Qirong Yao, Hyunjin Jung, Kijeong Kong, Chandan De, Jaeyoung Kim, Jonathan D. Denlinger, Han Woong Yeom
We report on the Tomonaga-Luttinger liquid (TLL) behavior in fully degenerate 1D Dirac fermions. A ternary van der Waals material Nb$_9$Si$_4$Te$_{18}$ incorporates in-plane NbTe$_2$ chains, which produce a 1D Dirac band crossing Fermi energy. Tunneling conductance of electrons confined within NbTe2 chains is found to be substantially suppressed at Fermi energy, which follows a power law with a universal temperature scaling, hallmarking a TLL state. The obtained Luttinger parameter of ~0.15 indicates strong electron-electron interaction. The TLL behavior is found to be robust against atomic-scale defects, which might be related to the Dirac electron nature. These findings, as combined with the tunability of the compound and the merit of a van der Waals material, offer a robust, tunable, and integrable platform to exploit non-Fermi liquid physics.
Banik Rai, Sandip Kumar Kuila, Rana Saha, Sankalpa Hazra, Chandan De, Jyotirmoy Sau, Venkatraman Gopalan, Partha Pratim Jana, Stuart S. P. Parkin, Nitesh Kumar
Trigonal Cr$_5$Te$_8$, a self-intercalated van der Waals ferromagnet with an out-of-plane magnetic anisotropy, has long been known to crystallize in a centrosymmetric structure. However, optical second harmonic generation experiments, together with comprehensive structural analysis, indicate that this compound rather adopts a non-centrosymmetric structure. Lorentz transmission electron microscopy reveals the presence of Néel-type skyrmions, consistent with its non-centrosymmetric structure. A large anomalous Hall conductivity of 102 ohm$^{-1}$cm$^{-1}$ at low temperature stems from intrinsic origin, which is larger than any previously reported values in the bulk Cr-Te system. Notably, spontaneous topological Hall resistivity arising from the skyrmionic phase has been observed. Our findings not only elucidate the unique magnetic and magneto-transport properties of non-centrosymmetric trigonal Cr$_5$Te$_8$, but also open new avenues for investigating the effects of broken inversion symmetry on material properties and their potential applications.
Y. Fujisawa, P. Wu, R. Okuma, B. R. M. Smith, D. Ueta, R. Kobayashi, N. Maekawa, T. Nakamura, C-H. Hsu, Chandan De, N. Tomoda, T. Higashihara, K. Morishita, T. Kato, Z. Y. Wang, Y. Okada
Following the discovery of graphene, interest in van der Waals (vdW) materials has surged; however, advancing physics beyond graphene requires quantum vdW materials platforms that host versatile, strongly interacting many-body states. Here, using scanning tunneling microscopy and spectroscopy at 300 mK, we uncover multiple competing electronic states in the van der Waals metal CeTe3: charge-ordered antiferromagnetic phases forming stripe and checkerboard orders. Remarkably, their competition is tuned by a modest in-plane magnetic field (~1.5 T), revealing strongly intertwined multiple frustrations involving antiferromagnetism, charge order, and Fermi-surface instabilities. Quasiparticle-interference imaging directly identifies the momentum-space origin of these competitions on the representative semimetals Fermi surface. While the observations can be understood at a basic level in terms of Kondo coupling between localized Ce 4f moments and itinerant Te 5p electrons, our results reveal a much richer phenomenology: an unusually broad electronic reconstruction extending to an energy scale of roughly 30 meV from EF, which realizes and deforms antiferromagnetic charge-ordered states and signals strongly correlated interactions beyond a weak-coupling description. Beyond establishing CeTe3 as a model platform, our results demonstrate that competing instabilities in antiferromagnetic two-dimensional metals/semimetals generate versatile electronic phases, opening a route to tunable nanoscale quantum states governed by the intertwined effects of correlation, symmetry, and topology.
Chandan De, Rabindranath Bag, Surjeet Singh, Fabio Orlandi, P. Manuel, Sean Langridge, Milan K Sanyal, C. N. R. Rao, Maxim Mostovoy, A. Sundaresan
Recent progress in the field of multiferroics led to the discovery of many new materials in which ferroelectricity is induced by cycloidal spiral orders. The direction of the electric polarization is typically constrained by spin anisotropies and magnetic field. Here, we report that the mixed rare-earth manganite, Gd$_{0.5}$Dy$_{0.5}$MnO$_3$, exhibits a spontaneous electric polarization along a general direction in the crystallographic ac-plane, which is suppressed below 10 K but re-emerges in an applied magnetic field. Neutron diffraction measurements show that the polarization direction results from a large tilt of the spiral plane with respect to the crystallographic axes and that the suppression of ferroelectricity is caused by the transformation of a cycloidal spiral into a helical one, a unique property of this rare-earth manganite. The freedom in the orientation of the spiral plane allows for a fine magnetic control of ferroelectricity, i.e. a rotation as well as a strong enhancement of the polarization depending on the magnetic field direction. We show that this unusual behavior originates from the coupling between the transition metal and rare-earth magnetic subsystems.
Chandan De, A. Sundaresan
We report the occurrence of both \emph{ab} and \emph{bc} cycloidal ordering of Mn-spins at different temperatures and their possible coexistence at low temperatures in the polycrystalline mixed rare-earth compounds, \emph{R}$_{0.5}$Dy$_{0.5}$MnO$_3$ (\emph{R} = Eu and Gd), which exhibit extraordinary magnetoelectric properties. While the polarization of Gd$_{0.5}$Dy$_{0.5}$MnO$_3$ is comparable to TbMnO$_3$, the compound Eu$_{0.5}$Dy$_{0.5}$MnO$_3$ shows high value of polarization. However, both of them show giant magnetic tunability and exhibit large magnetocapacitance whose sign changes across the two cycloidal ordering temperatures. Intriguingly, the electric polarization can be reversed upon ramping up or ramping down the magnetic field, which has not been observed for any of the \emph{R}MnO$_3$ system. Most strikingly, these compounds show non-volatile ferroelectric memory effect even in the paraelectric and paramagnetic region (T$_C$ $\leq$ T $\leq$ 80 K). We attribute these remarkable properties to the coexistence of \emph{ab} and \emph{bc} cycloidal ordered phases.
Subrata Ghosh, Rosalin Mohanty, Yuwei Sun, Soumi Mondal, Chandan De, Jose G. Jimenez, Weiwei Xie, Cheng Gong, Zhiqiang Mao
Two-dimensional (2D) van der Waals (vdW) multiferroics have emerged as a promising platform for next-generation multifunctional devices. Although recent studies have demonstrated that artificial heterostructures can combine dual ferroic orders and exhibit strong magnetoelectric coupling, their performance is sometimes limited by poor interface quality and inadequate long-term stability. By contrast, the realization of intrinsic single-phase materials with coexisting ferromagnetism and ferroelectricity remains a longstanding challenge in the field. Here we report the realization of a single-phase 2D vdW multiferroic system, CuIn0.2V0.8P2S6, which exhibits both ferromagnetism and room-temperature ferroelectricity. The intrinsic ferroelectric nature of CuIn0.2V0.8P2S6 was probed using ferroelectric tunnel junctions, which exhibit a large tunneling electroresistance with an ON/OFF ratio of 107 at 295 K. CuIn0.2V0.8P2S6 develops ferromagnetic ordering with the Curie temperature (TC) of 14.6 K, as evidenced by pronounced magnetic hysteresis and a relatively large remanent magnetization. Notably, the appearance of a magnetodielectric response below TC is consistent with the anticipated interplay between the ferromagnetic and ferroelectric orders. These results highlight a promising route toward single-phase van der Waals multiferroics with coexisting ferroic orders.
Xiaoyu Guo, Rachel Owen, Austin Kaczmarek, Xiaochen Fang, Chandan De, Youngjun Ahn, Wei Hu, Nishkarsh Agarwal, Suk Hyun Sung, Robert Hovden, Sang-Wook Cheong, Liuyan Zhao
Domain walls are ubiquitous in materials that undergo phase transitions driven by spontaneous symmetry breaking. Domain walls in ferroics and multiferroics have received tremendous attention recently due to their emergent properties distinct from their domain counterparts, for example, their high mobility and controllability, as well as their potential applications in nanoelectronics. However, it is extremely challenging to detect, visualize and study the ferro-rotational (FR) domain walls because the FR order, in contrast to ferromagnetism (FM) and ferroelectricity (FE), is invariant under both the spatial-inversion and the time-reversal operations and thus hardly couple with conventional experimental probes. Here, an FR candidate $\mathrm{NiTiO_{3}}$ is investigated by ultrasensitive electric quadrupole (EQ) second harmonic generation rotational anisotropy (SHG RA) to probe the point symmetries of the two degenerate FR domain states, showing their relation by the vertical mirror operations that are broken below the FR critical temperature. We then visualize the real-space FR domains by scanning EQ SHG microscopy, and further resolve the FR domain walls by revealing a suppressed SHG intensity at domain walls. By taking local EQ SHG RA measurements, we show the restoration of the mirror symmetry at FR domain walls and prove their unconventional nonpolar nature. Our findings not only provide a comprehensive insight into FR domain walls, but also demonstrate a unique and powerful tool for future studies on domain walls of unconventional ferroics, both of which pave the way towards future manipulations and applications of FR domain walls.
Kai Du, Xiaochen Fang, Choongjae Won, Chandan De, Fei-ting Huang, Fernando J. Gomez-Ruiz, Adolfo Del Campo, Sang-Wook Cheong
The formation of topological defects after a symmetry-breaking phase transition is an overarching phenomenon that encodes rich information about the underlying dynamics. Kibble-Zurek mechanism (KZM), which describes these nonequilibrium dynamics, predicts defect densities of these second-order phase transitions driven by thermal fluctuations. It has been verified as a successful model in a wide variety of physical systems, finding applications from structure formation in the early universe to condensed matter systems. However, whether topologically-trivial Ising domains, one of the most common and fundamental types of domains in condensed matter systems, also obey the KZM has never been investigated in the laboratory. We examined two different kinds of three-dimensional (3D) structural Ising domains: clockwise (CW)/counter-clockwise (CCW) ferro-rotation domains in NiTiO3 and up/down polar domains in BiTeI. While the KZM slope of ferro-rotation domains in NiTiO3 agrees well with the prediction of the 3D Ising model, the KZM slope of polar domains in BiTeI surprisingly far exceeds the theoretical limit, setting an exotic example where possible weak long-range dipolar interactions play a critical role in steepening the KZM slope of non-topological quantities. Our results demonstrate the validity of KZM for Ising domains and reveal an enhancement of the power-law exponent and a possible reduction of the dynamic critical exponent z for transitions with long-range interactions.
Chandan De, Somnath Ghara, A. Sundaresan
Pyro-current measurements have been widely used to study ferroelectric properties in multiferroic materials. However, determination of intrinsic polarization by this method is not straightforward because of leakage current and trapped charge carriers. Here, we demonstrate the formation of internal electric field due to thermally stimulated charge carriers and its influence on ferroelectric polarization in a polycrystalline sample of the well known multiferroic TbMnO$_3$. While an electric field ($E_{ext}$) poling across the ferroelectric transition ($T_C$ $\sim$ 26 K) is essential to obtain depolarization current at $T_C$, the sample poled only in the paraelectric state ($T_{pole}$ = 130 $-$ 50 K) also exhibits a pyro-current peak at $T_C$ but with the same polarity ($-$ $I_{pyro}$) as that of the external field ($-$ $E_{ext}$). We demonstrate that these unusual behavior of pyro-current are caused by a positive internal electric field ($+$ $E_{int}$) which in turn is created by thermally stimulated free charge carriers during the poling process in the paraelectric state. We also show that a combination of DC-biased current and pyro-current measurements is a promising method to study the intrinsic ferroelectric properties in multiferroic materials.