arXiv subjects

Fernando de Juan

Publications and source records attributed to Fernando de Juan.

At least 19 recordsLinked to original sources

arXiv2026

Stacking-tuned superconductivity and competing charge-density-wave states in NbSe$_2$

Layer stacking provides a powerful yet underexplored route for reshaping collective quantum order in van der Waals materials. Here we use high-resolution scanning tunneling microscopy and spectroscopy to show that the stacking sequence alone can qualitatively transform the charge density and superconducting orders in NbSe$_2$, while preserving the same in-plane atomic structure. Comparing the 4Ha and 2H polytypes, we find that, unlike the ubiquitous triangular incommensurate $3Q^\mathrm{I}$ order of 2H-NbSe$_2$, 4Ha-NbSe$_2$ hosts two competing CDW states with no measurable correlation with local strain: a unidirectional commensurate $1Q^\mathrm{C}$ phase and a triangular incommensurate $3Q^\mathrm{I}$ phase, with $Q^\mathrm{I}=Q^\mathrm{C}+\delta$. We introduce a phase-resolved analysis that directly maps the gradient of the CDW phases and reveals vortices bound to the $1Q^\mathrm{C}$ - $3Q^\mathrm{I}$ phase boundaries. These vortices accommodate the momentum mismatch $\delta$ through abrupt $2\pi$ phase slips, providing a mechanism by which distinct charge orders coexist. Superconductivity is also reshaped by stacking, while both polytypes exhibit multiband pairing.

cond-mat.supr-con↗

arXiv2026

Observation of magnetically switchable quantum geometric photocurrents

In non-centrosymmetric materials, light can be rectified into two types of DC photocurrents, known as injection and shift currents, through the bulk photovoltaic effect. Recent theory has uncovered their deep relation with the two-state quantum geometry of resonant transitions: In non-magnetic crystals, where these currents have been routinely observed, the injection current responds to circular light and probes the Berry curvature, while the shift current responds to linear light and probes the geometric connection. Magnetic crystals have been predicted to show a new set of hitherto unobserved magnetically switchable photocurrents, with the roles of linear and circular light interchanged: A linear injection current, which probes the quantum metric, and a circular shift current, which probes the geometric torsion. In this work, we demonstrate the existence of such currents for the first time, demonstrating the switching of the current by flipping the N\'eel vector in a van der Waals antiferromagnet. Furthermore, their specific frequency and temperature dependence confirm the assignment of circular shift and linear injection currents. Our work demonstrates a new way to control photocurrents in magnets that are directly tied to geometry and have promising applications in antiferromagnetic spintronics and light harvesting.

cond-mat.mtrl-sci↗

arXiv2026

Transport signatures of valley polarization in graphene multilayers: In-plane linear magnetoconductivity vs anomalous Hall effect

In two-dimensional materials where interacting Fermi pockets occur in valleys related by time-reversal symmetry, a spontaneous valley imbalance results in a novel state known as an orbital magnet. Due to the breaking of time-reversal symmetry, this state can be probed in transport experiments by the violation of Onsager relations, most often done through the anomalous Hall effect (AHE). Here we propose that odd-in-field, in-plane linear magnetoconductivity (LMC) is an alternative probe of valley polarization which can occur even when the AHE vanishes. In multilayer structures, the effect originates from in-plane orbital moments and Berry curvatures enabled by interlayer tunneling and dominates over the spin response. After a classification of many recently studied multilayers, we focus on two valley polarized examples: twisted bilayer graphene, where LMC is finite but the AHE vanishes unless additional symmetry breaking from the substrate is present, and rhombohedral graphene multilayers, where LMC and AHE both track valley polarization because they have the same symmetry. Using self-consistent Hartree-Fock and semiclassical transport calculations, we present detailed predictions of LMR for these two examples and analyze the implications for recent experiments.

cond-mat.mes-hall↗

arXiv2026

Four-layer charge density waves and chirality in CsV$_3$Sb$_5$

The kagome superconductor CsV$_3$Sb$_5$ is the only one in the AV$_3$Sb$_5$ family (A=K,Rb,Cs) that shows a $2\times2\times4$ charge-density-wave (CDW) ground state competing with the more common $2\times2\times2$. In addition, it is also the only one that shows second-harmonic transport and thus broken inversion symmetry, suggesting these two features are connected. In this work, we test whether the $2\times2\times4$ CDW can break inversion symmetry by analyzing its stacking energetics with a real-space Ginzburg-Landau free energy. In the limit where each layer is forced to adopt a fixed, threefold symmetric tri-hexagonal configuration, we find an analytical phase diagram mostly occupied by inversion preserving AB and ABCD stacked solutions. Relaxing this rigid layer constraint in a physically meaningful way consistent with \emph{ab initio}--calculated parameters, we find several new phases including an AABC solution and a distorted ABCD solution, which break inversion and all mirrors and are thus chiral. However, these phases occupy only small fractions of phase space, suggesting other mechanisms for inversion symmetry breaking may be at play in CsV$_3$Sb$_5$.

cond-mat.str-el↗

arXiv2026

Anomalous Nonlinear Magnetoconductivity in van der Waals Magnet CrSBr

Nonlinear magnetoconductivity (NLMC) is a nonreciprocal transport response arising in non-centrosymmetric materials. However, this ordinary NLMC signal vanishes at zero magnetic field, limiting its potential for applications. Here, we report the observation of an anomalous NLMC controlled by internal order parameters such as the magnetization or N\'eel vectors. We achieve this response by breaking both inversion and time-reversal symmetry in artificial van der Waals heterostructures based on the magnetic CrSBr and insulating hBN. The nonreciprocal signal can be tuned between two different states in ferromagnetic monolayer CrSBr and among four different states in antiferromagnetic bilayer CrSBr, thanks to its metamagnetic transition. Remarkably, this output signal in the ferromagnetic (antiferromagnetic) state of CrSBr is three (one) orders of magnitude higher than those previously measured. A conductivity scaling analysis reveals the Berry connection polarizability as the origin of the anomalous NLMC. Our results pave the way for high-frequency rectifiers with magnetically switchable output polarity as well as for an efficient electrical readout of the magnetic state of antiferromagnetic materials.

cond-mat.mes-hall↗

arXiv2026

Ferroaxial and nematic transitions in the charge density wave phase of 1T-TiSe$_2$

Charge density waves (CDWs) with multi-component order parameters can break unexpected symmetries through the interplay of nearly degenerate instabilities. In the widely investigated material 1T-TiSe$_2$, a central question is whether the observed CDW has a chiral character, which would manifest as the spontaneous breaking of mirror and inversion symmetries. Previous experiments have reported conflicting results about the broken symmetries in the CDW phase of 1T-TiSe$_2$. Here, we resolve this controversy by identifying the bulk broken symmetry as ferroaxial, corresponding to the breaking of vertical mirrors while preserving inversion symmetry. Using symmetry-resolved elastoresistivity, we detect the spontaneous emergence of intrinsic off-diagonal elastoresistivity coefficients that satisfy an antisymmetric relation ($m_{xx-yy,xy} \approx -m_{xy,xx-yy}$), providing an unambiguous bulk transport signature of a macroscopic electric toroidal moment. Simultaneous elastocaloric measurements reveal that the onset of ferroaxial order occurs just below the CDW transition. As the temperature is lowered further, a diverging nematic susceptibility signals a distinct rotational symmetry-breaking instability inside the ferroaxial CDW state. Our findings demonstrate that the proposed ``chiral'' CDW in 1T-TiSe$_2$ is actually a centrosymmetric ferroaxial state, reconciling previous surface-sensitive observations with bulk symmetry constraints.

cond-mat.str-el↗

arXiv2026

Topologically Protected Surface Altermagnetism on Antiferromagnets

Altermagnetism (AM) and its associated spin-transport phenomena are typically linked to spin-split electronic band structures in bulk materials. However, the crystal surface has a reduced symmetry with respect to the bulk, which can induce AM at the surface of conventional antiferromagnets (AFMs) $\unicode{x2013}$ a local effect which cannot be detected using bulk properties. In this work we define the symmetry conditions necessary for surface AM and show how it can be topologically protected, rendering it a robust effect. We provide a minimal model for one trivial and two topological examples of surface AM. We show that the spin spectral density, accessible by spin- and angle-resolved photoemission spectroscopy, can exhibit a $d$-wave-like altermagnetic character at the surface, even when the full band structure is completely spin degenerate. Our topological model describes the Dirac semimetal CuMnAs, which provides an existing realization of our theory. Our results identify crystal surfaces as a platform to realize robust, topology- and symmetry-driven unconventional magnetism beyond the bulk classification of magnetic materials.

cond-mat.str-el↗

arXiv2025

Doping-induced nematic and stripe orders within the charge density wave state of TiSe$_2$

In this work, we present a theory to address conflicting experimental claims regarding the charge density wave (CDW) state in TiSe$_2$, including whether there is a single or multiple CDW transitions and whether threefold rotation symmetry ($C_3$) is broken. Using a continuum $\boldsymbol{k}\cdot\boldsymbol{p}$ model coupled to the CDW order parameter, we show how commonplace conduction band doping induces a nematic transition from a $C_3$-symmetric $3Q$ CDW to a $C_3$-breaking $3Q$ CDW, which is favored by the large ellipticity of the conduction bands of TiSe$_2$. We also find that a $1Q$ stripe CDW is generically stabilized for sufficiently high electron doping. We then show how both stripe and nematic CDW states emerge self-consistently from a minimal interacting tight-binding model, for both positive and negative initial gaps. Our theory provides a new scenario in which, as temperature is lowered, a second $C_3$-breaking transition may occur or not depending on the doping level, potentially explaining the experimental variability. These predictions can be further verified with a variety of probes including transport, photoemission and tunneling.

cond-mat.str-el↗

arXiv2025

Layer-selective Cooper pairing in an alternately stacked transition metal dichalcogenide

Multigap superconductivity emerges when superconducting gaps form on distinct Fermi surfaces. Arising from locally overlapping atomic orbitals, multiple superconducting bands introduce a new internal degree of freedom in the material that, however, escapes external control due to their coexistence in real space in the known multigap superconductors. Here, we show that the layered superconductor 4Hb-TaSSe - composed of alternating trigonal (H) and octahedral (T) polymorph layers - is a multigap superconductor, featuring two weakly coupled superconducting condensates with distinct properties, spatially separated in alternating layers. Using high-resolution quasiparticle tunneling and Andreev reflection spectroscopy in the two polymorph layers, we identify two superconducting gaps that vary in size and internal structure. The intrinsic Cooper pairing in each polymorph is corroborated by the temperatures and magnetic fields at which the gaps open up, which differ in each polymorph layer and show opposing resilience to these parameters. This behavior enables selective external actuation upon the condensates. Our theoretical model based on ab-initio calculations reproduces key features of the observed superconducting gaps in the presence of finite interlayer hybridization and explains the unusually high critical field observed in the T-layer. Our results establish TMD polymorphs as platforms for engineering tunable multigap superconductors, offering new opportunities in layered superconducting device architectures.

cond-mat.supr-con↗

arXiv2025

In-plane anisotropy of charge density wave fluctuations in 1$T$-TiSe$_2$

We report measurements of anisotropic triple-$q$ charge density wave (CDW) fluctuations in the transition metal dichalcogenide 1$T$-TiSe$_2$ over a large volume of reciprocal space with X-ray diffuse scattering. Above the transition temperature, $T_{\text{CDW}}$, the out-of-plane diffuse scattering is characterized by rod-like structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. In addition, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations. At $T_{\text{CDW}}$, long range coherence is established in both the in-plane and out-of-plane directions, consistent with the large observed value of the CDW gap compared to $T_{\text{CDW}}$, and the predicted presence of a hierarchy of energy scales.

cond-mat.str-el↗

arXiv2025

Optical probes of two-component pairing states in transition metal dichalcogenides

Signatures of unconventional superconductivity have been recently observed in certain transition metal dichalcogenides (TMDs), including 4H$_b$-TaS$_2$ and monolayer 2H-NbSe$_2$. While the pairing channel remains unknown, it has been argued that spin fluctuations can stabilize pairing in the two-component $E'$ channel, a $p$-wave spin-triplet state which could be consistent with some of the reported signatures. Exploiting the particular multi-orbital character of the Fermi surface and the presence of Ising spin-orbit coupling, which enable finite optical conductivity in the clean limit, in this work we predict clear-cut optical signatures to detect and distinguish the chiral and nematic ground states of the $E'$ pairing. We quantify how nematic $E'$ states produce a diagonal anisotropy $\sigma_{xx}\!\neq\!\sigma_{yy}$ due to the broken threefold symmetry ($C_3$), while chiral $E'$ states yield a finite optical Hall conductivity $\sigma_{xy}^H$ due to broken time-reversal symmetry, and find both signals could be detected in current experiments. For instance, for realistic gaps in the meV range, we predict a relative anisotropy $\Delta\sigma/\sigma\sim10^{-5}$ in the nematic states, and a polar Kerr rotation of $\theta_K\!\sim\!10^{-5}$ rad in the chiral states. These symmetry fingerprints provide a practical route to distinguish nematic and chiral superconducting order in TMD superconductors.

cond-mat.supr-con↗

arXiv2025

Magnetic injection photocurrents in valley polarized states of twisted bilayer graphene

Magic-angle twisted bilayer graphene displays a complex phase diagram as a function of flat band filling, featuring compressibility cascade transitions and a variety of competing ground states with broken spin, valley and point group symmetries. Recent THz photocurrent spectroscopy experiments have shown a dependence on the filling which is not consistent with the simplest cascade picture of sequential filling of equivalent flat bands. In this work, we show that when time-reversal symmetry is broken due to valley polarization, a magnetic injection photocurrent develops which can be used to distinguish different spin-valley polarization scenarios. Using the topological heavy fermion model we compute both shift and injection currents as a function of filling and argue that current experiments can be used to determine the spontaneous valley polarization.

cond-mat.mes-hall↗

arXiv2025

Probing the Quantized Berry Phases in 1H-NbSe$_2$ Using Scanning Tunneling Microscopy

Topologically trivial insulators are classified into two primary categories: unobstructed and obstructed atomic insulators. While both types can be described by exponentially localized Wannier orbitals, a defining feature of obstructed atomic insulators is that the centers of charge of these orbitals are positioned at empty sites within the unit cell, rather than on atoms. Despite extensive theoretical predictions, the unambiguous and quantitative experimental identification of an obstructed atomic phase has remained elusive. In this work, we present the first direct experimental evidence of such a phase in 1H-NbSe$_2$. We develop a novel method to extract the inter-orbital correlation functions from the local spectral function probed by scanning tunneling microscopy (STM), leveraging the orbital wave functions obtained from ab initio calculations. Applying this technique to STM images, we determine the inter-orbital correlation functions for the atomic band of 1H-NbSe$_2$ that crosses the Fermi level. Our results show that this band realizes an optimally compact obstructed atomic phase, providing the first unambiguous experimental identification of such a phase. Our approach of deconvolving the STM signal using ab initio orbital wave functions is broadly applicable to other material platforms, offering a powerful tool for exploring other electronic phases.

cond-mat.mes-hall↗

arXiv2024

Unveiling intrinsic bulk photovoltaic effect in atomically thin ReS2

The bulk photovoltaic effect (BPVE) offers a promising avenue to surpass the efficiency limitations of current solar cell technology. However, disentangling intrinsic and extrinsic contributions to photocurrent remains a significant challenge. Here, we fabricate high-quality, lateral devices based on atomically thin ReS2 with minimal contact resistance, providing an optimal platform for distinguishing intrinsic bulk photovoltaic signals from other extrinsic photocurrent contributions originating from interfacial effects. Our devices exhibit large bulk photovoltaic performance with intrinsic responsivities of 1 mA/W in the visible range, without the need for external tuning knobs such as strain engineering. Our experimental findings are supported by theoretical calculations. Furthermore, our approach can be extrapolated to investigate the intrinsic BPVE in other non-centrosymmetric van der Waals materials, paving the way for a new generation of efficient light-harvesting devices.

cond-mat.mes-hall↗

arXiv2024

Ising domain wall networks from intertwined charge density waves in single-layer TiSe2

When the period of an incommensurate charge density wave (ICDW) approaches an integer multiple of a lattice vector, the energy gain obtained from locking the period to the lattice can lead to a fascinating transition into a commensurate state. This transition actually occurs through an intermediate near-commensurate (NC) phase, with locally commensurate regions separated by an ordered array of phase slips of a complex CDW order parameter. TiSe2 is a paradigmatic CDW system where incommensuration is believed to be induced by carrier doping, yet its putative NC state has never been imaged or its nature established. Here we report the observation of a striking NC state in ultraclean, slightly doped monolayers of TiSe2, displaying an intricate network of coherent, unidirectional CDW domain walls over hundreds of nanometers. Detailed analysis reveals these are not phase slips of a complex CDW, but rather sign-changing Ising-type domain walls of two coupled real CDWs of previously known symmetry, consistent with the period doubling nature of the parent commensurate state. In addition, we observe an unexpected nematic modulation at the original lattice Bragg peaks which couples to the CDW order parameters. A Ginzburg-Landau analysis naturally explains the couplings and relative modulations of all order parameters, unveiling TiSe2 as a rare example of an NC-CDW of two intertwined real modulations and emergent nematicity.

cond-mat.str-el↗

arXiv2024

Atomic-scale mapping of superconductivity in the incoherent CDW mosaic phase of a transition metal dichalcogenide

The emergence of superconductivity in the octahedrally coordinated (1T) phase of TaS2 is preceded by the intriguing loss of long-range order in the charge density wave (CDW). Such decoherence, attainable by different methods, results in the formation of nm-sized coherent CDW domains bound by a two-dimensional network of domain walls (DW) - mosaic phase -, which has been proposed as the spatial origin of the superconductivity. Here, we report the atomic-scale characterization of the superconducting state of 1T-TaSSe, a model 1T compound exhibiting the CDW mosaic phase. We use high-resolution scanning tunneling spectroscopy and Andreev spectroscopy to probe the microscopic nature of the superconducting state in unambiguous connection with the electronic structure of the mosaic phase. Spatially resolved conductance maps at the Fermi level at the onset of superconductivity reveal that the density of states is mostly localized on the CDW domains compared to the domain walls, which suggests their dominant role in the formation of superconductivity. This scenario is confirmed within the superconducting dome at 340 mK, where superconductivity is fully developed, and the subtle spatial inhomogeneity of the superconducting gap remains unlinked to the domain wall network. Our results provide key new insights into the fundamental interplay between superconductivity and CDW in these relevant strongly correlated systems.

cond-mat.supr-con↗

arXiv2024

Charge transfer in T/H heterostructures of transition metal dichalcogenides

The $\sqrt{13}\times\sqrt{13}$ charge density wave state of the T polytype of MX$_2$ (M=Nb,Ta, X=S, Se) is known to host a half-filled flat band, which electronic correlations drive into a Mott insulating state. When T polytypes are coupled to strongly metallic H polytypes, such as in T/H bilayer heterostructures or the bulk 4H$_b$ polytype, charge transfer can destabilize the Mott state, but quantifying its magnitude has been a source of controversy. In this work, we perform a systematic ab-initio study of charge transfer for all experimentally relevant T/H bilayers and bulk 4H$_b$ structures. In all cases we find charge transfer from T to H layers which depends strongly on the interlayer distance but weakly on the Hubbard interaction. Additionally, Se compounds display smaller charge transfer than S compounds, and 4H$_b$ bulk polytypes display more charge transfer than isolated bilayers. We rationalize these findings in terms of band structure properties, and argue they might explain differences between compounds observed experimentally. Our work reveals the tendency to Mott insulation and the origin of superconductivity may vary significantly across the family of T/H heterostructures.

cond-mat.mtrl-sci↗

arXiv2024

Universal features of entanglement entropy in the honeycomb Hubbard model

The entanglement entropy is a unique probe to reveal universal features of strongly interacting many-body systems. In two or more dimensions these features are subtle, and detecting them numerically requires extreme precision, a notoriously difficult task. This is especially challenging in models of interacting fermions, where many such universal features have yet to be observed. In this paper we tackle this challenge by introducing a new method to compute the Rényi entanglement entropy in auxiliary-field quantum Monte Carlo simulations, where we treat the entangling region itself as a stochastic variable. We demonstrate the efficiency of this method by extracting, for the first time, universal subleading logarithmic terms in a two dimensional model of interacting fermions, focusing on the half-filled honeycomb Hubbard model at $T=0$. We detect the universal corner contribution due to gapless fermions throughout the Dirac semi-metal phase and at the Gross-Neveu-Yukawa critical point, where the latter shows a pronounced enhancement depending on the type of entangling cut. Finally, we observe the universal Goldstone mode contribution in the antiferromagnetic Mott insulating phase.

cond-mat.str-el↗