While a tenant is under lease, the tenant is, for practical purposes, the owner of the property. Thus, a landlord cannot enter the premises without permission. There are a couple of exception to this rule. Landlords are allowed to enter in an emergency and to protect property abandoned by a tenant. Also, unless the lease specifically provides so, a landlord – in most states – cannot enter without permission even to make investigate for repairs or even make the repairs.
Also, a tenant cannot, in most cases, withhold consent to a landlord for landlord to enter to show the property to prospective renters or buyers. At any rate, it is advisable for a landlord to provide ample notice and give specific times before entering to make repairs or allow prospective renters or buyers to see the property.