The electronic landscape of LaAs
In his first PhD publication, Hung-Yu presented the first ever grown single crystals of LaAs. Through an elaborate analysis of magneto-transport data including resistivity and Hall effect, he searched for the origin of extreme magnetoresistance – a large and positive magnetoresistance recently observed in several topological semimetals. Two classes of explanations have been offered for this: the presence of nontrivial topology and the compensation between electron and hole densities. By synthesizing a new member of the family, LaAs, and performing transport measurements, angle-resolved photoemission spectroscopy, and density functional theory calculations, we showed that (a) LaAs retains all qualitative features characteristic of the XMR effect but with a significant reduction in magnitude compared to LaSb and LaBi, (b) the absence of a band inversion or a Dirac cone in LaAs indicates that topology is insignificant toXMR, (c) the equal number of electron and hole carriers indicates that compensation is necessary for XMR but does not explain its magnitude, and (d) the ratio of electron and hole mobilities is much different in LaAs compared to LaSb and LaBi. We argue that the compensation is responsible for the XMR profile and the mobility mismatch constrains the magnitude of XMR.
Getting closer to a spin liquid state in honeycomb iridates
Our postdoc, Mykola Abramchuk, succeeded to synthesize the first copper iridium binary metal oxide synthesized by a topotactic reaction from sodium iridate and copper chloride under soft conditions. Chemical analysis and crystallographic refinement reveal the composition Cu2IrO3 in a monoclinic space group with layered honeycomb structure Resistivity measurements show insulator behavior with variable-range hopping. Magnetization measurements reveal that the effective iridium moment (1.9 Bohr magneton) and the Curie-Weiss temperature (-110 K) in Cu2IrO3 are close to its parent compound Na2IrO3. A subtle magnetic transition with weak hysteresis and without a sharp heat capacity peak is observed at 2.7 K in Cu2IrO3 different from the robust antiferromagnetic order at 15 K in Na2IrO3. The resulting frustration index f=40 in Cu2IrO3 is higher than f=8 in Na2IrO3 which brings copper iridate in closer proximity to a Kitaev spin liquid than its parent compound.
Superconductivity in LaBi under pressure
We discover superconductivity at 3.5 GPa, XMR in LaBi. Superconductivity emerges by suppressing magnetic or charge order in correlated electron systems. However, LaBi does not have a charge or magnetic order. At 11 GPa, it shows a structural transition concurrent with a 40% increase in the superconducting critical temperature but the first appearance of superconductivity at 3.5 GPa is not associated with any structural transitions. LaBi is the archetypal semimetal with extreme magnetoresistance (XMR). The appearance of superconductivity at 3.5 GPa coincides with the disappearance of XMR in this topological semimetal. A new route to discovering superconductivity is proposed by suppressing extreme magnetoresistance (XMR) in topological semimetals such as LaBi. Pressure is a clean tuning parameter to vary the crystalline and the electronic structure of materials. In the case of LaBi, the effect of pressure is to induce superconductivity first, then induce a structural transition. Similar effects are observed in WTe2 and ZrTe5 which are also topological semimetals with extreme magnetoresistance.
Introducing the triangular phase diagram of XMR
Over the past two years, a number of topological semimetals have been found to exhibit extremely large magnetoresistance (XMR). At first glance, these materials seem completely unrelated both from structural and from a chemical point of view. But the phenomenology of XMR is quite similar regardless of material difference. This is a typical example in physics where a robust universal phenomenon is observed in seemingly unrelated systems. We show that a triangular temperature-field phase diagram explains the phenomenology of XMR in all these materials. From band structure calculations in four different compounds, we show that these materials share one common feature in their electronic structure: they all have a mixing between d and p orbitals on their Fermi surfaces. We propose that XMR is the result of interaction between the magnetic field and this d-p orbital texture. This is the first time that a unified picture is presented for topological semimetals with XMR.
Extreme magnetoresistance in a rock salt material
Magnetoresistance is the percentage increase of the electrical resistance of a material in response to a magnetic field. In most metals, magnetoresistance is a weak effect less than 10 %. Here we report the discovery of one million percent magnetoresistance in Lanthanum antimonide LaSb. This unusual effect is called extreme magnetoresistance (XMR). Apart from applications of XMR, there is a fundamental importance to our discovery in LaSb. Recently, several intermetallic compounds have been reported with XMR, all of which are based on transition metals. LaSb is the first rare-earth based compound with XMR. In the transition metal-based compounds, XMR has been attributed to topological properties of the band structure. The complex structures of these compounds and the particular topology of their band structures hinders an easy theoretical understanding of XMR and its relation to the band structure topology. LaSb has a simple rock-salt structure, composition, and band structure. This compound seems to hold the key to a theoretical understanding of topological semimetals and their relations to XMR.
Universal V-shaped phase diagram in AFe2As2
Discovering universal trends that do not depend on details of a specific material are of great importance in condensed matter physics. In this work, we present a universal V-shaped phase diagram for the pressure dependence of the superconducting transition temperature Tc in AFe2As2 where A could be any of the Alkali metals: K, Rb, or Cs. We show that as a function of pressure, Tc in AFe2As2 decreases initially until a critical pressure Pc , where it reverses direction and increases. We have shown similar behaviors in KFe2As2 , RbFe2As2 , and CsFe2As2 with Pc = 18, 11, and 14 kbar. Comparing the V-shaped pressure dependence of Tc in these three materials reveals a universal trend in which and are identical in AFe2As2 independent of the chemical composition and the structural parameters. Our interpretation of the Tc reversal at Pc in AFe2As2 is a change of pairing state induced by pressure. Several recent theoretical works proposed close competition between d-wave and s-wave pairing in these fully hole-doped AFe2As2 systems. Our serial pressure experiments on three related compounds provide firm experimental ground to support these ideas.
Probing fluctuations with the Nernst effect
In most metals, below a critical temperature Tc, electrons form bound states called Cooper pairs and condense into a coherent superconducting state characterized by zero resistance to electrical current. Surprisingly, we can learn a lot about a superconductor at temperatures far above Tc. This is because Cooper pairs, the building blocks of a superconductor, can fluctuate into the normal state and survive high temperatures (T > Tc) for a short while. They cannot live long enough to induce the superconducting order but we can probe their brief existence by transport measurements and they would tell us the properties of the superconductor that is yet to come at lower temperatures (T < Tc). A great probe of superconducting fluctuations is the Nernst effect which is the transverse electric voltage in response to a heat current in a material that is exposed to a magnetic field as shown in the figure. We measured the Nernst effect in cuprates, the family of superconductors with the highest Tc. For this particular work, we chose the electron-doped cuprate superconductor Pr2−xCexCuO4 (PCCO) at four concentrations, from underdoped (x=0.13) to overdoped (x=0.17), for a wide range of temperatures above the critical temperature Tc. Our goal was to address one of the biggest questions in the field of high-temperature superconductivity: why do cuprates have a Tc dome?
We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc=1.0 K and Hc2=1.6 T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. In the absence of inversion symmetry, the superconducting state can potentially become a mixture of triplet and singlet pairing. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. The combination of strong spin-orbit coupling and noncentrosymmetric crystal structure make LuPtBi a strong candidate for 3D topological superconductivity.
Tuning the pairing symmetry of KFe2As2 by pressure
Proximity to an antiferromagnetic phase suggests that pairing in iron-based superconductors is mediated by spin fluctuations, but orbital fluctuations have also been invoked. The former typically favor a pairing state of extended s-wave symmetry with a gap that changes sign between electron and hole Fermi surfaces (s±), whereas the latter yield a standard s-wave state without sign change (s++). Here we show that applying pressure to KFe2As2 induces a sudden change in the critical temperature Tc, from an initial decrease with pressure to an increase above a critical pressure Pc. The smooth evolution of the resistivity and Hall coefficient through Pc rules out a change in the Fermi surface. We infer that there must be a change of pairing symmetry at Pc. Below Pc, there is compelling evidence for a d-wave state. Above Pc, the high sensitivity to disorder rules out an s++state. Given the near degeneracy of d-wave and s± states found theoretically, we propose an s±state above Pc. A change from d-wave to s-wave would probably proceed through an intermediate s+id state that breaks time-reversal symmetry.
We have studied the effect of pressure on the pyrochlore iridate Eu2Ir2O7, which, at ambient pressure, has a thermally driven insulator to metal transition at TMI∼120K. As a function of pressure, the insulating gap closes, apparently continuously near P∼6GPa. However, rather than TMI going to zero as expected, the insulating ground state crosses over to a metallic state with a negative temperature coefficient of resistivity, suggesting that these ground states have a novel character. The high-temperature state also crosses over near 6 GPa from an incoherent to a conventional metal, implying that there is a connection between the high- and the low-temperature states.