# Inclusive Privacy Design for Older Adults Living in Ambient Assisted Living

Nyteisha Bookert, May Almousa, and Mohd Anwar
Department of Computer Science , College of Engineering
North Carolina A&T State University
North Carolina, USA

###### Abstract

Ambient assisted living (AAL) environments support independence and quality of
life of older adults However, in an AAL environment, privacy-related issues
(e.g., unawareness, information disclosure, and lack of support) directly
impact older adults and bystanders (e.g., caregivers, service providers,
etc.). We explore the privacy challenges that both older adults and bystanders
face in AAL. We call for inclusive privacy design and recommend following
areas of improvement: consent, notification, and consideration for cultural
differences.

Copyright is held by the author/owner. Permission to make digital or hard
copies of all or part of this work for personal or classroom use is granted
without fee.

7th Workshop on Inclusive Privacy and Security (WIPS 2022) at _USENIX
Symposium on Usable Privacy and Security (SOUPS)_ 2022\.
August 7, 2022, Boston, MA, United States.

##  1 Introduction

The aging population of the world is rapidly growing due to an increased life
expectancy and declining fertility rates. Individuals 65 and older are
expected to outnumber children under five for the first time in history [1].
This demographic shift raises several issues, such as an increase in age-
related diseases and health care costs, a shortage of caregivers, and a rise
in dependency [15]. We need solutions that reduce the burden on society and
enable older adults to manage their health and safely maintain their autonomy.

The Internet of Medical Things (IoMT) enables medical adherence, activity
recognition, and disease management [6, 7]. Furthermore, IoMT supports remote
health monitoring to share health-related and other relevant information with
caregivers, healthcare providers, and family members. IoMT reduces the strain
of nursing homes by supporting Ambient Assisted Living, allowing elderly
adults to remain in their own homes [19], [5]. AAL environments contain
different IoMT devices and other technologies that use interrelated computing
devices to transfer data without requiring human-to-computer interaction to
facilitate a seamless living environment. These technologies include emergency
help systems, fall detection systems, and vital signs monitoring.

When used correctly, the emerging technology supports older adults to be more
organized [13]. Everyday tasks, such as cleaning, can become challenging for
seniors, technologies including robot vacuums, can make tasks more manageable.
Other technologies such as automated lights can significantly lessen the odds
of injury and risk of falls at night. The adoption of innovative home
technology can support aging at home. Intelligent devices offer an
alternative, less costly solution by leveraging technological developments. In
addition, while preserving their autonomy, older adults can deepen their
relationships with friends and family through smart home technology [8].

However, IoMT and AAL environments expose user data to the risk of known
networking vulnerabilities and introduce new threats against patients,
devices, and medical infrastructure. Hence, data privacy stands as one of the
barriers to adoption [17]. Privacy concerns and issues must be addressed in
the design process. Moreover, privacy preferences and expectations are
diverse. We explore the privacy challenges that older adults and bystanders
face in AAL. To address these challenges, we propose a research agenda with
the following research questions. What privacy issues directly impact older
adults and bystanders in an AAL environment? How do cultural differences
impact the IoMT settings in a smart home (or AAL environment)? Then we suggest
further evaluating how effective are current privacy-enhancing technologies to
addressing the issues identified through the first two questions.

The remainder of this paper is organized as follows. We introduce the older
adult population group and their needs for IoMT in section 2. Section 3
presents the AAL scenario. A discussion on the privacy challenges is in
section 4, and key considerations for inclusive design are in section 5.
Finally, we summarize our recommendations in section 6.

##  2 Adoption of IoMT among Older Adult Population Group

According to the American Association of Retired Persons (AARP) [2], in the
US, about 90 percent of adults over 65 prefer to age at home. This creates an
enormous demand for IoMT devices. The IoMT devices with the software and
health systems form an ecosystem that improves patient care and helps
streamline the workflow in the healthcare systems. Patients are, through these
technologies, able to save on medical-related costs and have a timely
intervention on their health conditions [16]. According to Khan et al. (2021)
[10], coupling healthcare with IoMT facilitates care administration and the
quality of life while still making the frameworks cost-effective.

The ability to track key health indicators remotely became especially critical
during the peak of the COVID-19 pandemic. Now, older adults and their
caregivers increasingly rely on remote health monitoring to stay in their
homes safely.

As the “Care Anywhere” healthcare trend continues with IoMT technology,
elderly people are left particularly vulnerable to privacy issues. Because
most older adults are overburdened with their health concerns, they tend to
use any IoMT device recommended to them. In many cases, the devices take their
privacy and control away, which is especially concerning in a population that
is less technologically literate. Yet, IoMT helps the aging population remain
independent for as long as possible.

##  3 Ambient Assisted Living Environment

Ambient Assisted Living (AAL) uses technologies to enable and support older
adults and those with special needs to live independently and provide more
autonomy. Health and activity monitoring sensors, such as biosensors (heart
rate, blood pressure, body temperature) and environmental (temperature, light,
humidity), allow for continuous health and cognitive status evaluation.
Evaluating the changes in activities or abnormal activities may suggest a
health decline. PIR, RFID, motion sensors, and GPS helps track and monitor
older adults’ movements throughout the space and enables wandering prevention
tools. For example, if an individual approaches a potentially dangerous area,
the relevant parties can receive an alert to intervene. A caregiver can
monitor audio and video with microphones and surveillance cameras, verify
medication adherence, and offer automated planning and scheduling [11, 15].
The environment includes multiple wireless access points and a smartphone with
an AAL App to manage the various sensors, actuators, and devices. AAL
technology can be implemented in traditional residential homes, nursing homes,
or outpatient facilities. Next, we provide three use cases in the AAL
environment.

Figure 1:  Ambient Assisted Living Environment

  1. 1.

An adult child procures an AAL home for their older parent, as shown in
Figure1. The child explains to the parent the capabilities of the home, such
as being able to control home automation (e.g., opening and closing the
blinds, the lights automatically turning on when entering a room), track
medication usage, and detect emergencies (e.g., detects falls, detects no
movement when movement is expected). The child helps the parent access the app
but does not detail the types of devices that support and enable those
applications in the home.

  2. 2.

After moving into the home, the older adult has a caregiver who visits the
home daily to assist with grocery shopping, errands, laundry, and light
housekeeping tasks.

  3. 3.

Each week, the older adult hosts a game night with a few friends who bring a
dish to share. They use the kitchen area to store the food and occasionally
use the restroom.

##  4 Privacy Concerns/Challenges

###  4.1 Bystanders

The literature has established that smart technologies have primarily focused
on end-users without considering other individuals, such as bystanders
affected by the privacy settings and postures of the IoMT devices. The
bystanders are not either target users or owners of the devices. Yao et al.
[20] define bystanders as individuals who do not own or directly use these
devices but are potentially involved in the use of smart home devices, such as
other family members who do not purchase the devices, guests, tenants,
passersby, etc. Besides, in an AAL environment, an older adult or a person
with special needs may not have decided to purchase the IoMT device or own the
living environment, which creates yet another unique scenario for bystanders
to consider. Even though older adults directly interact with and use these
devices, they may not have insight or control over the use scenarios or
privacy configurations. Consideration must be given to the interaction
patterns between target end-users and bystanders – potential guests or
visitors (e.g., caregivers, visiting friends, or family).

###  4.2 Unawareness

In an AAL system, various sensors, actuators, and devices collect health
information. Traditionally, these devices operate in the background without
human interaction, creating the risk of unawareness. Unawareness refers to a
person not knowing about the collection and sharing practices or purposes of a
system [4]. In an AAL environment, the risk is compounded for bystanders and
future users. It is imperative to consider how an individual gains access to
an AAL environment and the impact it may have on their privacy. For example,
someone who moves into an AAL home may not know or remember each item
(sensors, actuators, devices). It may be difficult for an occupant to reveal
the devices, sensors, and actuators making the technology possible. For
instance, an occupant may understand that a system monitors their recovery
after knee surgery but cannot discover which elements support the process.
Additional challenges include discovering the privacy settings for the system
and locating privacy notices of the respective device - which may fail to
support users’ preferences. Cultural differences may further expand the
misconceptions about the data flow. Individuals from a collectivism [9]
culture may expect their family members to have the ability to monitor their
day-to-day activities, while others may find this intrusive.

###  4.3 Information Disclosure

Disclosure of information is the ability of an unauthorized party or system to
learn the contents or information about a person or entity [21]. Some examples
of the disclosure include a nurse accessing records of a patient not under
their charge, an adversary gaining access to pharmaceutical research data, and
a company selling data without the consumers’ permission. Additionally, IoT
devices may contain privacy-violating interactions and presentations that
reveal information to others. The definition of an authorized person may be
unclear in an AAL environment. Consider the adult child and older adult in
scenario 1. Who decides who has access to the older adults’ information? Is it
the person who creates the account for the older adult or the older adult who
will live in the environment? If the adult child is not an authorized user but
gains access to the older adults’ system and eavesdrops during the weekly game
night, it is considered unauthorized disclosure.

###  4.4 Need for Support

The use of IoMT devices poses some challenges for older adults because they
have limited technical experiences, knowledge, and declining physical and
cognitive abilities. These are attributes that put older adults at a higher
risk with the use of this technology. The ever-changing nature of the
cybersecurity and technology landscape, which are designed with less
consideration of their age group, leads to some level of mismatched mental
models causing challenges such as privacy and usability[12]. Another issue is
that older adult tends to rely on other people for many things due to their
age-related challenges, including the operation or interaction with
technological devices such as the IoMT[14]. Therefore, while seeking help on
the devices in the AAL environment, their health information which should be
private can be exposed to extended family members, neighbors, or caregivers.

The limited knowledge regarding technical issues such as privacy settings of
an IoMT device makes the elderly less concerned about these aspects compared
to other groups. As a result, even when their privacy got compromised while
using IoMT devices, they will not be able to address the problem. Yao et al.
(2019)[20] argue that older adults do not engage in privacy-seeking behaviors
to mitigate privacy concerns. They tend to place a high degree of trust in the
manufacturers of devices and systems, which leaves them with the perception
that the developers of devices will factor in all their privacy and safety
needs.

The IoMT devices also have a design challenge that makes it hard to maintain
privacy. IoMT devices pose serious risks in terms of privacy for older adults
who are less aware, inexperienced, and less concerned with the risks involved.
There should be considerations from the device developers in dealing with
privacy issues. The privacy control settings of these devices are left to the
users by default. Whereas, in most cases, the older adults age group shares
these devices with other users, especially those supporting them. Therefore,
the design of IoMT needs to consider the socio-cultural factors in the privacy
design for the older adults age group.

##  5 Call for Inclusive Privacy Design in AAL/IoMT

  1. 1.

Consent + Data Collection. As a result of using the IoMT devices, a trove of
data is collected by the device manufacturers. The collected data is used to
assess the health status of the user and offer a suitable and immediate
response by the device. Currently, significant research around privacy
concerns has focused on end-users of smart home devices. For many older
adults, privacy concerns are overshadowed by a strong desire to remain
independent while keeping their health under control. For IoMT, the industry
must go beyond just users’ privacy expectations. Desired privacy mechanisms
must also cover the bystander’s perspective. Even though the bystanders are
not the owner or primary users of the device, they may be affected through
data collection settings. Because house guests and other family members have
not consented to data collection, privacy issues may arise. Even when an older
adult may not have privacy concerns, IoMT devices must consider the
possibility of accidental data collection from bystanders or conflicting
privacy preferences of a bystander.

Designers should consider implementing cooperative mechanisms between owners
and bystanders, such as easy access for users and bystanders, and provide
bystanders an opportunity to share privacy preferences with owners. There is
an increased need for privacy awareness at the smart home level compared to
the device level [18]. Furthermore, bystander-centric mechanisms are necessary
to aid bystanders in detecting nearby devices, revealing device behaviors,
limiting data collection, and controlling personal data [20]. The IoT Privacy
Infrastructure [3] serves as an example at a large scale.

  2. 2.

Notifications. Notifications are typical features of IoMT devices and AAL
systems. For example, a user may receive alerts to take their medication or to
exercise. If a system detects a fall, the user may receive a prompt to
determine whether the incident was a false alarm or an actual event. Emergency
contacts and emergency personnel may receive an alert during a confirmed
incident. There are several ways to present notifications within the
environment. However, they introduce unique potential privacy concerns. For
example, the medication reminder could be a visual notification on the
smartphone or an audio reminder shared in the room the person is currently
occupying. If the audio reminder shares the information while a guest is
present, it may reveal sensitive information. An appropriate level of
specificity should be included in the notification that provides appropriate
details to the individual but does not disclose sensitive information to
bystanders. Mechanisms should also be in place to allow the user to silence
notifications to limit potential exposure. Additionally, devices and sensors
in an AAL environment that collect sensitive data (audio, video, and health
information) should notify users of their presence with indicators. Indicators
range from blinking lights on a camera to an audible before a recording
begins. However, the indicator’s design should be inclusive to accommodate
individual needs and consider unobtrusive modalities [18].

  3. 3.

Cultural Differences. System designers should consider how demographic and
other cultural factors (e.g., age, relationship dynamics, sexuality, norms,
race, gender) alter preferences and expectations of the environment. For
example, the parent-child relationship scenario introduces several privacy
threats, such as unawareness about specific devices and sensors in the
environment and thereby disclosure of information. Yet, the dynamic of that
relationship and cultural norms may impact how the parent views this
interaction. The parent may want to know more about the environment. Flexible
controls should be in place for flexibility depending on the relationship
dynamics and accepted cultural beliefs. Furthermore, the proper controls may
also benefit other bystanders as they navigate the home. Parents may also
choose to limit access to their information. The controls should enable
bystanders and users to balance privacy and other key factors, such as safety
and health.

##  6 Summary

As IoMT and AAL emerge as solutions to mitigate the challenges of the aging
population, it is imperative to identify and address the privacy concerns of
all individuals, including bystanders. By considering sample scenarios of
bystanders in AAL environments interacting with IoMT devices, we establish the
need for the research community to investigate the privacy issues directly
impacting bystanders in an AAL environment and how cultural differences affect
IoMT settings in an AAL environment. Yet, effective solutions must account for
challenges such as unawareness and reliance on others for support and consider
cultural differences to provide better tools. Furthermore, we must examine how
effectively current privacy-enhancing technologies reduce privacy risks in AAL
environments.

## Acknowledgments

This work was supported partially by the U. S. Department of Education under
the Title III Historically Black Graduate Institutions (HBGI) grant. The views
and conclusions contained herein are those of the authors and should not be
interpreted as necessarily representing the official policies or endorsements,
either expressed or implied, of the U. S. Government. The U. S. Government is
authorized to reproduce and distribute reprints for governmental purposes
notwithstanding any copyright annotation therein.

———-

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# Importance of the semimetallic state for the quantum Hall effect in HfTe5

M. M. Piva<EMAIL_ADDRESS>Max Planck Institute for Chemical Physics
of Solids, Nöthnitzer Straße 40, D-01187 Dresden, Germany  R. Wawrzyńczak  Max
Planck Institute for Chemical Physics of Solids, Nöthnitzer Straße 40, D-01187
Dresden, Germany  Nitesh Kumar  Max Planck Institute for Chemical Physics of
Solids, Nöthnitzer Straße 40, D-01187 Dresden, Germany  S. N. Bose National
Centre for Basic Sciences, Salt Lake City, Kolkata, 700 106 India  L. O.
Kutelak  Brazilian Synchrotron Light Laboratory (LNLS), Brazilian Center for
Research in Energy and Materials (CNPEM), Campinas 13083-970, SP, Brazil  Max
Planck Institute for Chemical Physics of Solids, Nöthnitzer Straße 40, D-01187
Dresden, Germany  G. A. Lombardi  Brazilian Synchrotron Light Laboratory
(LNLS), Brazilian Center for Research in Energy and Materials (CNPEM),
Campinas 13083-970, SP, Brazil  R. D. dos Reis  Brazilian Synchrotron Light
Laboratory (LNLS), Brazilian Center for Research in Energy and Materials
(CNPEM), Campinas 13083-970, SP, Brazil  C. Felser  Max Planck Institute for
Chemical Physics of Solids, Nöthnitzer Straße 40, D-01187 Dresden, Germany  M.
Nicklas<EMAIL_ADDRESS>Max Planck Institute for Chemical Physics
of Solids, Nöthnitzer Straße 40, D-01187 Dresden, Germany

###### Abstract

At ambient pressure, HfTe5 is a material at the boundary between a weak and a
strong topological phase, which can be tuned by changes in its crystalline
structure or by the application of high magnetic fields. It exhibits a
Lifshitz transition upon cooling, and three-dimensional (3D) quantum Hall
effect (QHE) plateaus can be observed at low temperatures. Here, we have
investigated the electrical transport properties of HfTe5 under hydrostatic
pressure up to 3 GPa. We find a pressure-induced crossover from a semimetallic
phase at low pressures to an insulating phase at about 1.5 GPa. Our data
suggest the presence of a pressure-induced Lifshitz transition at low
temperatures within the insulating phase around 2 GPa. The quasi-3D QHE is
confined to the low-pressure region in the semimetallic phase. This reveals
the importance of the semimetallic groundstate for the emergence of the QHE in
HfTe5 and thus favors a scenario based on a low carrier density metal in the
quantum limit for the observed signatures of the quasi-quantized 3D QHE.

The two-dimensional (2D) quantum Hall effect (QHE) occurs due to the tuning of
the Fermi level through Landau levels created by the application of high
magnetic fields in 2D materials, resulting in a quantized Hall conductivity
with values of $\nu e^{2}/h$, where $\nu$ is the filling factor, $e$ is the
electron charge, and $h$ is Planck’s constant. Along with these plateaus in
the Hall conductivity, the longitudinal resistivity vanishes due to conduction
without scattering at the edges of the material. Remarkably, the bulk remains
insulating because this effect occurs when the Fermi energy is in the gap
between different Landau levels. This effect was first observed in 1980 by von
Klitzing et al. [1] and its discovery led to the application of topological
concepts to solid-state physics [2].

Materials with non-trivial topology in momentum space or real space have great
potential for applications in many fields, from catalysis and solar cells to
spintronics and even high-energy physics [3]. In particular, topological
insulators possess an insulating bulk and a metallic surface due to the
presence of metallic surface states similar to the state responsible for the
QHE. These materials can be classified as weak topological insulators (WTIs)
or strong (STIs) topological insulators, depending on whether the surface
states are present only on the side surfaces of the material or on all
surfaces [4, 5].

The 2D QHE has been extensively studied and found in graphene, for example [6,
7]. However, its three-dimensional (3D) counterpart was proposed theoretically
in 1987 [8], but it is still difficult to realize and observe experimentally.
One way to achieve a 3D QHE analog state is to stack 2D quantum Hall layers in
a highly anisotropic environment [9, 10, 11]. However, this leads to physics
similar to that observed in the 2D QHE and is referred to as quasi-2D QHE.
This effect has been observed in Bechgaard salts [12, 13], $\eta$-Mo4O11 [14],
graphite [15, 16, 17], Bi2Se3 [18], EuMnBi2 [19] and BaMnSb2 [20, 21].

Recently, the 3D QHE has been observed in the transition metal pentatellurides
ZrTe5 and HfTe5 [22, 23, 24, 25]. Both materials crystallize in an
orthorhombic $Cmcm$ structure with TMTe5 (TM = transition metal) layers
stacked along the $b$ axis [26, 27]. The groundstates of these compounds
depend strongly on the volume of the unit cell [28, 29], which can be
influenced by the growth conditions of the crystals. They can be considered as
Dirac semimetals closely related to an STI or WTI phase [24, 25, 11].
Moreover, upon cooling, a Lifshitz transition can be observed in both, which
is a change in the Fermi surface topology [30], changing the dominant carrier
type from holes to electrons and driving the materials into a semiconductor
phase and back to a semimetallic phase at low temperatures [31, 32]. The
origin of the QHE is still under debate. Previous reports found that the
quantized Hall effect is caused by Fermi surface instabilities such as a
charge density wave (CDW) [22, 23, 33]. On the other hand, several reports
claim that quasi-quantized Hall plateaus and longitudinal resistivity minima
exist in the quantum limit of 3D metals with low carrier density and closed
Fermi surfaces [25, 11]. The main difference between these two scenarios is
the need for a gapped groundstate (in the first scenario) or a metallic
groundstate (in the second scenario).

One way to distinguish between these two scenarios is to study the evolution
of the QHE as the material groundstate is tuned from metal to insulator.
Electronic band structure calculations revealed that the activation energy of
HfTe5 increases upon compression [28], suggesting that it may be possible to
tune this material from its Dirac semimetallic state at ambient pressure to an
insulating state under compression. In contrast, previous electrical
resistivity measurements under pressure have found a metallic state with
increasing pressure [34, 21]. A possible explanation could be non-hydrostatic
pressure conditions, since the previous pressure experiments were performed
with either no [34] or a solid [21] pressure-transmitting medium. This
possibility is further supported by recent measurements under uniaxial stress
and calculations which found that a metallic groundstate is favored for
uniaxial compressive stress [35, 36], i.e., non-hydrostatic pressure
conditions.

In this Letter we focus on the effects of hydrostatic pressure on the
electronic properties of HfTe5. We show that HfTe5 can be tuned from its Dirac
semimetallic state at ambient pressure to a gapped groundstate under
hydrostatic compression. We find the signature of the quasi-quantized 3D QHE
only at low pressures in the semimetallic region of the temperature – pressure
phase diagram. In addition, we find a change in the type of dominant charge
carriers at low temperatures and high pressures, indicating that HfTe5
exhibits another pressure-induced Lifshitz transition. Our results emphasize
the importance of hydrostatic conditions in pressure experiments and pinpoint
the origin of the 3D QHE in HfTe5 as a feature present in the quantum limit of
metals with low carrier density and closed Fermi surfaces.

Single crystals of HfTe5 were grown by chemical vapor transport method.
Stoichiometric amounts of Hf and Te were sealed in a quartz ampoule with
iodine and placed in a two-zone furnace for over a month. Electrical
resistivity measurements were performed with a four-probe configuration as
shown in the inset of Fig. 1 (a) using a commercial Physical Properties
Measurement System (Quantum Design) along with an AC resistance bridge (Linear
Research). Pressures up to 2.9 GPa were generated utilizing a self-clamped
piston-cylinder-type pressure cell. Silicon oil served as the pressure
transmitting medium in order to avoid non-hydrostatic pressure conditions in
the investigated pressure range [37] and lead was the pressure gauge.

Figure 1: (a) Longitudinal resistivity ($\rho_{xx}$) as a function of
temperature at various pressures. The inset presents a schematic drawing of
the sample used in the experiments. (b) Activation energy ($\Delta$) as a
function of temperature at selected pressures. The inset shows a detailed view
of the low-temperature region.

Figure 1(a) presents the temperature dependence of the longitudinal
resistivity ($\rho_{xx}$) at selected pressures. At 0.1 GPa, close to ambient
pressure, a large peak centered around 70 K is observed upon cooling due to
the presence of a temperature-induced Lifshitz transition, in agreement with
previous reports [38, 32, 23, 24]. The chemical potential of HfTe5 is strongly
temperature dependent. At room temperature the dominant carriers are hole-
like; upon cooling the system becomes a semiconductor and then a semimetal
again, but with dominant electron-like charge carriers below the Lifshitz
transition [32].

Application of external pressure first shifts the position of the peak
($T_{p}$) to higher temperatures, reaching a maximum of 79 K at 0.6 GPa,
further increasing pressure suppresses $T_{p}$ down to 47 K at 2.9 GPa (see
Fig. 2(a)), in good agreement with previous reports [38, 21] and similar to
observations in the sister compound ZrTe5 [39]. However, the strongest effect
of pressure is seen in the longitudinal resistivity at low temperatures (see
Fig. 1(a) ). The metallic behavior below $T_{p}$ first becomes weaker with
increasing pressure, and above 1.5 GPa $\rho_{xx}(T)$ starts to increase
exponentially toward low temperatures, indicating the opening of a gap below
$\approx 30$ K that has not been reported before, possibly due to non-
hydrostatic pressure conditions in the previous experiments. We will return to
this latter point below. Note that around 15 K $\rho_{xx}(T)$ tends to
saturate toward low temperatures, and a temperature $T^{*}$ can be defined as
an inflection point in $\rho_{xx}(T)$. It is almost pressure independent up to
2.9 GPa. The origin of this behavior is still unknown and could be related to
one of the following scenarios: a conduction dominated by topological surface
states [35], or the presence of impurity bands [40] or a side conduction band,
both of which can contribute to electronic transport [32].

To gain further insight into the electronic structure of HfTe5, we
investigated the evolution of the activation energy ($\Delta$) as a function
of temperature and pressure. The gap energy is usually obtained as the linear
slope of an Arrhenius plot $\ln(\rho_{xx})=\ln(\rho_{0})+\Delta/T$. However,
in HfTe5 the activation energy shows a strong evolution as a function of
temperature [32], so it is more appropriate to analyze the temperature
derivative of the Arrhenius plot $\ln(\rho_{xx})=\ln(\rho_{0})+\Delta(T)/T$,
which yields $\Delta(T)$, as shown in Fig. 1(b). At low pressures we observe
only the opening of a gap in the high-temperature region ($T\gtrsim 50$ K) and
metallic behavior below 50 K, but at 1.5 GPa and above we find the opening of
a second gap, or the reopening of the high-temperature gap, at low
temperatures ($T\lesssim 50$ K). We will refer to the maximum size of the
high-temperature gap as $\Delta_{HT}$ and the low-temperature gap as
$\Delta_{LT}$.

Figure 2: (a) Temperature-pressure phase diagram of HfTe5. (b) Maximum values
of the high-temperature gap ($\Delta_{HT}$) (left axis) and low-temperature
gap ($\Delta_{LT}$) (right axis) as a function of pressure. (c) Cross
sectional area of the Fermi surface ($S_{FZ}$) as a function of pressure. The
inset presents a Landau fan diagram at selected pressures. (d) Calculated
$\rho_{xy{\rm,plat.}}^{\rm cal.}$ and measured Hall resistivity
$\rho_{xy{\rm,plat.}}^{\rm exp.}$ at the last plateau of the QHE. (e) Carrier
density ($n$) and mobility ($\mu$) as a function of pressure. The jump in
$n(p)$ is accompanied by a change in carrier type from electron-like to hole-
like.

At 0.1 GPa near ambient pressure, the high-temperature gap opens at 200 K and
reaches a maximum size of $\Delta_{HT}=35$ meV at around 120 K, in agreement
with previous reports [32]. With increasing pressure, $\Delta_{HT}$ increases
to 61 meV at 1.1 GPa (see Fig. 2(b)), in agreement with electronic band
calculations upon unit cell compression [28] and with an early study of the
electrical resistivity HfTe5 under pressure [38]. Above 1.2 GPa $\Delta_{HT}$
decreases with further increasing pressure, but the gap opens already at
higher temperatures and beyond 1.5 GPa already above room temperature.
Furthermore, around 1.5 GPa the temperature dependence of the resistivity also
indicates a gap at low temperatures, $\Delta_{LT}$ reaching a value of 2.8 meV
at about 25 K at 1.5 GPa. By increasing the pressure $\Delta_{LT}$ increases
strongly to 4.4 meV at 2.4 GPa and then decreases again to 2.9 meV at 2.9 GPa
(see Fig. 2(b)).

A possible scenario for the existence of two different gaps in the high-
temperature and low-temperature range is the occurrence of a topological phase
transition from a WTI to a STI. This transition leads to the closing and
reopening of the gap and it was theoretically proposed for HfTe5 [41] upon
cooling, but has not yet been observed experimentally [32]. Similar
topological transitions in HfTe5 have been observed as a function of uniaxial
strain [35] or magnetic fields [42]. Therefore, it is quite feasible that the
application of external pressure tunes the Fermi energy of HfTe5 to a level at
which a topological phase transition can be induced by cooling.

We note that other reports have found metallic behavior with increasing
applied pressure. In these experiments, a solid [21] transmitting medium or
even no transmitting medium [34] was used, both of which are expected to
induce a large pressure gradient in the sample chamber. These conditions show
similarities to uniaxial stress experiments which favored a metallic state
[35, 36] and therefore the contrasting results of the aforementioned reports
compared to our Letter are not surprising. This demonstrates the importance of
hydrostatic pressure conditions for measurements on HfTe5 crystals.

At ambient pressure, the magnetoresistance (${\rm MR}$)
$\frac{\rho_{xx}(H)-\rho_{xx}(0)}{\rho_{xx}(0)}$ of the investigated HfTe5
sample shows an extremely high value of 1650% at 9 T, indicating the high
quality of the crystal (see the Supplemental Material - SM for details [43]).
At small magnetic fields $\rho_{xx}(H)$ shows the well-known quantum
oscillations (QOs) and the quantum limit is reached at 1.4 T (see Fig. 3(a)).
The application of external pressure suppresses the oscillations and the
magnitude of the MR. At 1.5 GPa and above, where a second gap opens at low
temperatures, no QOs are visible in $\rho_{xx}(H)$ (see Fig. 3(c)). In
addition, at low temperatures we find a change from electron-like charge
carriers to hole-like charge carriers in the Hall resistivity $\rho_{xy}$
around 2.2 GPa (see the SM [43]). This suggests the possibility of a pressure-
induced Lifshitz transition. Since previous experiments were performed under
less homogeneous pressure conditions than in our experiment, it may be
sensitive to the pressure homogeneity. We note that our X-ray powder
diffraction data do not show any structural phase transition at room
temperature (see the SM [43]). In the following, we discuss the pressure
evolution of the quasi-quantized 3D QHE and its relationship to the pressure-
induced changes in the electronic properties.

Figure 3: Longitudinal ($\rho_{xx}$) and Hall resistivity ($\rho_{xy}$) as a
function of magnetic field at 1.8 K and at (a) 0.1, (b) 0.8, and (c) 1.5 GPa.
The magnetic field was applied parallel to the $b$ axis in both measurements.
The arrows mark the last plateau.

To get more information about the changes in the Fermi surface, we analyze the
QO (see the SM for details [43]). The frequency of the oscillations $B_{Fz}$
was extracted from the slope of the Landau fan diagrams, as shown in the inset
of Fig. 2(c). At 0.1 GPa the extracted frequency of 1.2 T is in good agreement
with previous reports [24, 23, 42], the application of external pressure
enhances the oscillation frequency to 2.7 T at 1.2 GPa. At higher pressures,
the data no longer show QO. The cross-sectional area of the Fermi surface
$S_{Fz}=B_{Fz}\times\left(\frac{2\pi e}{\hbar}\right)$, where $\hbar$ is the
reduced Planck constant, first remains constant with increasing pressure
before it starts to increase above 0.5 GPa. The dependence of the Fermi
surface area on pressure is depicted in Fig. 2(c).

Looking more closely at the low-field region, evidence for the 3D integer QHE
in HfTe5 can be seen in Fig. 3(a). Plateaus in the Hall resistivity
$\rho_{xy}$ occur at the same fields as the minima in the longitudinal
resistivity $\rho_{xx}$. The experimental $\rho_{xy}$ of the last plateau is
close to $\rho_{xy{\rm,plat.}}^{\rm cal.}=\frac{h}{2e^{2}}\lambda_{Fz}$, where
$h$ is Planck’s constant, $e$ is the electron charge, and $\lambda_{Fz}$ is
the Fermi wavelength along the $z$ direction. However, the longitudinal
resistivity does not vanish completely as it should for a fully quantized 3D
QHE. With increasing pressure the features in $\rho_{xy}$ become less
pronounced. The plateaus in $\rho_{xy}$ can only be identified in the metallic
regime, which is indicated by the temperature dependence of the resistivity
$\rho_{xx}$ [see, e.g., Figs 3(a) and 3(b)]. At 1.5 GPa, the temperature
dependence of the resistivity shown in Fig. 1(a) indicates the opening of a
low-temperature gap, i.e., an insulating behavior, and no signs of a 3D QHE
are visible in $\rho_{xx}(H)$ and $\rho_{xy}(H)$ (Fig. 3(c)).

Two possible scenarios are currently being discussed to explain a quasi-3D QHE
in HfTe5. The first considers the material as a stack of 2D layers that
generate a Fermi surface instability, such as a CDW, which opens a gap giving
rise to a quantization of the quantum Hall effect proportional to the Fermi
wavelength along the $z$ direction $\lambda_{Fz}$ [22, 23]. The finite
longitudinal resistivity $\rho_{xx}$ is then explained by carriers from other
Fermi surface pockets contributing to the electrical transport [23]. The
second scenario achieves a quantization by considering a bulk 3D metallic
material, rather than a stack of 2D layers, with a closed Fermi surface and
low carrier density [25, 11]. When the system enters the quantum limit, the
only allowed dispersion is parallel to the applied magnetic field, resulting
in metallic Landau bands that give rise to the finite longitudinal
resistivity. In both cases, the value of $\rho_{xy}$ at the last plateau is
given by $\frac{h}{2e^{2}}\lambda_{Fz}$ [11].

At low pressures, the Hall resistivity at the last plateau agrees quite well
with the prediction in both scenarios
$\rho_{xy}=\frac{h}{2e^{2}}\lambda_{Fz}$. But at 0.8 GPa the calculated value
starts to become larger than the experimental one (see Fig. 3(d)). In this
pressure range, HfTe5 shows no fundamental changes in its electronic
properties at low temperature. It continues to exhibit metallic behavior with
low carrier density. We do not find any additional QO frequencies that could
indicate abrupt changes in the Fermi surface. Moreover, the cross-sectional
area of the Fermi surface $S_{FZ}$ increases monotonically with pressure;
i.e., we can assume that the closed 3D anisotropic Fermi pocket identified at
ambient pressure, which is a prerequisite for the model assuming a bulk 3D
metal [24], does not change significantly in this pressure range. The
characteristic changes in the low-temperature behavior occur at higher
pressures. The low-temperature gap opens only above 1.5 GPa and a change in
the dominant type of charge carrier takes place above 2 GPa. It is therefore
surprising that $\rho_{xy{\rm,plat.}}^{\rm exp.}$ deviates from
$\rho_{xy{\rm,plat.}}^{\rm cal.}$. The amplitude of the QO is strongly
suppressed with pressure, which is caused by the strong decrease of the charge
carrier mobility (see Fig. 3(e)). However, the increased uncertainty in
determining the position of the last plateau does not explain the observed
discrepancy. Our results support the scenario that the quasi-3D QHE in HfTe5
originates from a bulk 3D metallic material with a closed Fermi surface and
low carrier density, and make a scenario based on a stack of 2D layers
generating a Fermi surface instability less likely.

In conclusion, we have demonstrated that the signatures of the quasi-quantized
3D QHE in pressurized HfTe5 are confined to the semimetallic state. This
supports a scenario based on a bulk 3D metal with low carrier density in the
quantum limit instead of a scenario based on stacked 2D layers and Fermi
surface instabilities for the origin of the quasi-quantized 3D QHE in HfTe5.
Furthermore, we find a change from electron-like to hole-like carriers at low
temperatures around 2.2 GPa, pointing to the existence of a pressure-induced
Lifshitz transition.

## DATA AVAILABILITY

Data that underpin the ﬁndings of this Letter are available at Edmond – the
open research data repository of the Max Planck Society at [44].

###### Acknowledgements.

We acknowledge fruitful discussions with J. Gooth. This project has received
funding from the European Union’s Horizon 2020 research and innovation
programme under the Marie Skłodowska-Curie grant agreement No 101019024.
R.D.dR. and L.O.K. acknowledges financial support from the Brazilian agencie
FAPESP (Grants: 2018/00823-0, 2021/02314-9 and 2022/05447-2). R.D.dR. and
G.A.L. has received financial support from the Max Planck Society under the
auspices of the Max Planck Partner Group R. D. dos Reis of the MPI for
Chemical Physics of Solids, Dresden, Germany. This research used facilities of
the Brazilian Synchrotron Light Laboratory (LNLS), part of the Brazilian
Center for Research in Energy and Materials (CNPEM), a private non-profit
organization under the supervision of the Brazilian Ministry for Science,
Technology, and Innovations (MCTI). The EMA beamline and LCTE staff are
acknowledged for the assistance during the experiments 20220918. C.F. was
financially supported by Deutsche Forschungsgemeinschaft (DFG) under SFB 1143
(Project No. 247310070) and Würzburg-Dresden Cluster of Excellence on
Complexity and Topology in Quantum Matter—ct.qmat (EXC 2147, project no.
39085490).

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