Forró Group
Scope
The Forró research group is seeking to build bridges between the physical, chemical, and biological sciences, the hard and soft matter themes. For example, a strong program in crystal and film growth and characterization is crucial for our studies of correlated electron materials; serious research on nanostructures gives us tools to address important issues on biomolecules; similarly, all improvements in spectromicroscopy instrumentation enhance our research on biological samples. Ultimately, biomedical imaging and cell-research benefit from the progress in many sub-areas of condensed matter physics.
Novel Quantum Materials
The demand for innovation generates a continuous need for new materials. Achieving these advancements requires a good understanding of how to synthesize novel, often exotic materials, what unique properties they possess, and how to fine-tune these properties for practical applications, such as fast and efficient electronics, high-density magnetic storage, spin-transistors, gas sensing, global positioning, energy harvesting and storage, magnetic trains, wearable electronics, medical imaging and therapy, infocommunication, and so on.
Two-dimensional (2D) materials have attracted interest because of the unusual properties that emerge in these confined structures. There is a growing family of 2D metal carbides and nitrides known as MXenes that contain an odd number of layers in which metals (M) encapsulate carbon or nitrogen (or both) (X) layers.
MXenes are formed by selectively etching the A layers from the MAX phases, leading to a structure of Mn+1XnTx, where M is an early transition metal (e.g., titanium, vanadium, chromium, molybdenum, etc.), A is an element, primarily from groups 13 and 14 (e.g., aluminum, silicon, or gallium), and X is either carbon or nitrogen (or both). Furthermore, n can be 1, 2, or 3 and Tx represents surface terminations that are typically -OH, -O, =O, -O-, or -F. These materials have emerged as leading candidates for a multitude of advanced technological applications due to their exceptional electrical conductivity, mechanical and tunable surface properties. However, one of the most significant challenges limiting their widespread adoption is their inherent susceptibility to oxidation, which can severely compromise their structural integrity and reduce their performance even under ambient conditions and especially under elevated humidity. Our group is focusing on the preparation and modification of Ti3C2, Ti2C, V2C, and Nb2C MXenes and to study their electronic, optical, and magnetic properties.
Spintronics in Carbonaceous Materials: Graphite, Graphene, and their derivatives
Spintronics is based on the concept of using the intrinsic angular momentum or spin of electrons as information carrier unit. The field has been active since the early 2000s and the 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter Grünberg for spintronics-related research. Concurrent HDD head readers employ a spintronics based technology and random-access memories (RAM), based on such principles, have been produced in prototype form. Electron spins interact with the environment through relativistic interactions, thus their coherence is preserved much longer than their collective motion (the electric current). The major governing factor in spintronics is the spin-coherence time. Its length determines whether a material can be used for spintronics. Researchers from our group observed that the spin-relaxation time in graphite is at least an order of magnitude longer than that observed in any other materials.
Graphite is a common material, which has been known since ancient times. It is used for several purposes including writing, as a lubricant and polishing material, in material sciences, energy storage, and even in nuclear technologies. Interest in graphite was revitalized by the discovery of a single graphite layer, graphene, in 2004. The latter material has been proposed for spintronics purposes but its spin-relaxation lifetime was limited to a few nanoseconds and it had to be cooled to liquid helium temperatures (4 K and below) for its observation. Our results showed a spin-relaxation lifetime in graphite around 100 ns at room temperature. Although it may not appear to be very long, it is still sufficient for spintronics applications.
Graphene is interesting for basic science, because it has a highly atypical band structure and its high tunability with chemical and electric field doping. It has raised notions like massless Dirac fermions, pseudospin, Berry phase, Klein paradox, Zitterbewegung and the spectacular manifestation of the integer and fractional quantum Hall effects.
But beyond the beauty for the basic research, graphene is even more interesting for applications. It is considered as a promising building block for prospective electronic technologies among which is spintronics. This is partially based on the high mobility of charge carriers and on a low spin-orbit coupling which is synonymous with a long spin lifetime. The goal of our research is to study the magnetic properties of doped graphene species and derivatives, and to extract the spin relaxation time by Electron Spin Resonance (ESR) measurements. It turns out that it is not so easy to pin down this quantity, because it depends on the environment of graphene, on the substrate by which it is supported, and last but not least, on the production method of it. One of the non-negligible goals of our work is to identify which production route of graphene gives the appropriate quality for spintronic applications and to find the intrinsic spin life-time of a real material. One of our previous work focuses on the chemical modification of mass producible, chemically exfoliated graphene. We were able to prove that lithium can successfully dope both the mono- and the few-layers, whileas sodium selectively dopes the single-layer flakes.
Electrons Leaving the Flatland: Magnetism and Superconductivity on the Nanoscale
It is generally believed that the simple charge modulation employed in MOSFET electronics will not be sufficient for future electronic devices. Controlling and manipulating new degrees of freedom, such as spin and orbits, will play a major role in the new generation of electronic materials including organic conductors, manganites, vanadates, and cuprates. Therefore, understanding the basic properties of such systems is crucial. Their engineering could have a significant impact on the electronics of the twenty-first century, and some of them are already in use. It turns out that in most of these materials, the electronic mean free path is very short, sometimes even shorter than the lattice spacing. In other words, despite the metallic-like temperature dependence of the resistivity, strictly speaking, they do not qualify as metals, and are often referred to as “bad” metals. Our goal is to investigate the bad metallicity with transport and magneto-transport studies in wide pressure (up to 20 GPa) and temperature (50 mK - 1000 K) ranges. In addition, lattice compression can affect the electron-phonon coupling, screening of the electron-electron interactions, alter the exchange interaction, and suppress low-dimensional fluctuations. We are confident that in some cases high pressures can tune the transition from a bad metal to a good metal (or from a non-Fermi liquid to a Fermi liquid), thus contributing towards an understanding of this peculiar state. Our research in novel electronic materials is described in this general context.
Relevant materials in our studies are transition metal dichalcogenides (TMD). Niobium Disulfide (NbS2) is one such material, and MoS2, WSe2, and NbSe2 are also common. TMDs, in bulk and as ultrathin layers, offer a wide variety of properties and phenomena to study, including magnetism, superconductivity, and charge density waves. Ionic gating is of notable interest to our group, in which the synthesis of ionic gels provide means to influence charge carrier density in a TMD.
Moreover, intercalation with magnetic ions such as iron (Fe) and cobalt (Co) can result in ferromagnetic, antiferromagnetic, and altermagnetic ordering. NbS2 and NbSe2 both have superconducting phases, depending on layer thickness, external magnetic fields, and temperature. TMDs are also excellent materials for studying topological phenomena due to their strong spin-orbit coupling, symmetry breaking, and Berry curvatures which can produce Hall effects depending on various tunable parameters (electric/magnetic fields, polarization, etc). Raman spectroscopy, ARPES, and scanning tunneling microscopy (STM) can map the band structure and Brillouin zones of TMDs.
In the figure above, the elements which comprise TMDs are displayed. Furthermore, important geometric structures are shown.
One particularly interesting compound is CoxNbS2 formed by the intercalation of cobalt ions in the parent compound NbS2. Depending on the concentration, the properties of NbS2 may change. For example, Co1/3NbS2 has an antiferromagnetic ordering at a temperature of 26 K and at ambient pressure [1], and increasing pressure at room temperature results in greater resistivity, contrary to conventional TMDs.
In blue is ambient-pressure resistivity of Co1/3NbS2 as a function of temperature and in red is thermoelectric power. At temperatures TN, antiferromagnetic ordering occurs.
We wish to see how intercalation changes the metallicity of NbS2. In general, sample preparation, including the use of the focused ion beam (FIB), allows for electrical and magneto-transport to be carried out along specific directions, the ab-plane and the c-axis, to observe magnetic ordering. The high stability and tunability of TMDs provide a plethora of experimental possibilities, studying key properties such as thermoelectric power and thermal resistivity as a function of temperature and magnetic fields, and intercalation may provide key insights into magnetic transitions and electronic behavior in various conditions.