Peer review of: Band insulator to Mott insulator transition in 1T-TaS2 (2020)

Reviewers' comments: 
Reviewer #1 (Remarks to the Author): 

The manuscript reports results of temperature-dependent electrical resistance, ARPES, and XRD 
measurements of the layered CDW reference material 1T-TaS2. The focus is on the temperature 
region where the pristine bulk material changes, upon heating, from a commensurate (C) CDW phase 
over a triclinic (T) CDW state to a nearly commensurate (NC), domain-like CDW phase. The major 
finding is a qualitative change in the ARPES spectral weight distribution and XRD intensity scan at an 
intermediate temperature (around 217 K) preceding the C-T CDW transition. Below this transition 
temperature, evidence is reported for an electronically gapped state with non-negligible band 
dispersion and a doubled unit cell in the z direction. In a small interval above the transition 
temperature (217-233 K), the band structure still exhibits a gap, but the dispersion appears more 2D-
like and c-axis doubling is absent. The observation is interpreted as a transition from a Peierls 
insulating ground state to a novel intermediate Mott insulating state. 

The study addresses an important open problem for 1T-TaS2, i.e., the nature of its ground state. A 
flurry of experimental and theoretical activities has recently looked into this question, inspired by the 
finding that interlayer coupling, specifically CDW stacking, can play an important role in determining 
the electronic structure at the Fermi level. A common view in the field currently seems to be that a 
single S-Ta-S layer is a Mott insulator, while the bulk crystal is a Peierls insulator due to CDW stacking 
effects, and the surface of the bulk maybe somewhere in between. But clear experimental verification 
is still lacking so that the results of the present study are, in principle, timely, relevant, and interesting. 

Unfortunately, however, there are a number of major issues that prevent me from concluding that the 
presented results can contribute to a better understanding of the phase transition behavior of the 
material and its ground state: 

(i) There is a significant inconsistency in the phase transition behavior observed with the three 
different techniques used. The measured resistance displays the known behavior: NC-C transition at 
180 K upon cooling and C-T transition at 220 K upon heating. The ARPES data, by contrast, show a 
NC-C transition upon cooling at 193 K (Fig. 2c) and the C-I transition upon heating at 217 K (Figs. 2e 
and 2f), whereas this latter transition is observed in the XRD data at 212 K (Fig. 4b). These 
differences call into question the quality of the samples and the reproducibility of the results. The 
authors explicitly state that “the temperature-dependent data are well reproducible in many different 
samples”, but they do not show the data. 

(ii) The ARPES evidence for non-negligible k_z dispersion in the low-temperature C phase is only 
indirect, provided by the supposedly k_z smearing-induced spectral weight shadow reflecting the 
projected band dispersion along the z direction (Fig. 3a, rightmost panel, taken at 175 K). What is 
disturbing here is that a different data normalization has apparently been applied to this ARPES 
intensity map compared to the one next to it taken in the intermediate “I” phase at 225 K (this can be 
seen in the photoemission intensity above the Fermi level). In order to convincingly establish the 
presence or absence of k_z dispersion in the C and I phase, respectively, photon energy-dependent 
measurements are required. 

(iii) Although the XRD evidence for c-axis doubling in the low-temperature C phase appears to be 
more direct, as given by the (0, 0, 5/2) and (0, 0, 7/2) superstructure reflections in Fig. 4b, the origin 
of these diffraction peaks, their intensities, and the underlying CDW stacking order remain completely 
unclear. This potentially novel result is also not put into the context of the results of previous 
structural determinations of the C phase. A convincing structural analysis of the transition to the 
intermediate phase is needed. 

(iv) A key difference between the applied experimental techniques is neither mentioned nor discussed 
in the manuscript: transport and XRD measurements are bulk sensitive, while ARPES is highly surface 
sensitive. So, an important question is to what extent the reported ARPES results are representative 
of bulk behavior. Considering the differences in transition temperatures and the absence of the 233 K 
transition from the reported bulk data, it may well be that the intermediate phase seen in ARPES is a 
surface effect, similar to the surface Mott transition that is known to occur in the sister material 1T-
TaSe2. That is, the intermediate phase would explicitly not reflect “an intrinsic property of 1T-TaS2”. 

Some minor points are the following: 

(v) The c lattice-parameter change occurring at 212 K should be quantified. 

(vi) Heating and cooling rates for the ARPES and XRD measurements should be stated. 

(vii) Experimental details of the transport measurements should be given. 

In conclusion, while the combined ARPES and XRD results are in principle interesting, the 
experimental evidence provided is mostly indirect and there is a lack of technical rigor so that the 
central conclusion (“transition from band insulator to Mott insulator at intermediate temperature”) is 
not substantiated by the data. Furthermore, the discussion and explanation of the results remains at a 
purely qualitative level, whereas quantitative insight is needed to make progress in better 
understanding this intriguing material. The manuscript in its present form cannot be recommended for 
publication. A major revision is needed. 



Reviewer #2 (Remarks to the Author): 

The authors of the article “Band insulator to Mott insulator transition in 1T-TaS2” have performed 
resistivity, ARPES and X-ray diffraction measurement of the transition metal dichalcogenide 1T-TaS2. 
Their data convincingly show that a dimerization of the unit cell in the stacking direction takes place at 
metal insulator transition. These data confirm important results that have been already published in 
Ref.8 and Ref.9 of the article. In addition, the authors discovered the presence of a new phase that 
shows up at 217 K while heating up the sample. After reading the article, we decided to perform the 
experiment our self and we confirm the existence of such new phase. 
This article is very interesting, well written and based on reproducible, high quality data. I have no 
doubt that will generate the interest of broad community. On the other hand, minor comments should 
be taken into account in order to improve the manuscript: 

1) The authors propose that the new phase is a Mott insulator. However, the resistivity shows a jump 
to down to the metallic value when heating above 217 K. The authors should explain this apparent 
incongruity. 
2) It would be very valuable if the authors could estimate the magnitude of displacement related to 
the dimerization. 
3) The authors have cited in Ref.23 a time resolved ARPES experiment on 1T-TaS2 but they 
overlooked to cite the first work on this subject (L. Perfetti, Physical review letters 97, 067402 (2006)). 
This negligence must be corrected. 


Reviewer #3 (Remarks to the Author): 

The authors performed temperature-dependent ARPES and XRD measurements, giving a band 
insulating ground state with interlayer dimerization of bulk 1T-TaS2 single crystal and existence of an 
intermediate Mott insulating state. This layered compound with simple crystal structure provides a 
great platform to investigate its complex electronic phase transitions (multi-type CDWs, 
superconductivity and so on) and the physical origins, from bulk crystals down to few-layer devices. 
Recent theoretical and STM studies on few-layer samples emphasize the importance of interlayer 
stacking order. I would suggest the following comments should be addressed before consideration. 

1) As stated in the end of this manuscript, it is intriguing that no transport anomaly responds to the 
observed intermediate Mott insulating state. Will that be observable in thin sample? What is the 
thickness of the sample measured in this study? Since the contribution of interlayer interaction would 
be more significant in thin sample, I would suggest the authors to compare the current results with 
that on thin sample. 
2) line 105: as noted, the temperature-dependent data are well reproducible in many different 
samples. I would suggest the authors plot all the data in comparison, at least presenting it in 
supplementary material as well as the differences between used samples. 
3) Figure 4(a): I would suggest that the intensity (y-axis) should be better presented in log scale since 
the (00l) peaks are too strong. Some other peaks other than (00l) and (00l/2) should also be 
addressed. 
4) The quality of 1T phase single crystals strongly depends on the quenching process and growing 
temperatures. More details of the quality of the single crystals should be addressed, such as SEM 
image, EDX result (usually S-deficiency exists), powder XRD and refinement and so on.