Exploratory Materials Synthesis and Characterization Group

The discovery of novel device functionalities and new model materials in physical sciences is tightly coupled to design, discovery and growth of novel materials. The core of our program is exploratory synthesis and characterization of quantum materials as well as their advanced synthesis for specialized experimental probes.

While at the microscopic property level all materials are described by quantum mechanics, in quantum materials properties of interest are produced by quantum effects such as topology of electronic wave function, quantum fluctuations, or quantum entanglement and coherence. A common thread in the study of quantum materials is the concept of emergence. Hence, quite often their properties are linked to strong electronic correlations and some type of electronic order.

We make crystals since many physical observables are tensor quantities and since fundamental science can be easily studied in such form by spectroscopy techniques. Moreover, layered and 2D van der Waals crystals provide an avenue for exfoliation, nanofabricaton and device making. An indispensable component of the program is daily round-the-clock rapid feedback of structural, transport, thermal, thermodynamic, and magnetic characterization using in-house instruments, combined with further studies at user facilities.  

We currently pursue several interpenetrating research directions.

1.      Dirac and Weyl materials

Dirac and Weyl semimetals will constitute the heart of future devices in many technologies such as photonics, catalysis, electronic, thermoelectric, spintronics or memory. My research group was the first to report quasi two-dimensional (quasi-2D) Dirac fermions in square Bi nets of SrMnBi₂ and CaMnBi₂ crystals,  a step beyond graphene and topological insulators. As stated in Nature Physics review article the roots of topological antiferromagnetic spintronics can be traced to our studies of layered antiferromagnets of the SrMnBi₂ – type materials. Since then, we discovered low-dimensional Dirac semimetal state in many crystals of ABX₂-type (A = alkaline or rare earth, B = transition metal, X = Bi, Sb, P) with common crystal structure motifs. This pointed to unique powerful inorganic building blocks (synthons) for assembly of Dirac matter in crystals where strong correlations and cooperative phenomena may exist. We have also discovered a series of 3D Dirac and Weyl semimetal states in crystals of ABX family (A = alkaline/rare earth, B = transition metal, X = pnictogen or Ge) which are heavy element compounds with high symmetries and strong SOC interaction. Examples include BaAuSb, SrAgBi and LaAlGe. A common characteristic of all is very small cyclotron mass, of the order of 1/100 of the bare electron mass. Lately, we have shifted our focus to Kagome lattices with Dirac and Weyl states. Ferromagnetic Fe₃Sn₂ with a geometrically frustrated kagome bilayer of Fe hosts massive Dirac fermions and skyrmionics bubbles at room temperature. Magnetic skyrmions in spin textures of chiral magnets are embodiment of strong correlations and topology. Though crystal synthesis of Fe₃Sn₂ is difficult due to nearby competing phases and small window of coexistence with the liquid, we have synthesized first crystals of Fe₃Sn₂. We used these crystals to demonstrate the first room-temperature thermoelectric signature of skyrmion bubbles in a simple thermal gradient and a wide temperature range of skyrmion signature for Kagome interlayer transport along crystal c-axis via topological Hall effect measurement. The results pave the way for future information storage and spin caloritronic devices using skyrmion manipulation by thermal gradients and crystal anisotropy.

2.      Iron superconductors

Past work was focused on discovery of first synchrotron-clean Fe(Te,S) and FeSe crystals, and wide variety of insulating states related to K_xFe_2-ySe₂ superconductors induced by pulsed magnetic field, sulfur substitution on Se atomic site or via engineering of other insulating Fe square (K_xFe_2-yS₂ and K(Fe,Ag)₂Te₂) or Fe ladder lattices (BaFe₂Se₃ and BaFe₂Se₂O). Our recent work is focused on the metallic resistivity at high-temperatures in the presence of strong disorder. There, as a consequence of phonon-unrelated scattering arising from unchanged Fe-Se and Fe-S bond distance lengths in individual FeCh₄ (Ch = Se, S) tetrahedra as S substitutes Se in Fe_1-ySe_1-xS_x alloys, resistivity in the normal state at high temperature exceeds the Mott limit. This provides an example of the violation of Matthiessen’s rule and the Mooij law, a dominant effect when adding moderate disorder past the Drude/Matthiessen’s regime in all materials.

3.      2D van der Waals materials

They feature weak van der Waals (vdW) bonds between atomic layers and strong covalent bonds in intralayer atomic units and provide an ideal platform for materials beyond graphene for future electronics. Transition metal dichalcogenides (TMDC’s) and other chalcogenide crystals may host Weyl states and are amenable not only for superconducting but also for  spintronic nano- and device-fabrication. My group has had keen interest in chalcogenide vdW materials in the past. We synthesized and characterized magnetic properties and magnetic critical behavior in wide variety of 2D van der Waals (vdW) magnetic crystals; examples include VI₃ or Mn₃Si₂Te₆. By introducing nanoscale dislocations and defects via FG crystal synthesis process we were able to enhance conductivity and clarify its mechanism in Cr₂Ge₂Te₆ and Cr₂Si₂Te₆. Work in this area resulted in an invited review article on the magnetic genome of 2D vdW magnets with a team of international experts.

4.      Heavy fermions

CeCoIn₅’s purity and accessible T_c at ambient pressure enabled rigorous investigation of the long-standing puzzle of high-temperature heavy fermion superconductivity. The discovery of 115 heavy fermion superconductors had considerable impact on the field. Innovative methods of crystal synthesis have resulted in intermetallic crystals with the smallest defect scattering known for any ternary compound. This enabled many landmark scientific experiments that must use very pure and large crystal such as the first measurements of the momentum (k) space structure of the superconducting energy gap Δ(k) for any heavy fermion superconductor, violation of Wiedemann – Franz (WF) law at the Quantum Critical Point, spin resonance and a full breadth of the vortex lattice in a d-wave superconductor. Lately, a combined experiment-theory study revealed a magnetic field-induced Quantum Critical Point and quantum-critical T/B-scaling of the Planckian metal Ce_1−xNd_xCoIn₅ in magnetic fields at x_c = 0.03. Meanwhile, the semiconducting gap in heavy fermions is of equal interest as the superconducting gap and for the very same reason: possibility that magnetic interactions are connected with gap formation. Discovery of FeSb₂ showed that, besides FeSi, there is a wider class of non-cubic crystals that may be used to study the applicability of Anderson model at half filling with reduced Coulomb repulsion U to 3d transition metal compounds with hybridizing gap. This influenced diverse areas, ranging from the problem of localized – itinerant crossover, hybridization gap and spectral weight transfer involving 4f and/or 3d electrons to thermoelectricity. Discovery of FeSb₂ is also important for its thermoelectric (TE) properties. This is because the most favorable TE electronic structure is the one that has a resonance in the density of states centered about 2–3 k_BT away from the Fermi energy. Kondo insulators (KI) represent a close approximation of such an ideal case since localized f or d states hybridize with conduction electron states leading to the formation of a small hybridization gap. The density of states just below and just above the hybridization gap becomes very large. The thermopower (TEP) is very sensitive to variations in density of states in the vicinity of the Fermi level hence very large absolute values of TEP can be expected and are reported in KI. Moreover, giant thermopower and giant thermoelectric power factor in FeSb₂ can be controlled by metallic flux growth (FG) synthesis-induced vacancies in crystal structure. Small KI-like gaps arise from the formation of covalent bonding/antibonding states and hence it is likely that high thermopower can be found in other materials with similar local crystallography, as exemplified in CoSbS crystals that shares similar local structural units to FeSb₂.