Zhen Zhan

• Doctor of Engineering
• PostDoc Position at Wuhan University

Moiré materials represent strongly interacting electron systems bridging topological and correlated physics. Despite notable advances, decoding wavefunction properties underlying the quantum geometry remains challenging. Here we utilize polarization-resolved photocurrent measurements to probe magic-angle twisted bilayer graphene, leveraging its sen...

We investigate the electronic structure of graphene monolayers subjected to patterned dielectricsuperlattices. Through a quantum capacitance model approach, we simulate realistic devices capable of imposing periodic potentials on graphene. By means of both tight-binding and continuummodels, we analyze the electronic structure across varied patterni...

We investigate the electronic properties of two-dimensional electron gases (2DEGs) subjected to a periodic patterned gate. By incorporating the superlattice potential (SL) induced by patterning into the Schrodinger equation, we develop a methodology for obtaining exact analytical solutions. These solutions enable us to construct a comprehensive pha...

Moiré structures formed by twisting three layers of graphene with two independent twist angles present an ideal platform for studying correlated quantum phenomena, as an infinite set of angle pairs is predicted to exhibit flat bands. Moreover, the two mutually incommensurate moiré patterns among the twisted trilayer graphene (TTG) can form highly t...

We investigate the electronic structure of graphene monolayers subjected to patterned dielectric superlattices. Through a quantum capacitance model approach, we simulate realistic devices capable of imposing periodic potentials on graphene. By means of both tight-binding and continuum models, we analyze the electronic structure across varied patter...

The relentless pursuit of band structure engineering continues to be a fundamental aspect in solid-state research. Here, we meticulously construct an artificial kagome potential to generate and control multiple Dirac bands of graphene. This unique high-order potential harbors natural multiperiodic components, enabling the reconstruction of band str...

Moir\'e materials represent strongly interacting electron systems bridging topological and correlated physics. Despite significant advances, decoding wavefunction properties underlying the quantum geometry remains challenging. Here, we utilize polarization-resolved photocurrent measurements to probe magic-angle twisted bilayer graphene, leveraging...

Experiments conducted on two-dimensional twisted materials have revealed a plethora of moiré patterns with different forms and shapes. The formation of these patterns is usually attributed to the presence of small strains in the samples, which typically arise during their fabrication. In this paper we find that the superlattice structure of such sy...

Twist-controlled moiré superlattices (MSs) have emerged as a versatile platform for realizing artificial systems with complex electronic spectra. The combination of Bernal-stacked bilayer graphene (BLG) and hexagonal boron nitride (hBN) can give rise to an interesting MS, which has recently featured a set of unexpected behaviors, such as unconventi...

Twisted bilayer graphene (TBG) represents a highly tunable, strongly correlated electron system owed to its unique flat electronic bands. However, understanding the single-particle band structure alone has been challenging due to complex lattice reconstruction effects and a lack of spectroscopic measurements over a broad energy range. Here, we prob...

In this study, we use a full tight-binding model to explore the behavior of graphene nanobubbles between the interface of twisted bilayer graphene (TBG-1.05∘), uncovering the simultaneous existence of confined states and pseudo-Landau level (pLL) states under minor strain conditions. We find that the energy separation of the confined states aligns...

Ultraflat bands have already been detected in twisted bilayer graphene and twisted bilayer transition-metal dichalcogenides, which provide a platform to investigate strong correlations. In this paper, the electronic properties of twisted trilayer molybdenum disulfide (TTM) are investigated via an accurate tight-binding Hamiltonian. We find that the...

Theoretical and experimental studies have verified the existence of “magic angles” in twisted bilayer graphene, where the rotation angle between layers gives rise to flat bands and consequently exotic correlated phases. Recently, magic-angle phenomena have been predicted and reported in other graphene systems, for instance, multilayers with alterna...

The discoveries of numerous exciting phenomena in twisted bilayer graphene (TBG) are stimulating significant investigations on moiré structures that possess a tunable moiré potential. Optical response can provide insights into the electronic structures and transport phenomena of non-twisted and twisted moiré structures. In this article, we review...

Moiré structures formed by twisting three layers of graphene with two independent twist angles present an ideal platform for studying correlatedquantum phenomena, as an infinite set of angle pairs is predicted to exhibit flat bands. Moreover, the two mutually incommensurate moiré patterns among the twisted trilayer graphene (TTG) can form highly t...

Twist-controlled moire superlattices (MS) have emerged as a versatile platform in which to realize artificial systems with complex electronic spectra. Bernal-stacked bilayer graphene (BLG) and hexagonal boron nitride (hBN) form an interesting example of the MS that has recently featured a set of unexpected behaviors, such as unconventional ferroele...

Experiments conducted on two-dimensional twisted materials have revealed a plethora of moiré patterns with different forms and shapes. The formation of these patterns is usually attributed to the presence of small strains in the samples, which typically arise during their fabrication. In this work we find that the superlattice structure of such sys...

Ultraflat bands have already been detected in twisted bilayer graphene (TBG) and twisted bilayer transition metal dichalcogenides (tb-TMDs), which provide a platform to investigate strong correlations. In this paper, the electronic properties of twisted trilayer molybdenum disulfide (TTM) are investigated via an accurate tight-banding Hamiltonian....

Motivated by the recent experimental detection of superconductivity in Bernal bilayer (AB) and rhombohedral trilayer (ABC) graphene, we study the emergence of superconductivity in multilayer graphene based on a Kohn-Luttinger (KL) -like mechanism in which the pairing glue is the screened Coulomb interaction. We find that electronic interactions alo...

Rydberg excitons, the solid-state counterparts of Rydberg atoms, have sparked considerable interest with regard to the harnessing of their quantum application potentials, but realizing their spatial confinement and manipulation poses a major challenge. Lately, the rise of two-dimensional moiré superlattices with highly tunable periodic potentials p...

Theoretical and experimental studies have verified the existence of ``magic angles'' in twisted bilayer graphene, where the twist between layers gives rise to flat bands and consequently highly correlated phases. Narrow bands can also exist in multilayers with alternating twist angles, and recent theoretical work suggests that they can also be foun...

Motivated by the recent experimental detection of superconductivity in Bernal bilayer (AB) and rhombohedral trilayer (ABC) graphene, we study the emergence of superconductivity in multilayer graphene based on a Kohn-Luttinger (KL)-like mechanism in which the pairing glue is the screened Coulomb interaction. We find that electronic interactions alon...

The recent observed anomalous Hall effect in magic angle twisted bilayer graphene (TBG) aligned to hexagonal boron nitride (hBN) and unconventional ferroelectricity in Bernal bilayer graphene sandwiched by hBN present a platform to tune the correlated properties in graphene systems. In these graphene-based moiré superlattices, the aligned hBN subst...

Rydberg excitons, the solid-state counterparts of Rydberg atoms, have sparked considerable interest in harnessing their quantum application potentials, whereas a major challenge is realizing their spatial confinement and manipulation. Lately, the rise of two-dimensional moir\'e superlattices with highly tunable periodic potentials provides a possib...

In two-dimensional small-angle twisted bilayers, van der Waals (vdW) interlayer interaction introduces an atomic-scale reconstruction, which consists of a moiré-periodic network of local subdegree lattice rotations. However, real-space measurement of the subdegree lattice rotation requires extremely high spatial resolution, which is an outstanding...

Twisted graphene multilayers have been recently demonstrated to share several correlation-driven behaviors with twisted bilayer graphene. In general, the van Hove singularities (VHSs) can be used as a proxy of the tendency for correlated behaviors. In this paper, we adopt an atomistic method by combining the tight-binding method with the semiclassi...

The recent observed anomalous Hall effect in magic angle twisted bilayer graphene (TBG) aligned to hexagonal boron nitride (hBN) and unconventional ferroelectricity in Bernal bilayer graphene sandwiched by hBN present a new platform to tune the correlated properties in graphene systems. In these graphene-based moir\'e superlattices, the aligned hBN...

Twisted graphene multi-layers have been recently demonstrated to share several correlation-driven behaviours with twisted bilayer graphene. In general, the van Hove singularities (VHSs) can be used as a proxy of the tendency for correlated behaviours. In this paper, we adopt an atomistic method by combining tight-binding method with the semi-classi...

TBPLaS is an open-source software package for the accurate simulation of physical systems with arbitrary geometry and dimensionality utilizing the tight-binding (TB) theory. It has an intuitive object-oriented Python application interface (API) and Cython/Fortran extensions for the performance critical parts, ensuring both flexibility and efficienc...

Strain-induced pseudomagnetic fields can mimic real magnetic fields to generate a zero-magnetic-field analog of the Landau levels (LLs), i.e., the pseudo-Landau levels (PLLs), in graphene. The distinct nature of the PLLs enables one to realize novel electronic states beyond what is feasible with real LLs. Here, we show that it is possible to realiz...

Twisted bilayer transition metal dichalcogenides are ideal platforms to study flat-band phenomena. In this paper, we investigate flat-band plasmons in the hole-doped twisted bilayer MoS2 (tb-MoS2) by employing a full tight-binding model and the random phase approximation. When considering lattice relaxations in tb-MoS2, the flat band is not separat...

In this work, we study heterostructures of TBG and hexagonal boron nitride (hBN) using an atomistic tight-binding model together with semi-classical molecular dynamics to consider relaxation effects. The hBN substrate has significant effects on the band structure of TBG even in the case where TBG and hBN are not aligned. Specifically, the substrate...

When two two-dimensional (2D) materials with different lattice constants or with different rotation angles are stacked on top of another, a moiré superlattice can be constructed. The electronic properties of the superlattice are strongly dependent on the stacking configurations, twist angles and substrates. For instance, theoretically, when the rot...

Strain-induced pseudo-magnetic fields can mimic real magnetic fields to generate a zero-magnetic-field analogue of the Landau levels (LLs), i.e., the pseudo-LLs, in graphene. The distinct nature of the pseudo-LLs enables one to realize novel electronic states beyond that can be feasible with real LLs. Here, we report the realization of one-dimensio...

Recent experimental and theoretical investigations demonstrate that twisted trilayer graphene (tTLG) is a highly tunable platform to study the correlated insulating states, ferromagnetism, and superconducting properties. Here we explore the possibility of tuning electronic correlations of the tTLG via a vertical pressure. A full tight-binding model...

Twisted bilayer graphene (TBG) has taken the spotlight in the condensed matter community since the discovery of correlated phases at the so-called magic angle. Interestingly, the role of a substrate on the electronic properties of TBG has not been completely elucidated. Up to now, most of the theoretical works carried out in order to understand thi...

Twisted bilayer transition metal dichalcogenides are ideal platforms to study flat-band phenomena. In this paper, we investigate flat-band plasmons in doped twisted bilayer {\mos} by employing a full tight-binding model and the random phase approximation. Twist-angle effects on flat-band plasmons in twisted bilayer {\mos} show that plasmons at a sm...

Recent experimental and theoretical investigations demonstrate that twisted trilayer graphene (tTLG) is a highly tunable platform to study the correlated insulating states, ferromagnetism, and superconducting properties. Here we explore the possibility of tuning electronic correlations of the tTLG via a vertical pressure. A full tight-binding model...

Twisted graphene multilayers exhibit strongly correlated insulating states and superconductivity due to the presence of ultraflat bands near the charge neutral point. In this paper, the response of ultraflat bands to lattice relaxation and a magnetic field in twisted trilayer graphene (tTLG) with different stacking arrangements is investigated by u...

Twisted bilayer graphene with tiny rotation angles have drawn significant attention due to the observation of the unconventional superconducting and correlated insulating behaviors. In this paper, we employ a full tight-binding model to investigate collective excitations in twisted bilayer graphene near the magic angle. The polarization function is...

Twisted graphene multilayers have been demonstrated to host strongly correlated insulating states and superconductivity due to the presence of ultraflat bands near the charge neutral point. In this paper, the response of ultraflat bands to the lattice relaxation and magnetic field in twisted trilayer graphene (tTLG) with different stacking arrangem...

In the emerging world of twisted bilayer structures, the possible configurations are limitless, which enables a rich landscape of electronic properties. In this paper, we focus on twisted bilayer transition metal dichalcogenides (TMDCs) and study their properties by means of an accurate tight-binding model. We build structures with different angles...

Ultraflat bands that have been theoretically and experimentally detected in a bunch of van der Waals stacked materials show some peculiar properties, for instance, highly localized electronic states and enhanced electron-electron interactions. In this Rapid Communication, using an accurate tight-binding model, we study the formation and evolution o...

Twisted bilayer graphene with tiny rotation angles have drawn significant attention due to the observation of the unconventional superconducting and correlated insulating behaviors. In this paper, we employ a full tight-binding model to investigate collective excitations in twisted bilayer graphene near magic angle. The polarization function is obt...

The electronic structures of 30∘ twisted double bilayer graphene, which loses the translational symmetry due to the incommensurate twist angle, are studied by means of the tight-binding approximation. We demonstrate the interlayer decoupling across the 30∘ twisted interface in the vicinity of the Fermi level from various electronic properties, incl...

In the emerging world of twisted bilayer structures, the possible configurations are limitless, which enables for a rich landscape of electronic properties. In this paper, we focus on twisted bilayer transition metal dichalcogenides (TMDCs) and study its properties by means of an accurate \textit{ab initio} tight-binding model. We build structures...

Ultraflatbands that have been theoretically and experimentally detected in a bunch of van der Waals stacked materials show some peculiar properties, for instance, highly localized electronic states and enhanced electron-electron interactions. In this Letter, using an accurate \textit{ab initio} tight-binding model, we study the formation and evolut...

A properly strained graphene monolayer or bilayer is expected to harbour periodic pseudo-magnetic fields with high symmetry, yet to date, a convincing demonstration of such pseudo-magnetic fields has been lacking, especially for bilayer graphene. Here, we report a definitive experimental proof for the existence of large-area, periodic pseudo-magnet...

Dodecagonal bilayer graphene quasicrystal has 12-fold rotational order but lacks translational symmetry which prevents the application of band theory. In this paper, we study the electronic and optical properties of graphene quasicrystal with large-scale tight-binding calculations involving more than ten million atoms. We propose a series of period...

In this paper, the electronic properties of several multilayer graphene quasicrystals are studied by means of the tight-binding method. The multilayer graphene quasicrystals are stacked in AA, AB or their combined orders, and the quasi-periodicity is introduced by rotating at least one layer by 30 degree. The periodic approximants of several multil...

Dodecagonal bilayer graphene quasicrystal has 12-fold rotational order but lacks translational symmetry which prevents the application of band theory. In this paper, we study the electronic and optical properties of graphene quasicrystal with large-scale tight-binding calculations involving more than ten million atoms. We propose a series of period...

Dodecagonal bilayer graphene quasicrystal has 12-fold rotational order but lacks translational symmetry which prevents the application of band theory. In this paper, we study the electronic and optical properties of graphene quasicrystal with large-scale tight-binding calculations involving more than ten million atoms. We propose a series of period...

A properly strained graphene monolayer or bilayer is expected to harbour periodic pseudo-magnetic fields with high symmetry, yet to date, a convincing demonstration of such pseudo-magnetic fields has been lacking, especially for bilayer graphene. Here, we report the first definitive experimental proof for the existence of large-area, periodic pseud...

An intrinsic electron injection model for two-dimensional (2D) Dirac materials, like graphene, is presented and its coupling to a recently developed quantum time-dependent Monte Carlo simulator for electron devices, based on the use of stochastic Bohmian conditional wave functions, is explained. The simulator is able to capture the full (dc, ac, tr...

Because of its large Fermi velocity, leading to a great mobility, graphene is expected to play an important role in (small signal) radio frequency electronics. Among other developments, graphene devices based on Klein tunneling phenomena have already been envisioned. The connection between the Klein tunneling times of electrons and the cut-off freq...

The proper modeling of the 2D materials requires a discussion on the boundary conditions between the device active region (open system) and the environment, to determine how and when electrons are injected. In devices with gapless materials, like graphene, the injection of electrons with positive and negative kinetic energies are needed for an accu...

Because of its large Fermi velocity, leading to a great mobility, graphene is expected to play an important role in (small signal) radio frequency electronics. Among other, graphene devices based on Klein tunneling phenomena are already envisioned. The connection between the Klein tunneling times of electrons and cut-off frequencies of graphene dev...

The simulation of electronic devices always implies a partition between an open system and its environment. The environment fixes the conditions at the borders of the open system through local and non-local thermodynamic arguments. For time dependent simulations, such boundary conditions determine how and when electrons are injected into the open s...

In this work we tackle two different issues. We first study the effects of the size of the cross section of the channel of silicon-based Gate-All-Around Field Effect Transistors (GAAFETs) on their performance. By computing the admittance parameters, |Y12| and |Y11|, and the hybrid parameter |H21|, we demonstrate a strategy to improve the intrinsic...

Without access to the full quantum state, modelling dissipation in an open system requires approximations. The physical soundness of such approximations relies on using realistic microscopic models of dissipation that satisfy completely positive dynamical maps. Here we present an approach based on the use of the Bohmian conditional wave function th...

Without access to the full quantum state, modeling dissipation in an open system requires approximations. The physical soundness of such approximations relies on using realistic microscopic models of dissipation that satisfy completely positive dynamical maps. Here we present an approach based on the use of the Bohmian conditional wave function tha...

The definition of the intrinsic cutoff frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) based on the current gain equals to one (0 dB) is critically analyzed. A condition for the validity of the quasistatic estimation of f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:...

The Wigner distribution function (WDF) is a very successful simulation tool for quantum phenomena. By construction (in spite of having negative values in its phase space), it exactly reproduces the charge and current densities in the physical space. In this work, the combination of the WDF and the Boltzmann collision operator for dissipative quantu...

The intrinsic cut-off frequency ($f_T$) of nanoscale transistors based on the current gain equals to one (0 dB) is critically analyzed. Since $f_T$ is a figure of merit when comparing different technologies, we suggest that predictions of $f_T$ have to be based on numerical simulations of the delay time in the total (particle plus displacement) dra...

In this work, the use of the Boltzmann collision operator for dissipative quantum transport is analyzed. Its mathematical role on the description of the time-evolution of the density matrix during a collision can be understood as processes of adding and subtracting states. We show that unphysical results can be present in quantum simulations when t...

Although time-independent models provide very useful dynamical information with a reduced computational burden, going beyond the quasi-static approximation provides enriched information when dealing with TeraHertz (THz) frequencies. In this work, the THz noise of dual-gate graphene transistors with DC polarization is analyzed from a careful simulat...

Although time-independent models provide very useful dynamical information with a reduced computational burden, going beyond the quasi-static approximation provides enriched information when dealing with TeraHertz (THz) frequencies. In this work, the THz noise of dual-gate graphene transistors with DC polarization is analyzed from a careful simulat...

An effective single-particle Schrodinger equation to include dissipation into quantum devices is presented. This effective equation is fully understood in the context of Bohmian mechanics, a theory of particles and waves, where it is possible to define unambiguously the wave function of a subsystem, the so-called conditional wave function. In parti...

The Wigner distribution function is a quasi-probability distribution. When properly integrated, it provides the correct charge and current densities, but it gives negative probabilities in some points and regions of the phase space. Alternatively, the Husimi distribution function is positive-defined everywhere, but it does not provide the correct c...

The Wigner distribution function is a quasi-probability distribution. When properly integrated, it provides the correct charge and current densities, but it gives negative probabilities in some points and regions of the phase space. Alternatively, the Husimi distribution function is positive-defined everywhere, but it does not provide the correct c...

The study of electron transport in quantum devices is mainly devoted to DC properties, while the fluctuations of the electrical current (or voltage) around these DC values, the so-called quantum noise, are much less analyzed. The computation of quantum noise is intrinsically linked (by temporal correlations) to our ability to understand/compute the...

A novel algorithm for a reduction of the computational burden associated to the time-dependent simulation of quantum transport with pure states is presented. The algorithm is based on using the superposition principle and the analytical knowledge of the free time-evolution of an initial state outside of the active region, together with absorbing la...

In open time-dependent quantum systems dealing with transport and pure states, the initial state is located outside of the active region of interest. Using the superposition principle and the analytical knowledge of the free time-evolution of such state outside the active region, together with absorbing layers and remapping, a model for a very sign...

Vanadium pentoxide V 2 O 5 was inserted between the donor layer and the anode as a hole-extracting nanolayer. Compared with devices without a hole-extracting layer, short-circuit current density (J SC), open-circuit voltage (V OC), fill factor (FF), and power conversion efficiency (PCE) of rubrene/C70-based heterojunction solar cells with 3 nm V 2O...

PIN organic solar cells with the structure of ITO/V2O5/rubrene/C70: rubrene/C70/BCP/Al were fabricated, using rubrene, rubrene and C70 mixing layer, and C70 as P, I and N layers, respectively. The thickness effect of the mixing layer on the performance of the cells is revealed, indicating that the optimized device performance is obtained at thickne...

Optical model based on transfer matrix method is employed to investigate the effects of active layer thickness and the configuration on the performance of organic solar cells based on P3HT:PCBM. Simulation result reveals that short circuit current density increases with active layer thickness and performance of inverted structure device is superior...

Bilayer cells with the structure of ITO/Rubrene/C 70/bathocuproine (BCP)/Al were fabricated. The effects of BCP layer on the performance of the cells and its mechanism were studied by inserting BCP with different thicknesses between C 70 and Al as buffer layer. The experiental results show that the power conversion efficiency of the cell is as high...

Optical modeling based on the transfer matrix method is employed to investigate the performance of the organic planar heterojunction solar cell with as the active layer. The detailed investigation is directed into the effects of layer thickness of the rubrene and on the total absorbed photon density in the active layer. It is revealed that the opti...