Physics of quantum and functional materials

Code: P1-0125

Duration: January 1, 2022–December 31, 2024

Although quantum effects have been exploited in a wide range of electronic devices for a long time, the past decade has seen a dramatic improvement in our understanding of how subtle quantum effects control macroscopic behaviour of a whole range of materials with different functionalities. The research programme "P1-0125: Physics of quantum and functional materials" will investigate fundamental physical phenomena in such materials and explore the possibility of emerging applications.

The research programme brings together a broad and complementary expertise of a large group of condensed-matter physicists with a prominent track-record in the field proved by numerous highly-cited publications in high-profile international journals (e.g., Science and Nature series), by various national and international awards, plenary and invited talks at the most prestigious international conferences, as well as by international patents.  The focus of the research programme will be around two strongly interlinked directions: materials and related technologies.

Our main aim will be to deepen the understanding of (i) the quantum entanglement phenomena in materials, (ii) the topological properties and their effect on the ordered states, (iii) the new quasiparticles predicted in low-dimensional quantum materials, (iv) the role of defects in stabilizing the quantum order, (v) the role of electron correlations in fuelling the competition between various types of quantum order, and (vi) the coupling of different degrees of freedom in order to take advantage of the (multi)functional behaviour, such as found in multiferroics and electrocalorics.

These phenomena will be investigated (i) in carefully selected families of quantum materials exhibiting unconventional superconductivity, quantum magnetism or exotic quantum spin-liquid ground states, (ii) in a range of topological materials, such as those with magnetic skyrmions, (iii) in multicaloric and multiferroic materials, and (iv) in high-entropy alloys.

The research group will use a broad arsenal of experimental techniques available at the home institution, such as the magnetic resonance and dielectric spectroscopy, thermal and magnetic property measurements, as well as other techniques available at various large-scale user facilities, such as the neutron scattering and the muon spectroscopy. Novel techniques will also be developed to address quantum and functional phenomena over the broad energy, length and time scales. Our experimental findings will not only be compared to the paradigmatic theoretical models, but will also stimulate the research of several potential applications. In particular, we will develop a novel highly sensitive optical magnetometer method challenging the current limitations in sensitivity, we will propose new methods for quantum computing using magnetic resonance techniques, and we will explore novel functionalised materials for 3D printing beyond the current state-of-the-art.

 

In 2018, members of the program group have published 38 original scientific papers in international peer-reviewed scientific journals, two book chapters and obtained two US patents. Among these, one paper was published in Nature Physics (IF = 22.7), one in Adv. Mater. (IF = 22), one in Nano Letters (IF = 12.1), one in Sci. Adv. (IF = 11.5), one in J. Mater. Chem. (IF = 9.9), two in Phys. Rev. Lett. (IF = 8.8) and 15 papers in the journals with the IF between 3.0 and 5.0.

 

The investigations were focused on the following research fields:

 

Quantum magnetism

 

Andrej Zorko, Peter Jeglič, Matej Pregelj, and Denis Arčon, in collaboration with partners from Switzerland, Germany, and Russia studied magnetic properties of the layered compound CuNCN with several experimental techniques, including NMR, NQR, and mu-SR. The investigation revealed a magnetically frozen and disordered magnetic ground state. The authors showed that regions of magnetically frozen and paramagnetic phases coexist on a microscopic level in this compound below the freezing temperature in a broad temperature range. The results were published in the paper A. Zorko et al. »Magnetic inhomogeneity in the copper pseudochalcogenide CuNCN«, Phys. Rev. B 97, 214432 (2018).

 

Andrej Zorko and Denis Arčon, in collaboration with partners from United Kingdom, Greece and Germany employed a combination of complementary experimental techniques, including heat-capacity measurements, NMR and elastic and inelastic neutron scattering to investigate structural and magnetic properties of the geometrically frustrated antiferromagnet β-NaMnO₂. The measurements disclosed the existence of novel structural degrees of freedom, which are incompatible with any commensurate order and are rather explained by an incommensurate compositionally modulated crystal structure. Such a structure leads to an incommensurate, that is inhomogeneous, cooperative magnetism. The discovery was published in the paper F. Orlandi et al. »Incommensurate atomic and magnetic modulations in the spin-frustrated β-NaMnO₂ triangular lattice«, Phys. Rev. Materials 2, 074407 (2018).

 

 

Figure 1: Incommensurate compositionally modulated crystal structure of β-NaMnO₂.

 

Andrej Zorko, in collaboration with partners from Croatia, France and USA, discovered the first crystal structures of oxo-bridged [Cr^IIITa^V] dinuclear complexes. The new structure complies with theoretical predictions based on DFT calculations. The compound was also magnetically characterized by the use of bulk SQUID magnetometry and a local-probe ESR technique. Also these experimental results agree well with DFT-based expectations. The discovery was published in the paper L. Androš Dubraja et al. »First crystal structures of oxo-bridged [Cr^IIITa^V] dinuclear complexes: spectroscopic, magnetic and theoretical investigations of the Cr–O–Ta core«, New J. Chem. 42, 10912 (2018).

 

Matej Pregelj, Andrej Zorko and Denis Arčon, in collaboration with partners from Switzerland and Austria, discovered coexistence of spinon and magnon excitations in the beta-TeVO₄ system. Their work is a rare demonstration of coexistence of fractional and collective excitations in a system of weakly coupled frustrated zigzag spin chains. The team reproduced the experimental dispersion relations, derived from inelastic neutron scattering, using the linear-spin-wave-theory calculations and precalculated spinon dispersion. This allowed them to quantitatively determine the main exchange interactions and their anisotropies. The discovery was published in the paper M. Pregelj et al. »Coexisting spinons and magnons in the frustrated zigzag spin-1/2 chain compound β-TeVO₄«, Phys. Rev. B 98, 094405 (2018).

 

 

 

Figure 2: Results of inelastic neutron scattering: (a) measurement and (b) theoretical model.

 

Matej Pregelj, Nejc Janša and Denis Arčon, in collaboration with partners from Italy and Brazil, investigated spin fluctuations in a high-spin state of cobalt valence tautomer. Reversible transition from low- to high-spin state can be induced by temperature, pressure and light-irradiation. The team investigated spin dynamics by nuclear-magnetic-resonance, muon-spin-relaxation and magnetization measurements. They found that at low temperatures (at 30 K) high-spin state can be induced by light irradiation, which has a lifetime of several hours and occurs in the MHz frequency range. The discovery was published in the paper F. Caracciolo et al. »Spin fluctuations in the light-induced high-spin state of cobalt valence tautomers«, Phys. Rev. B 98, 054416 (2018).

 

Nejc Janša, Andrej Zorko, Matjaž Gomilšek, Matej Pregelj and Martin Klanjšek, together with partners from Switzerland, experimentally demonstrated that a spin flip in the most promising Kitaev honeycomb magnet, in ruthenium trichloride, fractionalizes into a Majorana fermion and a pair of gauge fluxes, in line with the famous Kitaev prediction. Both types of fractional quasiparticles behave as neither pure fermions nor pure bosons, but rather as anyons. As they are both found to survive in a broad range of temperatures and magnetic fields, this discovery establishes ruthenium trichloride as a unique platform for future investigations of anyons. The work was published in the article N. Janša et al., »Observation of two types of fractional excitation in the Kitaev honeycomb magnet«, Nature Physics 14, 786 (2018).

 

 

Figure 3: In a Kitaev honeycomb magnet, a spin flip fractionalizes into three fractional quasiparticles: a Majorana fermion (red trace) and two excited gauge fluxes (blue hexagons).

 

Denis Arčon, Peter Jeglič and Tilen Knaflič discovered a Verwey-type charge ordering and electron localization transition in a compound, which is composed of negatively charged dioxygen molecules. One of the very first attempts to understand the charge dynamics in mixed-valence systems dates back to 1939 when Evert Verwey, a Dutch chemist, observed a sudden jump in resistivity near -150°C in magnetite. A research team of scientists from Germany and Slovenia reported a Verwey-type transition in a completely different class of mixed-valence compounds, which is composed of negatively charged dioxygen molecules. The compound Cs₄O₆ undergoes a phase transition from a state with indistinguishable molecular O₂^x- entities to a state with well-defined superoxide O₂^- and peroxide O₂^2- anions. The breakthrough of this study is the observation of such a charge ordering in a simple crystal structure where novel physical phenomena are expected to emerge from intertwining of degrees of freedom pertinent to electronically active oxygen molecular units. The work was published in P. Adler et al., »Verwey-type charge ordering transition in an open-shell p-electron compound«, Science advances 4, eaap7581 (2018).

 

 

Figure 4: Charge ordering in Cs₄O₆ is temperature dependent and is responsible for the change in crystal structure and the electrical conductivity.

 

Magnetism of CeGdTbDyHo high-entropy alloy

 

We have investigated the magnetism of the CeGdTbDyHo high-entropy alloy, composed of rare-earth elements that mix ideally in a solid solution. This high-entropy alloy forms an almost undistorted hexagonal crystal lattice (Figure 5), which possesses an enormous chemical disorder. The structure is stabilized entropically by the mixing entropy term T∆S_mix in the Gibbs free energy.

 

Figure 5: Schematic presentation of the crystal structure of a hexagonal high-entropy alloy, composed of five chemical elements that mix randomly on the lattice.

 

By measuring the magnetic susceptibility, the magnetoresistance and the specific heat, we have determined the (H, T) magnetic phase diagram, which contains a helical antiferromagnetic state at elevated temperatures and a disordered ferromagnetic state at low temperatures (Figure 6). 

Figure 6: Schematic presentation of (a) helical antiferromagnetic structure and (b) ferromagnetic structure.

 

Published in : S. Vrtnik, J. Lužnik, P. Koželj, A. Jelen, J. Luzar, Z. Jagličić, A. Meden, M. Feuerbacher, J. Dolinšek. Disordered ferromagnetic state in the Ce-Gd-Tb-Dy-Ho hexagonal high-entropy alloy. Journal of Alloys and Compounds 742 (2018), 877-886.

 

 

Study of nanostructured materials and materials with large caloric effects for solid state cooling applications

 

Ferroelectric relaxors are important class of material which exhibit extraordinary ferroelectric, dielectric, piezoelectric, and electrocaloric properties. The physical reason behind these extraordinary properties of relaxors are so called polar nanoregions (PNR’s). In this study we investigate the impact of PNR’s on polarization and electrocaloric properties by utilizing dynamic pair distribution function technique (DPDF). DPDF indicates the distance between a specific atomic pair, while the peak height corresponds to the probability of finding such an atomic pair at this distance. Hence, we obtained direct information about the specific atomic off centering corresponding to polar vectors in real space which was correlated with the dielectric, polarization and electrocaloric response of lead free relaxor system Ba(Ti,Zr)O₃. The study was published in Pramanick, A., Dmowski, W., Egami, T.I, Setiadi Budisuharto, A., Weyland, F., Novak, N., Christianson, A., Borreguero, J. M., Abernathy, D., Jørgensen, M. R. V.. Stabilization of Polar Nanoregions in Pb-free Ferroelectrics. Physical Review Letters 120 (2018), 207603.

We showed by direct measurements the existence of the large electrocaloric effect in novel bulk lead-free materials. In addition, we demonstrated that these materials can replace materials based on lead due to their large electrocaloric responsivity and large brakdown electric field. Patent application, which was bought by Company Gorenje d.d. in 2016, has been awarded a USA patent in 2018: patent Malič, B., Uršič, H., Kosec, M., Drnovšek, S., Cilenšek, J., Kutnjak, Z., Rožič, B., Flisar, U., Kitanovski, A., Ožbolt, M., Plaznik, U., Poredoš, A., Tomc, U., Tušek, J.. Method for electrocaloric energy conversion: United States Patent US9915446 (B2), 2018-03-13.

 

Figure 7:  Elastocaloric cooling cycle.

 

Enhanced electrical response in ferroelectric thin film capacitors with inkjet-printed LaNiO₃ electrodes

 

We have developed inkjet printing process of lanthanum nickelate (LaNiO₃, LNO) top electrodes onto ferroelectric Pb(Zr,Ti)O₃ (PZT) thin films on platinized silicon substrates. The evolved ink formulation enabled the deposition of well-defined, smooth, and flat layers with minimal inter-diffusion at the LNO–PZT interface. The capacitors exhibit better polarization switching characteristics, improved fatigue properties, and about 40 % larger dielectric constant than those with sputtered gold top electrodes. The Rayleigh analysis of the dielectric response revealed the strongly enhanced mobility of ferroelectric domain walls as the main contribution to improved characteristics of the LNO–PZT capacitors. Published in: A. Matavž, J. Kovač, M. Čekada, B. Malič, V. Bobnar, Applied Physics Letters 122, 214102 (2018).

 

Cellulose nanofibrils-reduced graphene oxide xerogels and cryogels for dielectric and electrochemical storage applications

 

Composites with reduced graphene oxide incorporated into the cellulose nanofibrils matrixes were fabricated as a dense film-like xerogel and well-aligned micro-to nano porous cryogels and evaluated related to their dielectric properties and electrochemical storage capacity. An outstanding dielectric performance and high flexibility of xerogel sample makes it a promising candidate as a highly-performing dielectric material for energy storage applications in engineering and electronic fields. On the other hand, high specific capacitance and electrochemical resistance indicate a suitability of porous cryogel as an electrode material in electrochemical storage devices. Published in: Y. Beeran, V. Bobnar, M. Finšgar, Y. Grohens, S. Thomas, V. Kokol, Polymer 147, 260 (2018).

 

Direct patterning of piezoelectric thin films by inkjet printing

 

We have developed a novel process for patterning of lead zirconate titanate (PZT) films on pristine platinized silicon through the use of inkjet-printed alkanethiolate-based templates. The technique requires neither lithography nor etching, respectively, before and after PZT printing. The described process allows for feature sizes in the sub-100 μm range with control over the thickness of the final film. Inkjet-printed PZT displays typical ferroelectric and piezoelectric properties of solution-derived thin films. Since substrate templating and functional material deposition are performed via additive manufacturing and using the same technology, we argue that our process could be an economically viable alternative to conventional deposition processes of patterned metal oxide films on high surface energy metal substrates. Published in: N. Godard, S. Glinšek, A. Matavž, V. Bobnar, E. Defay, Advanced Materials Technologies (2018), doi: 10.1002/admt.201800168.

 

Parameters optimization for synthesis of Al-doped ZnO nanodiscs by laser ablation in water

 

Al-doped ZnO crystalline colloidal nanodiscs were synthesized by laser ablation of ZnO:Al₂O₃ in MilliQ water. Experiments were performed systematically by changing the number of applied laser pulses and laser output energy with the aim to affect the nanoparticle size, composition (Al/Zn ratio) and characteristics (band-gap, crystallinity). Distinctly, set of nanoparticle syntheses was performed in deionized water for comparison. SEM investigation of colloidal nanoparticles revealed that the formed nanoparticles are 30 nm thick nanodiscs with average diameters ranging from 450 to 510 nm. It was found that craters in the target formed during the laser ablation influence the size of synthesized colloidal nanoparticles. This is explained by efficient nanoparticle growth through diffusion process, which takes place in spatially restricted volume of the target crater. When laser ablation takes place in deionized water, the synthesized nanoparticles have a mesh-like structure with sparse concentration of disc-like nanoparticles. Al/Zn ratio and band-gap energy of nanoparticles are highly influenced by the number and output energy of applied laser pulses (N. Krstulović, K. Salamon, O. Budimilja, J. Kovač, J. Dasović, P. Umek, I. Capan: Applied Surface Science 440 (2018) 916–925).

 

 

 

Figure 8: Representative SEM (a) and TEM (b) images of Al-doped ZnO nanodiscs. The particles were formed by irradiation of the ZnO:Al₂O₃ target with 10000 laser pulses and 300 mJ of laser output energy (l_laser=1064 nm).

 

Reorientational Motions and Ionic Conductivity in (NH₄)₂B₁₀H₁₀ and (NH₄)₂B₁₂H₁₂

 

Closo-boranes are promising materials for use in solid-electrolyte fuel cells due to their high ionic conductivity. In this study, we investigated two ammonium borane systems, containing 10 or 12 boron atoms in a boron cage (Figure 9). Molecular motions were studied by means of ¹H and ¹¹B NMR spectra and spin-lattice relaxation. We identified activation energies for rotations of boron cages around different axes. These rotations assist the long-range diffusion of NH₄ units. Independent ionic conductivity measurements uncovered that these two systems are bad conductors and that the conductivity cannot be explained solely by the rotations of boron cages. Published in: Anton Gradišek, Mitja Krnel, Mark Paskevicius, Bjarne R. S. Hansen, Torben R. Jensen, Janez Dolinšek, J. Phys. Chem. C, 2018, 122, 17073-17079.

 

 

Figure 9: Structural details of (NH₄)₂B₁₀H₁₀ in (NH₄)₂B₁₂H₁₂, containing 10 or 12 boron atoms in a boron cage.

 

NMR investigations of liquid-crystalline elastomers

 

We have investigated orientational ordering of molecular building blocks in liquid single crystal elastomers, using deuteron quadrupole perturbed nuclear magnetic resonance. By analysing temperature dependencies of spin-spin and spin-lattice magnetization relaxation rates, we have resolved differences in the reorientational dynamics of network-bound and free mesogen molecules, as well as of crosslinker molecules in selectively deuterated networks. We have found the dynamics of crosslinker to be substantially slower than the dynamics of mesogen, leading in the first case to strong homogeneous broadening of resonance lines. This supports the scenario of substantial local disorder in the nematic director for real liquid single crystal elastomer networks.

Research activities in the field of physics of liquid crystal elastomers have been extended to binary systems, consisting of two mesogen species, typically of a nematogen and of a smectogen, with controlled composition. In such systems, temperature profiles of elastic and thermomechanical response can be altered by changing the composition. We have shown that a relatively low external mechanical stress induces a transition from smectic to nematic state in the networks of composition close to 1:1, as observed through decrease in the elastic constant by at least one order of magnitude (Figure 10).

 

Figure 10: Temperature-composition-stress phase diagram of a binary smectic-nematic liquid crystal elastomer.

 

Published in: Dynamic investigation of liquid crystalline elastomers and their constituentas by ²H NMR spectroscopy, J. Milavec, A. Rešetič, A. Bubnov, B. Zalar, and V. Domenici, Liquid Crystals 45, 2158-2173 (2018);  Stress-strain and thermomechanical characterization of nematic to smectic A transition in a strongly-crosslinked bimesogenic liquid crystal elastomer, A. Rešetič, J. Milavec, V. Domenici, B. Zupančič, A. Bubnov, and B. Zalar, Polymer 158, 96-102 (2018).