Long-lifetime spin excitations near domain walls in 1T-TaS2

Abstract

SignificanceThere is an intense ongoing search for two-level quantum systems with long lifetimes for applications in quantum communication and computation. Much research has been focused on studying isolated spins in semiconductors or band insulators. Mott insulators provide an interesting alternative platform but have been far less explored. In this work we use a technique capable of resolving individual spins at atomic length scales, to measure the two-level switching of spin states in 1T-TaS₂. We find quasi-1D chains of spin-1/2 electrons embedded in 1T-TaS₂ which have exceptionally long lifetimes. The discovery of long-lived spin states in a tractable van der Waal material opens doors to using Mott systems in future quantum information applications.

Keywords: noise spectroscopy; scanning tunneling microscopy; spin chain.

Fig. 1.

CDW DW in 1T-TaS₂: structure, morphology, and localized quasi-1D electrons near the DW. (A) Schematic of the SD reconstruction in monolayer TaS₂ showing the positions of the tantalum (smaller purple dots) and sulfur (blue dots) atoms. (B) Large area STM topography (50 nm × 50 nm) showing the √13 × √13 reconstructed surface (V_S = 300 mV, I_t = 30 pA). (Scale bar, 5 nm.) (Bottom Inset) A zoomed in view of the reconstructed lattice with the SD overlay. (Scale bar, 1 nm.) (Top Inset) Typical dI/dV spectrum far from DWs. The two peaks around ±0.2 eV are associated with the UHB and LHB, respectively. (C) STM topography (18 nm × 18 nm) of an area with the DW studied in this work. The SD clusters on the two sides of the DW are marked in yellow and red (V_bias = 400 mV, I_t = 40 pA). (Scale bar, 5 nm.) (D) Topography obtained simultaneously with the LDOS maps on a DW. Green/blue circles show the location of the localized spins. (E) LDOS map at −50 mV. The green/blue colored dots label the location of the localized low-energy electrons. (F) LDOS map at −160 mV (close to the energy of the LHB). The dark area reflects a suppression of the DOS due to band bending. (Scale bar in D–F, 1 nm.) (G) dI/dV spectra obtained on the localized electron sites (blue and green dots in E) and a far-off site (pink). The spectra shown are averaged over similar sites. The shaded gray area denotes the Mott gap. The shaded blue area denotes the energy of the localized electrons inside the Mott gap.

Fig. 2.

Quasi-1D spin chains with two-level noise with a spin-polarized tip. (A) Topography of a DW obtained with spin-polarized tip at 300 mK. (Scale bar, 1 nm.) (B) Schematic of the electrons/spin chains along the DW inside the Mott gap. The red dashed line depicts the position of the DW. (C) Noise measurements, i.e., time traces of the tunnel current, obtained on the marked sites (as indicated in A). The time traces have been color-referenced to the clusters they were obtained on. The sweeps are offset by 30 pA for clarity. The blue and green sweeps show the presence of two-level telegraph noise on the SD cluster near the DW and an absence of it on the DW. The measurements were made with a spin-polarized tip, tunnel current I_t = 30 pA, and V_Bias = 30 mV with the feedback loop turned off. (D) The histogram plots of the current sweeps show a two-level distribution for the blue site (which is absent for the DW site).

Fig. 3.

Inelastic electron tunneling spectroscopy (IETS) signal in the dI/dV spectra at 300 mK obtained with a W-tip. (A) Topography of the DW showing clusters where the noise or dI/dV spectra in B were obtained. (Scale bar, 1 nm.) (B) dI/dV spectrum obtained on cluster with noise showing an inelastic signal (V_bias = 50 mV, I_t = 50 pA) (purple). dI/dV spectrum on a nearby cluster without noise which also does not show an inelastic signal. (V_bias = −70 mV, I_t = 50 pA) (green). (C) Derivative of the dI/dV signals showing the symmetric peak–dip feature at the energies associated with the inelastic mode (purple) but a featureless signal for the green curve. (D) IETS of different energies obtained at different locations near the DW shown in C. (E and F) Histogram of the energies of the peaks and dips of d²I/dV², respectively, obtained around the DW shown by the dotted rectangle in A. The fitted Gaussian shows that the IETS signals at different locations near the DW follow a distribution around a mean value of 11 meV.

Fig. 4.

Spin-based NDC on the trapped electron sites measured with a spin-polarized tip. (A) Tunnel current obtained with a spin-polarized tip as a function of bias showing NDC at low energies near a DW. The locations of the spectra are indicated by gray and red circles in C and D. (B) dI/dV spectrum corresponding to A. The NDC manifests as negative values of the dI/dV. (C) Density of states (LDOS) map near DW at −25 mV showing the trapped electrons. (D) NDC map obtained with a spin-polarized tip near DW. A strong correlation between position of the electrons and strong NDC below E_F is evident (the details of obtaining the NDC map are in SI Appendix). (E) Normalized dI/dV spectra as a function of set point tunneling current obtained with a spin-polarized tip. V_Bias = 400 mV. The spectra have been normalized at 400 mV. The gray dashed line indicates the zero conductance. The NDC is small when the tip is far away (60 pA) but becomes successively stronger as the tip is moved closer to the sample (480 pA).

Fig. 5.

Anticorrelated switching on nearby clusters measured with a spin-polarized tip. (A) Topography of the DW showing clusters where the noise spectra were obtained with the spin-polarized tip. (Scale bar, 1 nm.) (B and C) Schematic of the spin-polarized tunnel current when the spins flip from a state with a lower current to a higher current and vice versa. (D, F, H, and J) Current sweeps with feedback off obtained on clusters shown in C. All the sweeps were obtained at V_bias = 30 mV, I_t = 30 pA at T = 300 mK. A small linear background has been subtracted from all of them that occurs due to piezo relaxation when the feedback is turned off. D and J show current behavior similar to the scenario in B, and F and H show current behavior similar to the scenario in C. (E, G, I, and K) Histograms of the current sweeps that show anticorrelated switching behavior between the two states for neighboring clusters. E and G and I and K show pairwise anticorrelation between the occupation of the up and down states. In J, the switching rate is much lower compared to D, F, and H possibly because the spin is located farther from the other spins.