Kagome topological metals, such as AV3Sb5 (A = Cs, Rb, K), exhibit a rich array of competing electronic orders, including charge density waves (CDWs), superconductivity, and electronic nematicity. CsV3Sb5 (CVS), in particular, shows a CDW transition around 90 K and a superconducting transition around 2.5 K. However, evidence from various techniques (µSR, STM, NMR, elastoresistance, SHG) suggests additional phases between these transitions, particularly around 35 K. These studies indicate a hidden phase that breaks both rotational and time-reversal symmetries, but with conflicting reports on the exact nature and timing of these symmetry breakings. This study uses in-plane magnetotransport measurements on CVS thin flakes to probe this hidden phase and elucidate its symmetry and underlying mechanism.

Previous research on AV3Sb5 compounds has revealed a complex interplay of electronic orders. Studies have reported observations of topological surface states, superconductivity with a pair density wave, electronic nematicity, charge density waves, chiral transport, anomalous Hall effect, and time-reversal symmetry breaking. The existence of a hidden phase below ~35 K has been suggested by several groups using different experimental techniques, including muon spin rotation (µSR), scanning tunneling microscopy (STM), nuclear magnetic resonance (NMR), elastoresistance measurements (EM), and second-harmonic generation (SHG). However, these studies have yielded inconsistent conclusions regarding the timing and nature of the symmetry breaking associated with this hidden phase. The exact mechanism remains elusive, prompting further investigation.

Thin flakes of CsV3Sb5 crystals were obtained using Al2O3-assisted exfoliation. Devices were fabricated using e-beam lithography and e-beam evaporation of Ti/Au electrodes. In-plane magnetoresistance (MR) measurements were performed using an Oxford Teslatron cryostat and a Quantum Design PPMS, with magnetic fields up to 14 T and temperatures ranging from 1.5 K to 300 K. The magnetic field angle was varied in the x-y plane, and the thickness of the samples was measured using atomic force microscopy (AFM). Hall effect measurements were also conducted to investigate time-reversal symmetry breaking.

The in-plane MR of CVS thin flakes exhibits a complex angular dependence that changes with both magnetic field strength and temperature. At low fields, a two-fold symmetry (C2') is observed, attributed to the classical Lorentz force effect. At intermediate and high fields, more complex patterns emerge, including a prominent six-fold symmetry (C6) at low temperatures (T < 35 K). This C6 component is associated with the hidden phase and is linked to the spatial symmetry of an orbital current order. The C2 symmetry persists to higher temperatures and is linked to the CDW order. Furthermore, a large negative in-plane MR is observed below ~35 K, independent of the current direction relative to the magnetic field. This negative MR is linked to the suppression of orbital current fluctuations by the magnetic field, further supporting the existence of a three-dimensional orbital current ordered phase. The temperature dependence of the C6 component and the negative MR coincide with a sudden increase in both Hall and anomalous Hall resistances below ~35 K, confirming the link to the hidden phase.

The observed six-fold symmetry (C6) in the in-plane MR and the large negative MR strongly suggest a three-dimensional orbital current ordered phase below ~35 K. The negative MR can be explained by the suppression of interlayer scattering by the in-plane magnetic field, implying strong interlayer interactions. The simultaneous emergence of the C6 component, negative MR, and anomalous Hall effect at ~35 K suggests a complex interplay between different electronic orders. This supports the presence of a three-dimensional hidden phase that is both electronically nematic and exhibits time-reversal symmetry breaking. The thickness dependence of the MR further reinforces the three-dimensional nature of this hidden phase, as it is absent in thinner samples where a dimensional crossover occurs.

This study reveals a three-dimensional hidden phase in CsV3Sb5 thin flakes below ~35 K, characterized by a six-fold symmetric in-plane magnetoresistance and a large negative magnetoresistance. This phase is linked to orbital current order and exhibits strong interlayer interactions. Further investigations into the microscopic details of this complex interplay of electronic orders are warranted.

The study focused on thin flakes of CsV3Sb5, which might exhibit different properties compared to bulk samples due to surface effects and altered interlayer coupling. The analysis relies on fitting the in-plane MR data to specific symmetry components, which might be affected by the presence of other, smaller, harmonic terms. Furthermore, the exact nature of the interaction between the orbital current order and the CDW order requires further investigation.