants is distinct anisotropy of their physical properties. Here we are particulary interested in electrical and thermal transport properties (electrical resistivity, thermoelectric power, Hall coefficient, thermal conductivity) when measured along different crystalline directions. The research is realized on the high quality single crystals produced by the leading experts in this field.

Our research is completed with the investigation of the magnetic properties, as well as with the computer calculations of the charge and heat transport in the same materials.

For some our experimental results for the anisotropic transport properties in Al-TM based approximants to the decagonal quasicrystals see Laboratory equipment & Selected results, and for some interestinf problems see Stil open questions.

The policrystalline samples of these materials with unusal physical properties are, of course, no less interesting particulary because of the unconventional dependence of these properties on the chemical composition and structural disorder.

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For the recent Lectures Notes on CMAs visit the Archives of the European School in Material Science

Selected Links to Qasicrystals

Through the research of the Laboratory on CMAs the Institute of Physics is a member of European Integrated Center for Metallic Alloys and Compounds (C-MAC)

Highly frustrated magnets

Research is a part of ESF (The European Science Foundation) Program: Highly Frustrated Magnets

For some interesting details of the structure and properties of highly frustrated magnets see and download some Lectures Notes from the HFM - School

Our research is focused on the studies of the heat transport in one-dimensional (1D) spin systems which are of strong current interest. From the theoretical side, there is consensus that the intrinsic spin-mediated heat transport of integrable spin models is ballistic, while the situation in nonintegrable spin models is less clear. Experimentally, such studies were stimulated by the observation of a strong anisotropy of the thermal conductivity κ(T) in some spin 1/2 ladder compounds, which has been explained by a large spin contribution κs along the ladder direction. In order to relate model calculations to experimental data, theory has to incorporate the coupling between spin excitations and the underlying lattice, while experimentally it is necessary to separate κs from the measured total κ. Here, studies of the magnetic-field dependent κ(B,T) can provide much more information than just the zero field κ(T) since strong enough magnetic fields change the spin excitation spectra and cause transitions between different quantum phases.

Layered materials

Layered materials, as are YBCO or BSCCO cuprates and iron arsenic pnictides, have emerged as the dominant source of new electronic states of matter. These materials are known to show superconductivity at exceptionally high temperatures, high thermoelectric power, and a variability of chemical composition that comes from the ability to intercalate various atoms and molecules between layers. They also proved as useful and promising from the technological standpoint, regarding the storage, the conversion and the transport of energy.

One of the objective of the research is to address the layered systems where the competition between different mechanisms (interactions) may lead to the state of frustration. This would be the state  where none of the mechanisms completely wins in imposing a particular ordering, thus resulting in the particular state, often, but not necessarily, reflected in the “textured” state on the nano-scale. The strong electronic correlations and state-stabilization through the coupling to lattice deformations often decisively shape new states.

Example - Phase diagram of Ba(Fe1-xCox)2As2
J.-H. Chu et al., Phys Rev B 79, p. 014506 (2009)

Parent compounds of iron pnictides undergo a structural phase transition, and a SDW transition, at a slightly lower temperature. Doping suppresses structural and SDW transition and superconductivity appears.
 

ρ(T) and χ(T) exhibit sharp superconducting transitions at 25 K and 20 K for Ba(Fe0.92Co0.08)2As2 and Ba(Fe0.95Ni0.05)2As2 respectively.

Several classical layered Transition Metal Dichalcogenides (TMD) have been known for decades. The possibility of novel electronic phases in their phase diagram has been recently addressed. Pressure has been applied at EPFL and in Zagreb, to push systems towards new phases.

 
TMD can be intercalated with a wide variety of molecules and atoms, including 3d transition metals. In the case of Co1/3NbS2 cobalt atoms occupy octahedral positions between the trigonal prismatic layers of the parent compound 2H-NbS2, resulting in a √3 × √3 superlattice.
 

The parent compound 2H-NbS2 is metallic and superconducting below Tc 6 K. Magnetic moments on the Co atoms destroy the superconductivity. These moments order antiferromagnetically (known as the hexagonal ordering of the first kind) below the Néel temperature TN of 26 K.
 

Complicated pressure dependences of dc-resistivity and thermoelectric power of Co1/3NbS2 indicate the presence of competing RKKY and super-exchange interactions in this system.

Research is performed with UKF(1B) Crossing Borders Grant through the Program

New electronic states driven by frustration
in layered materials

   
Institute of Physics
Zagreb, Croatia

C-MAC Days 2014
December 8 – 11, 2014
Zagreb, Croatia
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C-MAC Days 2013
9th-12th December 2013
Ljubljna, Slovenia
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Visit of the Nobel Prize Laureate Daniel Shechtman
18 -20 April, 2013
Zagreb
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Open day
22 March 2013
Institute of Physics
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C-MAC Days 2012
10th-13th December 2012
Kraków, Poland
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2012 MRS Fall Meeting
November 25 - 30, 2012,
Boston, Massachusetts
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Junior Travel Avwards
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Collaborative Workshop
March 28-31, 2012
Zagreb, Croatia
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Physics of Low-Dimensional Conductors: Problems & Perspectives
March 25-28, 2012
Zagreb, Croatia
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With a contribution by
Ana Smontara

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Institute of Physics, Zagreb, 2013.