Tuning charge-density wave order and superconductivity in the kagome metals KV3_3Sb5−x_{5-x}Snx_x and RbV3_3Sb5−x_{5-x}Snx_x

The family of AV$_3$Sb$_5$ (A = K, Rb, Cs) kagome metals exhibit charge density wave (CDW) order, proposed to be chiral, followed by a lower temperature superconducting state. Recent studies have proposed the importance of band structure saddle points proximal to the Fermi energy in governing these two transitions. Here we show the effects of hole-doping achieved via chemical substitution of Sn for Sb on the CDW and superconducting states in both KV$_3$Sb$_5$ and RbV$_3$Sb$_5$, and generate a phase diagram. Hole-doping lifts the $\Gamma$ pocket and van Hove singularities (vHs) toward $E_F$ causing the superconducting $T_C$ in both systems to increase to about 4.5 K, while rapidly suppressing the CDW state.Comment: 6 pages, 4 figure

Emergence of unidirectional coherent quasiparticles from high-temperature rotational symmetry broken phase of AV3Sb5 kagome superconductors

Kagome metals AV3Sb5 display a rich phase diagram of correlated electron states, including superconductivity and novel density waves. Within this landscape, recent experiments reveal signs of a new transition below T ~ 35 K attributed to the highly sought-after electronic nematic phase that spontaneously breaks rotational symmetry of the lattice. We use spectroscopic-imaging scanning tunneling microscopy to study atomic-scale signatures of electronic symmetry breaking as a function of temperature across several materials in this family: CsV3Sb5, KV3Sb5 and Sn-doped CsV3Sb5. We find that rotational symmetry breaking onsets universally at a high temperature in these materials, toward the 2 x 2 charge density wave (CDW) transition temperature T*. At a significantly lower temperature of about 30 K, we discover a striking emergence of the quantum interference of coherent quasiparticles, a key signature for the formation of a coherent electronic state. These quasiparticles display a pronounced unidirectional reciprocal-space fingerprint, which strengthens on approaching the superconducting state. Our experiments reveal that the high-temperature charge ordering states are separated from the superconducting ground state by an intermediate-temperature regime with coherent unidirectional quasiparticles. Their emergence that occurs significantly below the onset of rotational symmetry breaking is phenomenologically different compared to high-temperature superconductors, shedding light on the complex nature of electronic nematicity in AV3Sb5 kagome superconductors

Incommensurate charge-stripe correlations in the kagome superconductor CsV3_3Sb5−x_{5-x}Snx_x

We track the evolution of charge correlations in the kagome superconductor CsV$_3$Sb$_5$ as its parent, long-ranged charge density order is destabilized. Upon hole-doping doping, interlayer charge correlations rapidly become short-ranged and their periodicity is reduced by half along the interlayer direction. Beyond the peak of the first superconducting dome, the parent charge density wave state vanishes and incommensurate, quasi-1D charge correlations are stabilized in its place. These competing, unidirectional charge correlations demonstrate an inherent electronic rotational symmetry breaking in CsV$_3$Sb$_5$, independent of the parent charge density wave state and reveal a complex landscape of charge correlations across the electronic phase diagram of this class of kagome superconductors. Our data suggest an inherent 2$k_f$ charge instability and the phenomenology of competing charge instabilities is reminiscent of what has been noted across several classes of unconventional superconductors.Comment: 6 pages, 4 figure

Small Fermi pockets intertwined with charge stripes and pair density wave order in a kagome superconductor

The kagome superconductor family AV3Sb5 (A=Cs, K, Rb) emerged as an exciting platform to study exotic Fermi surface instabilities. Here we use spectroscopic-imaging scanning tunneling microscopy (SI-STM) and angle-resolved photoemission spectroscopy (ARPES) to reveal how the surprising cascade of higher and lower-dimensional density waves in CsV3Sb5 is intimately tied to a set of small reconstructed Fermi pockets. ARPES measurements visualize the formation of these pockets generated by a 3D charge density wave transition. The pockets are connected by dispersive q* wave vectors observed in Fourier transforms of STM differential conductance maps. As the additional 1D charge order emerges at a lower temperature, q* wave vectors become substantially renormalized, signaling further reconstruction of the Fermi pockets. Remarkably, in the superconducting state, the superconducting gap modulations give rise to an in-plane Cooper pair-density-wave at the same q* wave vectors. Our work demonstrates the intrinsic origin of the charge-stripes and the pair-density-wave in CsV3Sb5 and their relationship to the Fermi pockets. These experiments uncover a unique scenario of how Fermi pockets generated by a parent charge density wave state can provide a favorable platform for the emergence of additional density waves

Exploring the Spin State of Cobalt in Oxide Perovskites

Controlling the spin state of a magnetic ion is important in the growing market of spintronic devices. The temperature dependent low spin to high spin transition of Co$^{3+}$ in LaCoO$_{3}$ has been commonly studied, but the factors that determine the spin state of Co$^{2+}$ and Co$^{3+}$ in a particular environment are unclear. In attempt to induce a spin state transition of these ions, Co$^{2+}$ and Co$^{3+}$ were introduced in small amounts in various cubic and orthorhombic perovskites SrTiO$_{3}$, LaAlO$_{3}$, CaSnO$_{3}$, SrSnO$_{3}$, and BaSnO$_{3}$. Here I report that 4% doping of Co into SrTiO$_{3}$ shows a high spin state of Co$^{2+}$ but an intermediate spin state of Co$^{3+}$ with a graduate decrease of effective moment as the cobalt oxidation state is increased from +2 to +3. In a system of LaAlO$_{3}$–SrTiO$_{3}$ doped with 5% Co$^{3+}$, LaAl$_{0.95}$Co$_{0.05}$O$_{3}$ is nonmagnetic, but a mixture of 70% LaAlO$_{3}$ and 30% SrTiO$_{3}$ shows a high spin state of Co$^{3+}$. Finally, the effect of M-O bond length on the spin state of Co$^{3+}$ was examined by doping 10% Co$^{3+}$ into ASnO$_{3}$ (A = Ca, Sr, and Ba). While CaSn$_{0.8}$Co$_{0.1}$Nb$_{0.1}$O$_{3}$ showed Co$^{3+}$ in intermediate spin, increasing the cation size to Ba induced a high spin state. The work presented in this thesis shows the effect of crystal structure, such as M-O bond length, on the spin state of Co$^{2+}$ and Co$^{3+}$. Future work would provide further understanding of factors that determine the spin state of Co ions, allowing for more targeted spintronic materials to be made for various electronic device applications

Structural Effects and Compositional Tuning in Magnetocalorics and Kagome Superconductors

Understanding the impact of structure on observable magnetic and electronic properties is an important aspect in materials chemistry. These insights can be used to develop theoretical models and predict new compounds that may be of interest in a wide range of applications. To study these structure–property relationships, many techniques are typically used, including X–ray diffraction, energy–dispersive X–ray spectroscopy, microscopy, magnetization and transport measurements, and density functional theory calculations. A combination of these measurements and computations can give a clearer picture of the underlying mechanisms that link structure and property so closely.Some materials exhibit a structural change concurrent with its magnetic ordering, and this magnetostructural coupling is proposed to enhance certain magnetic effects. This phenomenon has been explored as a potential route to discovering new materials that exhibit desirable effects, such as in magnetocaloric materials with applications in solid state refrigeration. One such material is AlFe2B2, which exhibits a large magnetocaloric effect that was previously not linked to its structure. Chapter 2 of this dissertation reports an in–depth variable temperature synchrotron X–ray diffraction study that establishes the importance of magnetostructural coupling in the magnetocaloric effect observed in AlFe2B2. In other systems, substituting an element with an isovalent, similar sized element can have pronounced changes on the magnetic and electronic properties. Chapter 3 explores the newly established magnetocaloric MnPdGa and the differences between MnPdGa and MnPtGa. While MnPtGa is also a magnetocaloric that adopts the same crystal structure and is isovalent to MnPdGa, DFT calculations elucidate electronic differences in the two compounds, which may cause their different magnetocaloric performances.Lastly, chapter 4 and appendix B contains a hole–doping study of the AV3Sb5 (A = K, Rb, and Cs) kagome superconductors which have a competing charge density wave order. With very careful and systematic Sn substitution on the Sb site, the superconducting and charge ordering critical temperatures are tuned, and the effects of hole–doping on A = K, Rb vs. A = Cs suggest that the different A site kagomes have subtle but important differences in their electronic structures. DFT calculations support this idea, and while the exact interaction mechanisms between the superconducting and charge density wave ordering phases in AV3Sb5 require further study, the results presented here show a strong structure–property dependence in AV3Sb5