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Pervasive electronic nematicity as the parent state of kagome superconductors

Muxian Xu, Siyu Cheng, Andrea Capa Salinas, Ganesh Pokharel, Alexander LaFleur, Hong Li, Hengxin Tan, Brenden R. Ortiz, Qinwen Deng, Binghai Yan, Ziqiang Wang, Stephen D. Wilson, Ilija Zeljkovic

TL;DR

This paper addresses whether electronic nematicity in kagome superconductors is a consequence of the $2\times 2$ CDW or an intrinsic parent state. Using spectroscopic-imaging STM, the authors suppress the CDW in CsV3Sb5 via Sn and Ti doping and map the electronic structure across the phase diagram. They find persistent short-range electronic nematic domains across wide dopant ranges even after CDW suppression, indicating nematicity is not tied to the $2\times 2$ CDW. The results position electronic nematicity as a generic, intrinsic feature of the kagome superconductor phase diagram and suggest it underpins the emergence of other low-temperature phenomena.

Abstract

Kagome superconductors $A$V$_3$Sb$_5$ ($A$ = Cs, K, Rb) have developed into an exciting playground for realizing and exploring exotic solid state phenomena. Abundant experimental evidence suggests that electronic structure breaks rotational symmetry of the lattice, but whether this may be a simple consequence of the symmetry of the underlying 2 $\times$ 2 charge density wave phase or an entirely different mechanism remains intensely debated. We use spectroscopic imaging scanning tunneling microscopy to explore the phase diagram of the prototypical kagome superconductor CsV$_3$Sb$_5$ as a function of doping. We intentionally suppress the charge density wave phase with chemical substitutions selectively introduced at two distinct lattice sites, and investigate the resulting system. We discover that rotational symmetry breaking of the electronic structure -- now present in short-range nanoscale regions -- persists in all samples, in a wide doping range long after all charge density waves have been suppressed. As such, our experiments uncover ubiquitous electronic nematicity across the $A$V$_3$Sb$_5$ phase diagram, unrelated to the 2 $\times$ 2 charge density wave. This further points towards electronic nematicity as the intrinsic nature of the parent state of kagome superconductors, under which other exotic low-temperature phenomena subsequently emerge.

Pervasive electronic nematicity as the parent state of kagome superconductors

TL;DR

This paper addresses whether electronic nematicity in kagome superconductors is a consequence of the CDW or an intrinsic parent state. Using spectroscopic-imaging STM, the authors suppress the CDW in CsV3Sb5 via Sn and Ti doping and map the electronic structure across the phase diagram. They find persistent short-range electronic nematic domains across wide dopant ranges even after CDW suppression, indicating nematicity is not tied to the CDW. The results position electronic nematicity as a generic, intrinsic feature of the kagome superconductor phase diagram and suggest it underpins the emergence of other low-temperature phenomena.

Abstract

Kagome superconductors VSb ( = Cs, K, Rb) have developed into an exciting playground for realizing and exploring exotic solid state phenomena. Abundant experimental evidence suggests that electronic structure breaks rotational symmetry of the lattice, but whether this may be a simple consequence of the symmetry of the underlying 2 2 charge density wave phase or an entirely different mechanism remains intensely debated. We use spectroscopic imaging scanning tunneling microscopy to explore the phase diagram of the prototypical kagome superconductor CsVSb as a function of doping. We intentionally suppress the charge density wave phase with chemical substitutions selectively introduced at two distinct lattice sites, and investigate the resulting system. We discover that rotational symmetry breaking of the electronic structure -- now present in short-range nanoscale regions -- persists in all samples, in a wide doping range long after all charge density waves have been suppressed. As such, our experiments uncover ubiquitous electronic nematicity across the VSb phase diagram, unrelated to the 2 2 charge density wave. This further points towards electronic nematicity as the intrinsic nature of the parent state of kagome superconductors, under which other exotic low-temperature phenomena subsequently emerge.

Paper Structure

This paper contains 4 sections, 1 equation, 4 figures.

Table of Contents

  1. Abstract
  2. Introduction
  3. Results
  4. Discussion

Figures (4)

  • Figure 1: Crystal structure and the evolution of the 2 $\times$ 2 CDW in CsV$_{3}$Sb$_{5-x}$Sn$_{x}$ - a, 3D ball model of the crystal structure of CsV$_{3}$Sb$_{5}$. b, A schematic phase diagram denoting the 2 $\times$ 2 CDW state and the superconducting state as a function of Sn doping. The four blue circles indicate the doping levels and temperatures at which the measurements in this study were performed. c-f, Representative STM topographs of the Sb-terminated surfaces for $x=0$ (c), $x=0.1$ (d), $x=0.33$ (e), $x=0.68$ (f). g-j, Fourier transforms (FTs) of the corresponding topographs in (c-f). FT peaks corresponding to the atomic lattice ($\textbf{Q}_{Bragg}$), the 2$\times$2 CDW ($\textbf{Q}_{CDW}$) and the 4$\times$1 charge stripes ($\textbf{Q}_{stripe}$) are labeled in g. In h–j, the atomic Bragg peaks and the expected CDW peak positions are marked using the same symbols as those in (g). 2 $\times$ 2 CDW peaks vanish for samples with $x \geq 0.1$. k, FT linecuts along the $\textbf{Q}_{stripe}$ direction (if present), showing the suppression of the 2 $\times$ 2 CDW peaks at higher Sn compositions. STM setup conditions: $V_{sample}$=100 mV, $I_{set}$=50 pA (c); $V_{sample}$=20 mV, $I_{set}$=200 pA (d); $V_{sample}$=20 mV, $I_{set}$=300 pA (e); $V_{sample}$=20 mV, $I_{set}$=200 pA (f).
  • Figure 2: Spectroscopic-imaging STM comparison of the $x = 0$ and $x = 0.33$ Sn-doped CsV$_3$Sb$_5$ samples. - a,c, Representative differential conductance maps ($g(r,V) = dI(r,V)/dV$) for samples with $x = 0$ and $x = 0.33$. The inset in the bottom right of a shows the Fourier transform (FT) of a typical differential conductance map for $x = 0$. b,d, Waterfall plot of FT linecuts of the $g(r,V)$ maps for $x = 0$ and $x = 0.33$ samples, starting from the FT center towards one atomic Bragg peak. e–h, FTs of $g(r,V)$ maps at selected energies for the $x = 0.33$ sample. The corresponding energies are indicated by the colored triangles in d. The line cuts in b and d are taken along the red dashed lines shown in the insets of a and g. i,k, Constant energy contours (CECs) obtained from DFT calculations for $k_z=0$. j,l, Autocorrelation of the CECs in i and k, respectively. Orange arrow in i denotes the part in the CEC that leads to the scattering and interference pattern marked by the orange arrow in f and j. STM setup conditions: $V_{sample}$=20 mV, $I_{set}$=40 pA, $V_{exc}$=3 mV (a); $R_{tip-sample}$=1.5 G$\Omega$, $I_{set}$=200 pA, $V_{exc}$=4 mV (b); $V_{sample}$=20 mV, $I_{set}$=300 pA, $V_{exc}$=1 mV (c); $R_{tip-sample}$=0.6 G$\Omega$, $I_{set}$=1 nA, $V_{exc}$=20 mV (d, e-h).
  • Figure 3: Nanoscale domains with anisotropic modulations.a,c, Representative high-resolution d$I$/d$V$(r, $V$) maps with $x = 0.33$ and $x = 0.68$. b,d, Selected regions of the d$I$/d$V$(r, $V$) maps (top row) and their Fourier transforms (FTs) (bottom row). Each selected region is dominated by the charge modulation along one crystalline direction (marked by the colored arrows) being substantially stronger compared to other directions. Scattering peaks marked by the colored arrows in the FTs further demonstrate the nematic nature of charge modulation with about $\frac{1}{2}\textbf{Q}_{Bragg}$ wave vector. STM setup conditions: $V_{sample}$=-200 mV, $I_{set}$=500 pA, $V_{exc}$=5 mV (a); $V_{sample}$=50 mV, $I_{set}$=200 pA, $V_{exc}$=10 mV (c).
  • Figure 4: Electronic nematic domains in the Ti-doped CsV$_{3-y}$Sb$_5$Ti$_y$ samples. - a, Representative STM topograph of CsV$_{3-y}$Sb$_{5}$Ti$_{y}$ with $y=0.2$, and b, Fourier transform (FT) of the area in (a). Colored circles mark the atomic Bragg peaks, while the blue squares mark the expected 2 $\times$ 2 CDW peak locations. The inset in the bottom left of b shows the FT line cuts of b, starting from the FT center towards each atomic Bragg peaks, showing the absence of 2 $\times$ 2 CDW. c,d, STM topographs of individual nematic domains. The insets in the bottom left of c and d are their FTs, with the atomic Bragg peaks with the strongest intensity marked by the colored circles. e,f, Amplitude map of the lattice modulations obtained by applying a Fourier-space filter that isolates a single pair of atomic Bragg peaks. Enhanced intensity in this map corresponds to regions where the lattice modulation is locally stronger along this direction. STM setup conditions: $V_{sample}$=100 mV, $I_{set}$=100 pA (a,c,d).